tag:theconversation.com,2011:/us/topics/sexual-selection-2521/articlesSexual selection – The Conversation2023-10-03T18:00:41Ztag:theconversation.com,2011:article/2147512023-10-03T18:00:41Z2023-10-03T18:00:41ZFemale animals teach each other to choose unusual males – new research<figure><img src="https://images.theconversation.com/files/551381/original/file-20231002-17-gnxcqd.jpg?ixlib=rb-1.1.0&rect=23%2C4%2C3056%2C1934&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/elegant-peacock-colors-1255144876">alexroch/Shutterstock</a></span></figcaption></figure><p>My friend recently changed their favourite celebrity crush from Anna Kendrick to
Lily James. While some people could see the attraction, others might
not. So, what is it that attracts us to potential mates? A <a href="http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002269">new study</a> suggests that female animals learn from other females to prefer distinctive males as mates.</p>
<p>Sexual selection involves the evolution of traits such as the <a href="https://www.youtube.com/watch?v=TTwT1-TpFhE">long, elaborate tail
of the peacock</a>. These traits have evolved to enhance an animal’s chance of attracting a mate, rather than to enhance survival ability. A weighty, colourful tail is a hindrance if you’re trying to evade a predator. </p>
<p>Generally, males compete for access to females, as the male investment in
offspring –- often only sperm -– is much less substantial than the effort a female puts in, producing eggs (which are larger than sperm), pregnancy and probably raising the offspring. So it is much more costly for a female to mate with a poor-quality male than vice versa, as the male can move quickly on to the next female. </p>
<p>This has led to the evolution of a plethora of sexually-selected male traits in the animal kingdom, and some very choosy females.</p>
<p>Historically, scientists focused on the interactions between males and often ignored the way females shaped evolution. But now researchers are paying more attention to the fascinating effects of female agency. </p>
<p>The new study, from Florida State University in the US, developed a mathematical model to try and explain some of the gaps in sexual selection theories.</p>
<p>First, it’s helpful to understand what research has shown about what makes a male attractive in the animal kingdom. In terms of looks, the males with the largest
<a href="https://www.sciencedaily.com/releases/2015/09/150901113432.htm">cheek pads or flanges</a> are most appealing to female orangutans, whereas <a href="https://www.sciencedirect.com/science/article/abs/pii/S0003347205809285">males with the longest “swords”</a> drive female swordtail fish wild. </p>
<h2>Not just about looks</h2>
<p>It is not just physical appearances that females choose their mates for.
The most <a href="https://www.theguardian.com/uk-news/2020/apr/16/fruity-and-irresistible-male-lemurs-wrist-scent-seduces-the-girls">pungent-smelling male ring-tailed lemurs</a> attract the most females. There are also plenty of examples of more complex traits, including the <a href="https://news.cornell.edu/stories/2018/11/females-drive-song-and-dance-male-birds-paradise">song and dance moves</a> of birds of paradise, or the crop-circle patterns made by male Japanese puffer fish to impress females.</p>
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<iframe width="440" height="260" src="https://www.youtube.com/embed/VQr8xDk_UaY?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>And, it’s not always the males that compete to be chosen by the females. Male stalk-eyed flies choose females based on the <a href="https://academic.oup.com/beheco/article/26/2/376/257321">distance between their eyes</a>, and find wider eyespans more attractive. </p>
<p>Current <a href="https://www.pnas.org/doi/10.1073/pnas.0608184103">theories of sexual selection</a> involve animals choosing mates due to signs they have good genes– like a long, elaborate tail. A strong, virile mate signals they will produce healthy offspring. </p>
<p>Alternatively, animals that have a hindersome trait, yet still survive, are probably of high genetic quality. There are also sensory bias theories, where mating preferences are a by-product of natural selection of the senses. </p>
<p>For example, <a href="https://link.springer.com/referenceworkentry/10.1007/978-3-319-19650-3_93">the hearing range of female Tungara frogs</a> is biased towards lower frequencies, coinciding with their preference for the lower-frequency calls produced by larger males.</p>
<p>Yet, none of these theories explain why there is so much variation in traits of males of the same species, or why female preferences can vary over time or within a species. </p>
<h2>Rare sex appeal</h2>
<p>The new study looked at whether females’ mate choices are based on watching more
experienced females choose their mates. It is well-known that animals can
learn from watching others. For example, young crows <a href="https://www.nature.com/articles/s41598-018-27405-1">learn how to make basic stick tools</a> by watching their parents. </p>
<p>Learning has also been shown in mate choice as <a href="https://royalsocietypublishing.org/doi/10.1098/rspb.2005.3054">females observing others</a> with
a male are more likely to choose that male themselves, or one with similar
traits.</p>
<p>The researchers based their model on the inferred attractiveness hypothesis,
where inexperienced females compare the qualities of a male, chosen by an
experienced female, to the qualities of all males. </p>
<p>For example, if a female sees an experienced female with a bright coloured male, she might seek a brightly coloured mate too. This would lead to bright colouration becoming more common, reducing variation. However, the inferred attractiveness hypothesis still doesn’t explain why there is so much divergence between males. </p>
<p>The US study was the first to consider that females may not be mind readers and may make mistakes when they try to copy other females. </p>
<p>In the hypothetical example illustrated below, the experienced female prefers males with redder plumage, so chooses to mate with male number three. The inexperienced female observer thinks that male number three’s long tail made him more attractive than his peers. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/551696/original/file-20231003-17-oibla1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Illustration of two female birds choosing which male bird to mate with." src="https://images.theconversation.com/files/551696/original/file-20231003-17-oibla1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/551696/original/file-20231003-17-oibla1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=266&fit=crop&dpr=1 600w, https://images.theconversation.com/files/551696/original/file-20231003-17-oibla1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=266&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/551696/original/file-20231003-17-oibla1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=266&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/551696/original/file-20231003-17-oibla1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=334&fit=crop&dpr=1 754w, https://images.theconversation.com/files/551696/original/file-20231003-17-oibla1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=334&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/551696/original/file-20231003-17-oibla1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=334&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Illustration from the Florida State University study.</span>
<span class="attribution"><a class="source" href="https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002269">Florida State University/PLOS Biology</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>The researchers used a computer simulation based on a mathematical model
of a population in which males mated with many females. The model involved males with two traits, with only two variants each (bright/dull colour and long/short tail length).</p>
<p>Their model showed that when females chose males based on the same trait that
the experienced female went for, these traits became fixed in the population, with
no variation. When females chose a more distinctive male, this caused the rare trait to become more common and, subsequently, less attractive. </p>
<p>This resulted in switches in female preferences over time, rather than a single attractive trait outcompeting the others.</p>
<p>We won’t know whether this happens in real life until scientists run field studies. But this is the first theory of sexual selection that explains how variation might be maintained in populations. </p>
<p>My current celebrity crush is Ryan Reynolds due to his good looks and
sense of humour. But I now wonder what other people might see in him – is it those eyes, or that smile, or something completely different?</p><img src="https://counter.theconversation.com/content/214751/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Louise Gentle works for Nottingham Trent University. </span></em></p>A new study modelled how a game of snog, marry, avoid, may play out in the animal kingdom.Louise Gentle, Principal Lecturer in Wildlife Conservation, Nottingham Trent UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1975312023-01-15T19:01:06Z2023-01-15T19:01:06ZBirdsong isn’t just competition for mates or territory. Zebra finches sing to bond<figure><img src="https://images.theconversation.com/files/504363/original/file-20230113-12-kcbftw.jpg?ixlib=rb-1.1.0&rect=0%2C87%2C3067%2C1577&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>When you hear beautiful birdsong, such as the warbling of the <a href="https://youtu.be/IQiuld1Z1i8">Australasian magpie</a>, you might believe it’s a sign of intense competition for territory or showing off to attract a mate. </p>
<p>After all, that’s the way birdsong is often thought of – a way for male birds to compete with each other. A prettier version of nature red in tooth and claw, as Tennyson <a href="https://wordhistories.net/2018/05/18/red-tooth-claw/">put it</a>. There’s some truth to it – in many species, even the most beautiful song by a male in another’s territory will invite attack. </p>
<p>But birdsong isn’t one dimensional. In some species, birdsong is much less about competition and much more about social cooperation. <a href="https://doi.org/10.1016/j.cub.2022.11.047">Our study</a> of highly social zebra finches found the song of the males boosts social cohesion in local populations. It even helps these tiny birds to coordinate their nest building, mating and reproduction when there’s about to be abundant food for their young. </p>
<p>So when you hear a melodious song, enjoy it. Nature isn’t just about competition. More and more, we’re learning cooperation is there too. In some species, birdsong promotes harmony at a species level – and boost bonding between a male and his partner. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/504361/original/file-20230113-28-18790e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Zebra finches" src="https://images.theconversation.com/files/504361/original/file-20230113-28-18790e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/504361/original/file-20230113-28-18790e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/504361/original/file-20230113-28-18790e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/504361/original/file-20230113-28-18790e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/504361/original/file-20230113-28-18790e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/504361/original/file-20230113-28-18790e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/504361/original/file-20230113-28-18790e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Flocks of zebra finches roam the interior, looking for seeds and water.</span>
<span class="attribution"><span class="source">Shutterstock</span></span>
</figcaption>
</figure>
<h2>What did we find?</h2>
<p>Zebra finches travel in large flocks through Australia’s arid interior, looking for seeds. They’re highly social, and are known as a bird species able to learn new songs. </p>
<p>Zebra finches are one of Australia’s most-studied birds, because they’re easy to keep in captivity. But to discover how they act in the wild, we had to go remote to a study area near Broken Hill. </p>
<p>To capture their song, we installed more than 20 microphones and recorded them from dawn to dusk every four days over a whole year. </p>
<p>We chose this species because the males sing their own individually distinctive songs – and they do it relatively quietly, compared to the loudspeaker bugling of, say, a currawong. </p>
<p>That’s important. When birds sing softly, it’s a sign the song isn’t a warning about territory. Zebra finch songs can be heard only by birds within a few metres, and they’re often sung when birds perch together in the same bush. So why do they do it? When each male sings their own unmistakable songs, it lets others in the group figure out who is singing – and gauge how many males are close by. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/tis-the-season-to-be-jolly-singing-christmas-carols-together-isnt-just-a-tradition-its-also-good-for-you-193855">'Tis the season to be jolly: singing Christmas carols together isn't just a tradition, it's also good for you</a>
</strong>
</em>
</p>
<hr>
<p>Listen for yourself. The two songs below are from male zebra finches – but they’re clearly very different from each other.</p>
<p><audio preload="metadata" controls="controls" data-duration="10" data-image="" data-title="Song of zebra finch male #1" data-size="119770" data-source="Hugo Loning (recorded at Fowlers Gap)" data-source-url="" data-license="Author provided" data-license-url="">
<source src="https://cdn.theconversation.com/audio/2722/single-male-file70.mp3" type="audio/mpeg">
</audio>
<div class="audio-player-caption">
Song of zebra finch male #1.
<span class="attribution"><span class="source">Hugo Loning (recorded at Fowlers Gap)</span>, <span class="license">Author provided</span><span class="download"><span>117 KB</span> <a target="_blank" href="https://cdn.theconversation.com/audio/2722/single-male-file70.mp3">(download)</a></span></span>
</div></p>
<p><audio preload="metadata" controls="controls" data-duration="7" data-image="" data-title="Song of zebra finch male #2" data-size="82864" data-source="Hugo Loning (recorded at Fowlers Gap)" data-source-url="" data-license="Author provided" data-license-url="">
<source src="https://cdn.theconversation.com/audio/2723/single-male-file81.mp3" type="audio/mpeg">
</audio>
<div class="audio-player-caption">
Song of zebra finch male #2.
<span class="attribution"><span class="source">Hugo Loning (recorded at Fowlers Gap)</span>, <span class="license">Author provided</span><span class="download"><span>80.9 KB</span> <a target="_blank" href="https://cdn.theconversation.com/audio/2723/single-male-file81.mp3">(download)</a></span></span>
</div></p>
<p>What happens when you play one male’s song to another male? In more territorial and less social species, it would provoke a aggressive response. But in zebra finches, it has the opposite effect. It actually draws in other males closer to share the same space. That’s why you can often hear multiple males sing together in the same bush. </p>
<p><audio preload="metadata" controls="controls" data-duration="12" data-image="" data-title="Zebra finch song in a social tree" data-size="72864" data-source="Hugo Loning (recorded at Fowlers Gap)" data-source-url="" data-license="Author provided" data-license-url="">
<source src="https://cdn.theconversation.com/audio/2724/song-in-social-context.mp3" type="audio/mpeg">
</audio>
<div class="audio-player-caption">
Zebra finch song in a social tree.
<span class="attribution"><span class="source">Hugo Loning (recorded at Fowlers Gap)</span>, <span class="license">Author provided</span><span class="download"><span>71.2 KB</span> <a target="_blank" href="https://cdn.theconversation.com/audio/2724/song-in-social-context.mp3">(download)</a></span></span>
</div></p>
<h2>Song for social bonding</h2>
<p>On our recordings, we heard zebra finches every day – even though it was in 2019, when one of the worst droughts in the last 100 years made conditions very poor. No finches even attempted to breed. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/504169/original/file-20230112-13-lj8qc1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/504169/original/file-20230112-13-lj8qc1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/504169/original/file-20230112-13-lj8qc1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/504169/original/file-20230112-13-lj8qc1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/504169/original/file-20230112-13-lj8qc1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/504169/original/file-20230112-13-lj8qc1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/504169/original/file-20230112-13-lj8qc1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Researcher checking one of the acoustic recording stations during a sandstorm at the height of the drought in February 2019.</span>
<span class="attribution"><span class="source">Simon Griffith</span></span>
</figcaption>
</figure>
<p>But then the climate cycle shifted from drought to rains and flood. Water drove grass growth – and that meant seeds were coming. The finches sensed this and began mating. </p>
<p>As the drought broke, we heard the males sing more, peaking around the time the female birds laid their eggs. This suggests zebra finch song is part of an ongoing “discussion” between a female and her partner, which helps synchronise their activity. Song is likely to be related to how birds feel. When food is abundant birds feel better and sing more. When individual finches hear lots of singing in the area they know others are also enjoying the conditions and it gives greater confidence in the decision to start breeding. </p>
<p>So if zebra finch males don’t sing to claim territory, are they doing it to show off to prospective mates? Nope. In these remarkable birds, the males tend to sing after they’ve found a partner. They sing to her while perching next to her on a branch, and they will sing regularly – even daily – over the years of their relationship. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/504168/original/file-20230112-17-gln6ej.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/504168/original/file-20230112-17-gln6ej.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=419&fit=crop&dpr=1 600w, https://images.theconversation.com/files/504168/original/file-20230112-17-gln6ej.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=419&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/504168/original/file-20230112-17-gln6ej.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=419&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/504168/original/file-20230112-17-gln6ej.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=526&fit=crop&dpr=1 754w, https://images.theconversation.com/files/504168/original/file-20230112-17-gln6ej.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=526&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/504168/original/file-20230112-17-gln6ej.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=526&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">A pair of zebra finches (the female is on the left)</span>
<span class="attribution"><span class="source">Simon Griffith</span></span>
</figcaption>
</figure>
<p>The females of most singing species <a href="https://www.scientificamerican.com/article/why-it-took-so-long-to-appreciate-female-birds-songs/">sing too</a>. Female zebra finches, however, don’t sing. </p>
<p>We believe the serenading of the female zebra finch by her partner may play a similar social function to the commonplace song duets you can hear from Australian songbirds in suburbs or in the bush. </p>
<p>For instance, magpie-lark couples will sing a duet while doing a kind of dance, making distinctive, synchronised wing spreading, shoulder raising and body flicking movements.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/aZ3MYBxG9d4?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Duetting magpie-larks.</span></figcaption>
</figure>
<p>These duets help to strengthen the pair’s bond and help them coordinate their behaviour. These synchronised songs and visual displays also show the strength of their partnership, which can make territory intruders <a href="https://www.anu.edu.au/news/all-news/reality-and-illusion-in-magpie-lark-song-and-dance-duets">think twice</a>. For zebra finches, it may be the male’s constant song is enough to keep the pair well bonded.</p>
<p>Australia is rightly famous for its birds, from raucous cockatoos to melodious songbirds. After all, birdsong <a href="https://www.australiangeographic.com.au/news/2016/08/the-worlds-songbirds-island-hopped-out-of-australia/">evolved here first</a> and spread to the rest of the world. </p>
<p>Now we know the zebra finch uses song as social bonding, we can look for this in other species around the world. We expect many other gregarious species will use song in ways more complex than marking territory or drawing in mates. </p>
<p>Birdsong is the animal sound we most regularly encounter – it’s part of the background of our lives. But for birds, it’s far beyond a pretty melody. It’s fundamental to how they live and who they are as a species. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/only-the-lonely-an-endangered-bird-is-forgetting-its-song-as-the-species-dies-out-156950">Only the lonely: an endangered bird is forgetting its song as the species dies out</a>
</strong>
</em>
</p>
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<img src="https://counter.theconversation.com/content/197531/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Simon Griffith receives funding from The Australian Research Council, and this work was supported by a grant to Marc Naguib from the Netherlands Organisation for Scientific Research (NWO). </span></em></p><p class="fine-print"><em><span>Hugo Loning is doing a PhD supported by a grant to Marc Naguib from the Netherlands Organisation for Scientific Research (NWO). </span></em></p>Birdsong plays a vital social role in the lives of these gregarious finches.Simon Griffith, Professor of Avian Behavioural Ecology, Macquarie UniversityHugo Loning, PhD candidate in Behavioural Ecology, Wageningen UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1947932022-11-21T13:15:49Z2022-11-21T13:15:49ZPeople don’t mate randomly – but the flawed assumption that they do is an essential part of many studies linking genes to diseases and traits<figure><img src="https://images.theconversation.com/files/496010/original/file-20221117-25-slwoe3.jpg?ixlib=rb-1.1.0&rect=110%2C96%2C4690%2C2134&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Statistical pitfalls in GWAS can result in misleading conclusions about whether some traits (like long horns or spotted skin, in the case of dinosaurs) are genetically linked.</span> <span class="attribution"><span class="source">@meanymoo</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span></figcaption></figure><p>The idea that <a href="https://doi.org/10.1002/0471667196.ess0209.pub2">correlation does not imply causation</a> is a fundamental caveat in epidemiological research. A classic example involves a hypothetical link between ice cream sales and drownings – instead of increased ice cream consumption causing more people to drown, it’s plausible that a third variable, summer weather, is driving up an appetite for ice cream and swimming, and hence opportunities to drown.</p>
<p>But what about correlations involving genes? How can researchers be sure that a particular trait or disease is truly genetically linked, and not caused by something else?</p>
<p>We are <a href="https://www.richardborder.com">statistical</a> <a href="https://scholar.google.com/citations?user=SPXgieEAAAAJ&hl=en">geneticists</a> who study the genetic and nongenetic factors that influence human variation. In our <a href="https://www.science.org/doi/10.1126/science.abo2059">recently published research</a>, we found that the genetic links between traits found in many studies might not be connected by genes at all. Instead, many are a result of how humans mate.</p>
<h2>Genome-wide association studies try to link genes to traits</h2>
<p>Because the genes you inherit from your parents remain unchanged throughout your life, with rare exception, it makes sense to assume that there is a causal relationship between certain traits you have and your genetics.</p>
<p>This logic is the basis for <a href="https://www.genome.gov/about-genomics/fact-sheets/Genome-Wide-Association-Studies-Fact-Sheet">genome-wide association studies, or GWAS</a>. These studies collect DNA from many people to identify positions in the genome that might be correlated with a trait of interest. For example, if you have certain forms of the <a href="https://www.cancer.gov/about-cancer/causes-prevention/genetics/brca-fact-sheet"><em>BRCA1</em> and <em>BRCA2</em> genes</a>, you may have an increased risk for certain types of cancer.</p>
<p>Similarly, there may be gene variants that play a role in whether or not someone has schizophrenia. The hope is to learn something about the complex mechanisms that link variation at the molecular level to individual differences. With a clearer understanding of the genetic basis of different traits, scientists would be better able to determine risk factors for related diseases. </p>
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<figcaption><span class="caption">GWAS studies seek to find genetic associations between individual traits.</span></figcaption>
</figure>
<p>Researchers have run <a href="https://doi.org/10.1093/nar/gky1120">thousands of GWAS to date</a>, identifying genetic variants associated with myriad diseases and disease-related traits. In many instances, researchers have identified genetic variants that affect more than one trait. This form of biological overlap, in which the same genes are thought to influence several apparently unrelated traits, is known as <a href="https://doi.org/10.1186/s13073-016-0332-x">pleiotropy</a>. For example, certain variants of the <a href="https://medlineplus.gov/genetics/gene/pah"><em>PAH</em> gene</a> can have <a href="https://medlineplus.gov/genetics/condition/phenylketonuria/">several distinct effects</a>, including altering skin pigmentation and causing seizures.</p>
<p>One way scientists assess pleiotropy is through <a href="https://doi.org/10.1038/ng.3604">genetic correlation analysis</a>. Here, geneticists investigate whether the genes associated with a given trait are associated with other traits or diseases by statistically analyzing large samples of genetic data. Over the past decade, genetic correlation analysis has become the primary method for assessing potential pleiotropy across fields as diverse as <a href="https://doi.org/10.1038/ng.3406">internal medicine</a>, <a href="https://www.thessgac.org">social science</a> and <a href="https://doi.org/10.1017/s0033291717002318">psychiatry</a>. </p>
<p>Scientists use the findings from genetic correlation analyses to figure out the potential shared causes of these traits. For instance, if <a href="https://doi.org/10.1126/science.aap8757">genes associated with bipolar disorders</a> also predict anxiety disorders, perhaps the two conditions may partially involve some of the same neural circuits or respond to similar treatments.</p>
<h2>Assortative mating and genetic correlation</h2>
<p>However, just because a gene is correlated with two or more traits doesn’t necessarily mean it causes them.</p>
<p>Virtually all the statistical methods researchers commonly use to assess genetic correlations <a href="https://doi.org/10.1046/j.1439-0388.2002.00356.x">assume that mating is random</a>. That is, they assume that potential mating partners decide who they will have children with based on a roll of the dice. In reality, many factors likely influence who mates with whom. The simplest example of this is geography – people living in different parts of the world are less likely to end up together than people living nearby.</p>
<p>We wanted to find out how much the assumption of random mating affects the accuracy of genetic correlation analyses. In particular, we focused on the potential confounding effects of <a href="https://doi.org/10.1038/s41562-018-0476-3">assortative mating</a>, or how people tend to mate with those who share similar characteristics with them. Assortative mating is a widely documented phenomenon seen across a broad array of traits, interests, measures and social factors, including <a href="https://doi.org/10.1002/ajhb.22917">height</a>, <a href="https://doi.org/10.2307/2095670">education</a> and <a href="https://doi.org/10.1016/j.biopsych.2019.06.025">psychiatric conditions</a>.</p>
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<figcaption><span class="caption">Humans do not mate randomly – rather, people tend to gravitate toward certain traits.</span></figcaption>
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<p>In <a href="https://doi.org/10.1126/science.abo2059">our study</a> we examined cross-trait assortative mating, whereby people with one trait (for example, being tall) tend to mate with people with a completely different trait (for example, being wealthy). From our database of 413,980 mate pairs in the U.K. and Denmark, we found evidence of cross-trait assortative mating for many traits – for instance, an individual’s time spent in formal schooling was correlated not only with their mate’s educational attainment, but also with many other characteristics, including height, smoking behaviors and risk for different diseases.</p>
<p>We found that taking into consideration the similarities across mates could strongly predict which traits would be considered genetically linked. In other words, just based on how many characteristics a pair of mates shared, we could identify around 75% of the presumed genetic links between these traits – all without sampling any DNA.</p>
<h2>Genetic correlation does not imply causation</h2>
<p>Cross-trait assortative mating shapes the genome. If people with one heritable trait tend to mate with people with another heritable trait, then these two distinct characteristics will become genetically correlated to each other in subsequent generations. This will happen regardless of whether or not these traits are truly genetically linked to each other.</p>
<p>Cross-trait assortative mating means that the genes you inherit from one parent will be correlated with those you inherit from the other. How people mate is not random, violating the key assumption behind genetic correlation analyses. This inflates the genetic association between traits that aren’t truly linked together by genes.</p>
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<a href="https://images.theconversation.com/files/495756/original/file-20221116-21-hyom6p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Illustration of dinosaurs with and without long horns or spiked backs." src="https://images.theconversation.com/files/495756/original/file-20221116-21-hyom6p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/495756/original/file-20221116-21-hyom6p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/495756/original/file-20221116-21-hyom6p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/495756/original/file-20221116-21-hyom6p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/495756/original/file-20221116-21-hyom6p.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/495756/original/file-20221116-21-hyom6p.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/495756/original/file-20221116-21-hyom6p.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=533&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">If dinosaurs with long horns preferentially mate with dinosaurs with spiked backs, genes for both of these traits can become associated with each other in subsequent generations even though the same gene doesn’t code for them.</span>
<span class="attribution"><span class="source">Aaqilah M</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">CC BY-NC-ND</a></span>
</figcaption>
</figure>
<p>Recent studies corroborate our findings. Earlier this year, researchers computed genetic correlations using a method that examines the association between the <a href="https://doi.org/10.1038/s41588-022-01062-7">traits and genes of siblings</a>. The genetic links between traits influenced by cross-trait assortative mating were substantially weakened.</p>
<p>But without accounting for cross-trait assortative mating, using genetic correlation estimates to study the biological pathways causing disease can be misleading. Genes that affect only one trait will appear to influence multiple different conditions. For example, a genetic test designed to assess the risk for one disease may incorrectly detect vulnerability for a broad number of unrelated conditions.</p>
<p>The ability to measure variation across individuals at the genetic and molecular level is truly a feat of modern science. However, genetic epidemiology is still an observational enterprise, subject to the same caveats and challenges facing other forms of nonexperimental research. Though our findings don’t discount all genetic epidemiology research, understanding what genetic studies are truly measuring will be essential to translate research findings into new ways to treat and assess disease.</p><img src="https://counter.theconversation.com/content/194793/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Richard Border receives funding from the National Institutes of Health.</span></em></p><p class="fine-print"><em><span>Noah Zaitlen receives funding from the NIH, NSF, DoD, and CZI. </span></em></p>People don’t randomly select who they have children with. And that means an underlying assumption in research that tries to link particular genes to certain diseases or traits is wrong.Richard Border, Postdoctoral Researcher in Statistical Genetics, University of California, Los AngelesNoah Zaitlen, Professor of Neurology and Human Genetics, University of California, Los AngelesLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1769972022-03-01T16:13:59Z2022-03-01T16:13:59ZFuture evolution: from looks to brains and personality, how will humans change in the next 10,000 years?<figure><img src="https://images.theconversation.com/files/448514/original/file-20220225-21-1ewvdn2.jpg?ixlib=rb-1.1.0&rect=26%2C40%2C2968%2C1931&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Where's next for Homo Sapiens?</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/kyiv-ukraine-june-16-2018-national-1120827521">Shutterstock</a></span></figcaption></figure><p><strong>READER QUESTION:</strong> <em>If humans don’t die out in a climate apocalypse or asteroid impact in the next 10,000 years, are we likely to evolve further into a more advanced species than what we are at the moment? Harry Bonas, 57, Nigeria</em></p>
<p>Humanity is the unlikely result of 4 billion years of evolution. </p>
<p>From self-replicating molecules in Archean seas, to eyeless fish in the Cambrian deep, to mammals scurrying from dinosaurs in the dark, and then, finally, improbably, ourselves – evolution shaped us. </p>
<p>Organisms reproduced imperfectly. Mistakes made when copying genes sometimes made them better fit to their environments, so those genes tended to get passed on. More reproduction followed, and more mistakes, the process repeating over billions of generations. Finally, <em>Homo sapiens</em> appeared. But we aren’t the end of that story. Evolution won’t stop with us, and <a href="https://www.newscientist.com/article/mg20126952-400-review-the-10000-year-explosion-by-gregory-cochran-and-henry-harpending/">we might even be evolving faster than ever</a>. </p>
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<img alt="" src="https://images.theconversation.com/files/313328/original/file-20200203-41485-1foofme.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/313328/original/file-20200203-41485-1foofme.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/313328/original/file-20200203-41485-1foofme.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/313328/original/file-20200203-41485-1foofme.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/313328/original/file-20200203-41485-1foofme.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/313328/original/file-20200203-41485-1foofme.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/313328/original/file-20200203-41485-1foofme.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<p><strong><em>This article is part of <a href="https://theconversation.com/uk/topics/lifes-big-questions-80040?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=LifesBigQuestionsUK">Life’s Big Questions</a></em></strong>
<br><em>The Conversation’s new series, co-published with BBC Future, seeks to answer our readers’ nagging questions about life, love, death and the universe. We work with professional researchers who have dedicated their lives to uncovering new perspectives on the questions that shape our lives.</em></p>
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<p>It’s hard to predict the future. The world will probably change in ways we can’t imagine. But we can make educated guesses. Paradoxically, the best way to predict the future is probably looking back at the past, and assuming past trends will continue going forward. This suggests some surprising things about our future.</p>
<p>We will likely live longer and become taller, as well as more lightly built. We’ll probably be less aggressive and more agreeable, but have smaller brains. A bit like a golden retriever, we’ll be friendly and jolly, but maybe not that interesting. At least, that’s one possible future. But to understand why I think that’s likely, we need to look at biology.</p>
<h2>The end of natural selection?</h2>
<p>Some scientists have argued that civilisation’s rise <a href="https://www.theguardian.com/science/2007/dec/15/genetics.evolution">ended natural selection</a>. It’s true that selective pressures that dominated in the past – <a href="https://onlinelibrary.wiley.com/doi/10.1002/ajpa.20415">predators</a>, famine, <a href="https://www.amazon.com/dp/B0047747QK/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1">plague</a>, <a href="https://www.amazon.com/dp/B00DTDFOFG/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1">warfare</a> – have mostly disappeared. </p>
<p>Starvation and famine were largely ended by <a href="https://www.amazon.com/dp/B003QP4BJM/ref=dp-kindle-redirect?_encoding=UTF8&btkr=1">high-yield crops, fertilisers</a> and family planning. Violence and war are less common than ever, despite modern militaries with nuclear weapons, or maybe <a href="https://www.amazon.com/Making-Atomic-Bomb-Richard-Rhodes-ebook/dp/B008TRU7SQ/ref=sr_1_1?crid=81PRQTJ0LOPH&keywords=making%20of%20the%20atomic%20bomb&qid=1644311120&s=digital-text&sprefix=making%20of%20the%20atomic%20bom%2Cdigital-text%2C264&sr=1-1">because of them</a>. The lions, wolves and sabertoothed cats that hunted us in the dark are endangered or extinct. Plagues that killed millions – smallpox, Black Death, cholera – were tamed by vaccines, antibiotics, clean water.</p>
<p>But evolution didn’t stop; other things just drive it now. Evolution isn’t so much about survival of the fittest as reproduction of the fittest. Even if nature is less likely to murder us, we still need to find partners and raise children, so sexual selection now plays a bigger role in our evolution.</p>
<p>And if nature doesn’t control our evolution anymore, the unnatural environment we’ve created – culture, technology, cities – produces new selective pressures very unlike those we faced in the ice age. We’re poorly adapted to this modern world; it follows that we’ll have to adapt. </p>
<p>And that process has already started. As our diets changed to include grains and dairy, we evolved genes to help us <a href="https://www.nature.com/articles/ng2123">digest starch</a> and <a href="https://www.annualreviews.org/doi/pdf/10.1146/annurev-genom-091416-035340">milk</a>. When dense cities created conditions for disease to spread, mutations for <a href="https://www.pnas.org/content/100/25/15276.short">disease resistance spread</a> too. And for some reason, <a href="https://www.karger.com/Article/Abstract/519504">our brains have got smaller</a>. Unnatural environments create unnatural selection.</p>
<p>To predict where this goes, we’ll look at our prehistory, studying trends over the past 6 million years of evolution. Some trends will continue, especially those that emerged in the past 10,000 years, after agriculture and civilisation were invented. </p>
<p>We’re also facing new selective pressures, such as reduced mortality. Studying the past doesn’t help here, but we can see how other species responded to similar pressures. Evolution in domestic animals may be especially relevant – arguably we’re becoming a kind of domesticated ape, but curiously, <a href="https://www.science.org/content/article/early-humans-domesticated-themselves-new-genetic-evidence-suggests?utm_campaign=SciMag&utm_source=Social&utm_medium=Facebook">one domesticated by ourselves</a>. </p>
<p>I’ll use this approach to make some predictions, if not always with high confidence. That is, I’ll speculate.</p>
<h2>Lifespan</h2>
<p>Humans will almost certainly evolve to live longer – much longer. Life cycles evolve in response to mortality rates, how likely predators and other threats are to kill you. When mortality rates are high, animals must reproduce young, or might not reproduce at all. There’s also no advantage to evolving mutations that prevent ageing or cancer - you won’t live long enough to use them. </p>
<p>When mortality rates are low, the opposite is true. It’s better to take your time reaching sexual maturity. It’s also useful to have adaptations that extend lifespan, and fertility, giving you more time to reproduce. That’s why animals with few predators - animals that live on islands or in the deep ocean, or are simply big - evolve longer lifespans. <a href="https://www.science.org/doi/full/10.1126/science.aaf1703">Greenland sharks</a>, <a href="https://www.newscientist.com/article/mg18324645-000-the-origin-of-harriet/?ignored=irrelevant">Galapagos tortoises</a> and <a href="https://www.sciencedirect.com/science/article/pii/S2211124714010195">bowhead whales</a> mature late, and can live for centuries.</p>
<p>Even before civilisation, people were unique among apes in having low mortality and <a href="https://onlinelibrary.wiley.com/doi/abs/10.1002/evan.1360010406">long lives</a>. Hunter-gatherers armed with spears and bows could defend against predators; <a href="https://www.sciencedirect.com/science/article/pii/S1090513800000660?casa_token=flfN2IIm4GMAAAAA%3A4wiyTcAbi4_ekhP3nx7AOBVKyWpHM-Yco6uu5x8jCVzhBp-4XhE7TNLbN_JLG29lpvrsp_sw2g">food sharing</a> prevented starvation. So we evolved delayed sexual maturity, and long lifespans - <a href="https://link.springer.com/referenceworkentry/10.1007/978-3-319-16999-6_2352-1">up to 70 years</a>.</p>
<p>Still, child mortality was high - <a href="https://escholarship.org/content/qt2zb2v4x9/qt2zb2v4x9.pdf">approaching 50%</a> or <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/j.1651-2227.2011.02573.x?casa_token=y-YSz5W2IRIAAAAA%3A4sZ98kY2GLRSs35WMNzJr_WqDQrxDqTuqwyIUYSe9N8Y6fx0XoYSAHCDxyrDqYyYuCl2apKeQErOUSc">more</a> by age 15. Average life expectancy was just <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/j.1651-2227.2011.02573.x?casa_token=y-YSz5W2IRIAAAAA%3A4sZ98kY2GLRSs35WMNzJr_WqDQrxDqTuqwyIUYSe9N8Y6fx0XoYSAHCDxyrDqYyYuCl2apKeQErOUSc">35 years</a>. Even after the rise of civilisation, child mortality stayed high until the 19th century, while life expectancy went down - <a href="https://ourworldindata.org/life-expectancy">to 30 years</a> - due to plagues and famines.</p>
<p>Then, in the past two centuries, better nutrition, medicine and hygiene reduced youth mortality to <a href="https://ourworldindata.org/grapher/youth-mortality-rate">under 1%</a> in most developed nations. Life expectancy soared to <a href="https://ourworldindata.org/life-expectancy#:%7E:text=The%20United%20Nations%20estimate%20a,life%20expectancy%20of%2072.3%20years.">70 years worldwide </a>, and 80 in developed countries. These increases are due to improved health, not evolution – but they set the stage for evolution to extend our lifespan.</p>
<p>Now, there’s little need to reproduce early. If anything, the years of training needed to be a doctor, CEO, or carpenter incentivise putting it off. And since our life expectancy has doubled, adaptations to prolong lifespan and child-bearing years are now advantageous. Given that more and more people live to <a href="https://ourworldindata.org/life-expectancy#:%7E:text=The%20United%20Nations%20estimate%20a,life%20expectancy%20of%2072.3%20years.">100</a> or even <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2895458/">110 years</a> - <a href="https://www.nature.com/articles/d41586-022-00070-1">the record being 122 years</a> - there’s reason to think our genes could evolve until the average person routinely lives 100 years or even more.</p>
<h2>Size, and strength</h2>
<p>Animals often evolve larger size over time; it’s a trend seen in <a href="https://www.pnas.org/content/113/13/3447">tyrannosaurs</a>, whales, <a href="https://www.cambridge.org/core/journals/paleobiology/article/abs/fossil-horses-from-eohippus-hyracotherium-to-equus-scaling-copes-law-and-the-evolution-of-body-size/9965A8A02408FBECB70D012A54930EA2">horses</a> and primates - including hominins. </p>
<p>Early hominins like <a href="https://humanorigins.si.edu/evidence/human-fossils/species/australopithecus-afarensis"><em>Australopithecus afarensis</em></a> and <a href="https://humanorigins.si.edu/evidence/human-fossils/species/homo-habilis"><em>Homo habilis</em></a> were small, four to five feet (120cm-150cm) tall. Later hominins - <a href="https://humanorigins.si.edu/evidence/human-fossils/species/homo-erectus"><em>Homo erectus</em></a>, Neanderthals, <em>Homo sapiens</em> - grew taller. We’ve <a href="https://onlinelibrary.wiley.com/doi/10.1111/brv.12165">continued to gain height</a> in historic times, partly driven by improved nutrition, but <a href="https://royalsocietypublishing.org/doi/10.1098/rspb.2015.0211">genes seem to be evolving too</a>. </p>
<p>Why we got big is unclear. In part, <a href="https://www.jstor.org/stable/10.1086/667694">mortality may drive size evolution</a>; growth takes time, so longer lives mean more time to grow. But human females also <a href="https://www.nature.com/articles/35003107">prefer</a> <a href="https://www.ingentaconnect.com/content/sbp/sbp/1996/00000024/00000002/art00005">tall males</a>. So both lower mortality and sexual preferences will likely cause humans to get taller. Today, the tallest people in the world are in Europe, led by the Netherlands. Here, men average 183cm (6ft); women 170cm (5ft 6in). Someday, most people might be that tall, or taller.</p>
<p>As we’ve grown taller, we’ve become more gracile. Over the past 2 million years, our skeletons became <a href="https://www.jstor.org/stable/27858865?seq=1#metadata_info_tab_contents">more lightly built</a> as we relied less on brute force, and more on tools and weapons. As farming forced us to settle down, our lives became more sedentary, so <a href="https://www.pnas.org/content/112/2/372">our bone density decreased</a>. As we spend more time behind desks, keyboards and steering wheels, these trends will likely continue.</p>
<p>Humans have also reduced our muscles <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4475937/">compared to other apes</a>, especially in our upper bodies. That will probably continue. Our ancestors had to slaughter antelopes and dig roots; later they tilled and reaped in the fields. Modern jobs increasingly require working with people, words and code - they take brains, not muscle. Even for manual laborers - farmers, fisherman, lumberjacks - machinery such as tractors, hydraulics and chainsaws now shoulder a lot of the work. As physical strength becomes less necessary, our muscles will keep shrinking.</p>
<p>Our jaws and teeth also got smaller. Early, plant-eating hominins had huge molars and mandibles for grinding fibrous vegetables. As we shifted to meat, then started cooking food, <a href="https://www.nature.com/articles/nature16972">jaws and teeth shrank</a>. Modern processed food – chicken nuggets, Big Macs, cookie dough ice cream – needs even less chewing, so jaws will keep shrinking, and we’ll likely lose our wisdom teeth.</p>
<h2>Beauty</h2>
<p>After people left Africa 100,000 years ago, humanity’s far-flung tribes became isolated by deserts, oceans, mountains, glaciers and sheer distance. In various parts of the world, different selective pressures – different climates, lifestyles and beauty standards – caused our appearance to evolve in different ways. Tribes evolved distinctive skin colour, eyes, hair and facial features. </p>
<p>With civilisation’s rise and new technologies, these populations were linked again. Wars of conquest, empire building, colonisation and trade – including trade of other humans – all shifted populations, which interbred. Today, road, rail and aircraft link us too. Bushmen would walk 40 miles to find a partner; we’ll go 4,000 miles. We’re increasingly one, worldwide population – freely mixing. That will create a world of hybrids – light brown skinned, dark-haired, Afro-Euro-Australo-Americo-Asians, their skin colour and facial features tending toward a global average.</p>
<p>Sexual selection will further accelerate the evolution of our appearance. With most forms of natural selection no longer operating, mate choice will play a larger role. Humans might become more attractive, but more uniform in appearance. Globalised media may also create more uniform standards of beauty, pushing all humans towards a single ideal. Sex differences, however, could be exaggerated if the ideal is masculine-looking men and feminine-looking women. </p>
<h2>Intelligence and personality</h2>
<p>Last, our brains and minds, our most distinctively human feature, will evolve, perhaps dramatically. Over the past 6 million years, hominin <a href="https://humanorigins.si.edu/human-characteristics/brains">brain size roughly tripled</a>, suggesting selection for big brains driven by tool use, complex societies and language. It might seem inevitable that this trend will continue, but it probably won’t.</p>
<p>Instead, <a href="https://www.frontiersin.org/articles/10.3389/fevo.2021.742639/full">our brains are getting smaller</a>. In Europe, brain size peaked <a href="https://www.jstor.org/stable/41464021">10,000—20,000 years ago, just before we invented farming</a>. Then, brains got smaller. Modern humans have brains smaller than our ancient predecessors, or even medieval people. It’s unclear why.</p>
<p>It could be that fat and protein were scarce once we shifted to farming, making it more costly to grow and maintain large brains. Brains are also energetically expensive – they burn around 20% of our daily calories. In agricultural societies with frequent famine, a big brain might be a liability. </p>
<p>Maybe hunter-gatherer life was demanding in ways farming isn’t. In civilisation, you don’t need to outwit lions and antelopes, or memorise every fruit tree and watering hole within 1,000 square miles. Making and using bows and spears also requires fine motor control, coordination, the ability to track animals and trajectories — maybe the parts of our brains used for those things got smaller when we stopped hunting.</p>
<p>Or maybe living in a large society of specialists demands less brainpower than living in a tribe of generalists. Stone-age people mastered many skills – hunting, tracking, foraging for plants, making herbal medicines and poisons, crafting tools, waging war, making music and magic. Modern humans perform fewer, more specialised roles as part of vast social networks, exploiting division of labour. In a civilisation, we specialise on a trade, <a href="https://www.gutenberg.org/ebooks/3300">then rely on others for everything else</a>.</p>
<p>That being said, brain size isn’t everything: <a href="https://www.frontiersin.org/articles/10.3389/fnana.2014.00046/full">elephants</a> and <a href="https://phys.org/news/2010-03-smart-killer-whales-orcas-2nd-biggest.html">orcas</a> have bigger brains than us, and Einstein’s brain was <a href="https://phys.org/news/2005-01-einsteins-brain.html">smaller than average</a>. Neanderthals had brains comparable to ours, but <a href="https://royalsocietypublishing.org/doi/10.1098/rspb.2013.0168">more of the brain was devoted to sight and control of the body</a>, suggesting less capacity for things like language and tool use. So how much the loss of brain mass affects overall intelligence is unclear. Maybe we lost certain abilities, while enhancing others that are more relevant to modern life. It’s possible that we’ve maintained processing power by having fewer, smaller neurons. Still, I worry about what that missing 10% of my grey matter did. </p>
<p>Curiously, domestic animals also <a href="https://www.jstor.org/stable/23264664">evolved smaller brains</a>. Sheep lost 24% of their brain mass after domestication; for cows, it’s 26%; dogs, 30%. This raises an unsettling possibility. Maybe being more willing to passively go with the flow (perhaps even thinking less), like a domesticated animal, has been bred into us, like it was for them. </p>
<p>Our personalities must be evolving too. Hunter-gatherers’ lives required aggression. They hunted large mammals, <a href="https://www.sciencedirect.com/science/article/pii/S0047248406002193">killed over partners</a> and <a href="https://www.amazon.com/Constant-Battles-Why-We-Fight-ebook/dp/B00DTDFOFG/ref=sr_1_1?crid=BG0I0EJMQB9I&keywords=constant+battles&qid=1645453352&sprefix=wodehous%2Caps%2C316&sr=8-1">warred with neighbouring tribes</a>. We get meat from a store, and turn to police and courts to settle disputes. If war hasn’t disappeared, it <a href="https://ourworldindata.org/war-and-peace">now accounts for fewer deaths</a>, relative to population, than at any time in history. Aggression, now a maladaptive trait, could be bred out.</p>
<p>Changing social patterns will also change personalities. Humans live in much larger groups than other apes, forming tribes of around 1,000 in hunter-gatherers. But in today’s world people living in vast cities of millions. In the past, our relationships were necessarily few, and often lifelong. Now we inhabit seas of people, moving often for work, and in the process forming thousands of relationships, many fleeting and, increasingly, virtual. This world will push us to become more outgoing, open and tolerant. Yet navigating such vast social networks may also require we become more willing to adapt ourselves to them – to be more conformist.</p>
<p>Not everyone is psychologically well-adapted to this existence. Our instincts, desires and fears are largely those of stone-age ancestors, who found meaning in hunting and foraging for their families, warring with their neighbours and praying to ancestor-spirits in the dark. Modern society meets our material needs well, but is less able to meet the psychological needs of our primitive caveman brains. </p>
<p>Perhaps because of this, increasing numbers of people suffer from psychological issues such as <a href="https://www.nytimes.com/2016/09/06/health/lonliness-aging-health-effects.html">loneliness</a>, anxiety and depression. Many turn to alcohol and other substances to cope. Selection against vulnerability to these conditions might improve our mental health, and make us happier as a species. But that could come at a price. Many great geniuses had their demons; leaders like Abraham Lincoln and Winston Churchill fought with depression, as did scientists such as Isaac Newton and Charles Darwin, and artists like Herman Melville and Emily Dickinson. Some, like Virginia Woolf, Vincent Van Gogh and Kurt Cobain, took their own lives. Others - Billy Holliday, Jimi Hendrix and Jack Kerouac – were destroyed by substance abuse. </p>
<p>A disturbing thought is that troubled minds will be removed from the gene pool – but potentially at the cost of eliminating the sort of spark that created visionary leaders, great writers, artists and musicians. Future humans might be better adjusted – but less fun to party with and less likely to launch a scientific revolution — stable, happy and boring.</p>
<h2>New species?</h2>
<p>There were once <a href="https://theconversation.com/were-other-humans-the-first-victims-of-the-sixth-mass-extinction-126638">nine human species</a>, now it’s just us. But could new human species evolve? For that to happen, we’d need isolated populations subject to distinct selective pressures. Distance no longer isolates us, but reproductive isolation could theoretically be achieved by selective mating. If people were culturally segregated – marrying based on religion, class, caste, or even politics – distinct populations, even species, might evolve. </p>
<p>In <a href="https://www.gutenberg.org/ebooks/35">The Time Machine</a>, sci-fi novelist H.G. Wells saw a future where class created distinct species. Upper classes evolved into the beautiful but useless Eloi, and the working classes become the ugly, subterranean Morlocks – who revolted and enslaved the Eloi. </p>
<p>In the past, religion and lifestyle have sometimes produced genetically distinct groups, as seen in for example <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3032072">Jewish</a> and <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC31389">Gypsy</a> populations. Today, politics also divides us – could it divide us genetically? Liberals now move to be near other liberals, and <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0225405">conservatives to be near conservatives</a>; many on the left <a href="https://time.com/5896607/dating-political-ideology/">won’t date Trump supporters</a> and vice versa. </p>
<p>Could this create two species, with instinctively different views? Probably not. Still, to the extent culture divides us, it could drive evolution in different ways, in different people. If cultures become more diverse, this could maintain and increase human genetic diversity.</p>
<h2>Strange New Possibilities</h2>
<p>So far, I’ve mostly taken a historical perspective, looking back. But in some ways, the future might be radically unlike the past. Evolution itself has evolved.</p>
<p>One of the more extreme possibilities is directed evolution, where we actively control our species’ evolution. We already breed ourselves when we choose partners with appearances and personalities we like. For thousands of years, <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3083418/">hunter-gatherers arranged marriages</a>, seeking good hunters for their daughters. Even where children chose partners, men were generally <a href="https://link.springer.com/article/10.1007/s12110-004-1014-8">expected to seek approval of the bride’s parents</a>. Similar traditions survive elsewhere today. In other words, we breed our own children.</p>
<p>And going forward, we’ll do this with far more knowledge of what we’re doing, and more control over the genes of our progeny. We can already screen ourselves and <a href="https://www.nature.com/scitable/topicpage/embryo-screening-and-the-ethics-of-human-60561/#:%7E:text=Also%20known%20as%20embryo%20screening,forced%20to%20make%20the%20difficult">embryos for genetic diseases</a>. We could potentially choose embryos for desirable genes, as we do with crops. Direct editing of the DNA of a human embryo has been <a href="https://www.nature.com/articles/d41586-019-00673-1">proven to be possible</a> — but seems morally abhorrent, effectively turning children into subjects of medical experimentation. And yet, if such technologies were proven safe, I could imagine a future where you’d be a bad parent <em>not</em> to give your children the best genes possible. </p>
<p>Computers also provide an entirely new selective pressure. As <a href="https://www.statista.com/chart/20822/way-of-meeting-partner-heterosexual-us-couples/#:%7E:text=Surveys%20carried%20out%20and%20analyzed,2%20percent%20to%2039%20percent.">more and more matches are made on smartphones</a>, we are delegating decisions about what the next generation looks like to computer algorithms, who recommend our potential matches. <a href="https://tinder.com/">Digital code</a> now helps choose what genetic code passed on to future generations, just like it shapes what you stream or buy online. This might sound like dark science fiction, but it’s already happening. Our genes are being curated by computer, just like our playlists. It’s hard to know where this leads, but I wonder if it’s entirely wise to turn over the future of our species to iPhones, the internet and the companies behind them.</p>
<p>Discussions of human evolution are usually backward looking, as if the greatest triumphs and challenges were in the distant past. But as technology and culture enter a period of <a href="https://www.amazon.com/Sapiens-Humankind-Yuval-Noah-Harari-ebook/dp/B00ICN066A/ref=sr_1_1?crid=2QHPQXXL0YKXN&keywords=sapiens&qid=1645456786&sprefix=sapiens%2Caps%2C124&sr=8-1">accelerating change</a>, our genes will too. Arguably, the most interesting parts of evolution aren’t life’s origins, dinosaurs, or Neanderthals, but what’s happening right now, our present – and our future. </p>
<hr>
<p><em>More <a href="https://theconversation.com/uk/topics/lifes-big-questions-80040?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=LifesBigQuestionsUK">Life’s Big Questions</a>:</em></p>
<ul>
<li><p><em><a href="https://theconversation.com/happiness-is-feeling-content-more-important-than-purpose-and-goals-131503?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=LifesBigQuestionsUK">Happiness: is contentment more important than purpose and goals?</a></em></p></li>
<li><p><em><a href="https://theconversation.com/would-we-still-see-ourselves-as-human-if-other-hominin-species-hadnt-gone-extinct-166759">Would we still see ourselves as ‘human’ if other hominin species hadn’t gone extinct?</a></em></p></li>
<li><p><em><a href="https://theconversation.com/death-can-our-final-moment-be-euphoric-129648?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=LifesBigQuestionsUK">Death: can our final moment be euphoric?</a></em></p></li>
<li><p><em><a href="https://theconversation.com/how-could-the-big-bang-arise-from-nothing-171986">How could the Big Bang arise from nothing?</a></em></p></li>
<li><p><em><a href="https://theconversation.com/love-is-it-just-a-fleeting-high-fuelled-by-brain-chemicals-129201?utm_source=TCUK&utm_medium=linkback&utm_campaign=TCUKengagement&utm_content=LifesBigQuestionsUK">Love: is it just a fleeting high fuelled by brain chemicals?</a></em></p></li>
</ul><img src="https://counter.theconversation.com/content/176997/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nicholas R. Longrich does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>We’ll probably be less aggressive and more agreeable, but have smaller brains – a bit like a Golden Retriever, we’ll be friendly, but maybe not that interesting or bright.Nicholas R. Longrich, Senior Lecturer in Paleontology and Evolutionary Biology, University of BathLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1736342022-02-01T13:14:21Z2022-02-01T13:14:21ZDid male and female dinosaurs differ? A new statistical technique is helping answer the question<figure><img src="https://images.theconversation.com/files/443225/original/file-20220128-23-12zgv3p.jpg?ixlib=rb-1.1.0&rect=2%2C14%2C1950%2C1159&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">How can researchers tell if male and female dinosaurs, like the stegosaur, were different?</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Journal.pone.0138352.g001A.jpg#/media/File:Journal.pone.0138352.g001A.jpg">Susannah Maidment et al. & Natural History Museum, London</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>In most animal species, <a href="https://doi.org/10.2307/2407393">males and females differ</a>. This is true for people and other mammals, as well as many species of birds, fish and reptiles. But what about dinosaurs? In 2015, I proposed that variation found in the iconic back plates of stegosaur dinosaurs was <a href="https://doi.org/10.1371/journal.pone.0123503">due to sex differences</a>.</p>
<p>I was surprised by how strongly some of my colleagues <a href="https://doi.org/10.1017/pab.2016.51">disagreed</a>, arguing that differences between sexes, called sexual dimorphism, <a href="https://doi.org/10.2307/2407393">did not exist in dinosaurs</a>.</p>
<p><a href="https://scholar.google.com/citations?user=umU9KBMAAAAJ&hl=en&oi=ao">I am a paleontologist</a>, and the debate sparked by my 2015 paper has made me reconsider how researchers studying ancient animals use statistics. </p>
<p>The limited fossil record makes it hard to declare if a dinosaur was sexually dimorphic. But I and some others in my field are beginning to <a href="https://doi.org/10.1038/d41586-019-00857-9">shift away from traditional black-or-white statistical thinking</a> that relies on p-values and statistical significance to define a true finding. Instead of only looking for yes or no answers, we are beginning to consider the estimated magnitude of sexual variation in a species, the degree of uncertainty in that estimate and how these measures compare to other species. This approach offers a more nuanced analysis to challenging questions in paleontology as well as many other fields of science.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/443076/original/file-20220127-9640-1ercxvu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A very colorful duck standing next to a drab brown duck." src="https://images.theconversation.com/files/443076/original/file-20220127-9640-1ercxvu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/443076/original/file-20220127-9640-1ercxvu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/443076/original/file-20220127-9640-1ercxvu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/443076/original/file-20220127-9640-1ercxvu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/443076/original/file-20220127-9640-1ercxvu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/443076/original/file-20220127-9640-1ercxvu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/443076/original/file-20220127-9640-1ercxvu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">In many species, like these mandarin ducks, males (left) and females (right) look very different.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Pair_of_mandarin_ducks.jpg">Francis C. Franklin via WikimediaCommons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Differences between males and females</h2>
<p><a href="http://dx.doi.org/10.1007/978-3-319-47829-6_433-1">Sexual dimorphism</a> is when males and females of a certain species differ on average in a particular trait – not including their reproductive anatomy. Classic examples are how male deer have antlers and male peacocks have flashy tail feathers, while the females lack these traits.</p>
<p>Dimorphism can also be subtle and unflashy. Often the difference is one of degree, like differences in the average body size between males and females – as in <a href="https://doi.org/10.1007/s12110-012-9130-3">gorillas</a>. In these modest cases, researchers use statistics to determine whether a trait differs on average between males and females.</p>
<h2>The dinosaur dilemma</h2>
<p>Studying sexual dimorphism in extinct animals is fraught with uncertainty. If you and I independently dig up similar fossils of the same species, they are inevitably going to be slightly different. These differences could be due to sex, but they could also be driven by age – <a href="https://www.worldcat.org/title/avian-anatomy-integument/oclc/603445440&referer=brief_results">young birds are fuzzy, adult birds are sleek</a>. They could also be due to genetics unrelated to sex, like eye color in humans.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/437461/original/file-20211214-15-1gmw3ot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two drawings of dinosaurs showing different shaped horns and frills." src="https://images.theconversation.com/files/437461/original/file-20211214-15-1gmw3ot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/437461/original/file-20211214-15-1gmw3ot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=520&fit=crop&dpr=1 600w, https://images.theconversation.com/files/437461/original/file-20211214-15-1gmw3ot.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=520&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/437461/original/file-20211214-15-1gmw3ot.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=520&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/437461/original/file-20211214-15-1gmw3ot.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=653&fit=crop&dpr=1 754w, https://images.theconversation.com/files/437461/original/file-20211214-15-1gmw3ot.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=653&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/437461/original/file-20211214-15-1gmw3ot.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=653&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">It’s possible that variation among individual dinosaurs of the same species could be due to sexual dimorphism, but there are rarely good enough samples to assert so using traditional statistics.</span>
<span class="attribution"><span class="source">James Ormiston</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>If paleontologists had thousands of fossils to study of every species, the many sources of biological variation wouldn’t matter as much. Unfortunately, the <a href="https://doi.org/10.1002/bies.201700167">ravages of time</a> have left the fossil record painfully incomplete, often with less than a dozen good specimens for large, extinct vertebrate species. Additionally, there is currently no way to identify the sex of an individual fossil except in rare cases where obvious clues exist, like <a href="https://doi.org/10.1126/science.1110578">eggs preserved within the body cavity</a>. </p>
<p>So where does all this leave the debate on whether male and female dinosaurs had differences within traits? On the one hand, birds – which are direct descendants of dinosaurs – <a href="https://doi.org/10.1098/rspb.1998.0308">commonly show sexual dimorphism</a>. So do <a href="https://doi.org/10.18475/cjos.v45i1.a12">crocodilians</a>, dinosaurs’ next closest living relatives. Evolutionary theory also predicts that, since dinosaurs reproduced with sperm and egg, there would be a <a href="https://doi.org/10.1016/j.tree.2011.12.006">benefit to sexual dimorphism</a>.</p>
<p>These things all suggest that dinosaurs likely were sexually dimorphic. But in science you need to be quantitative. The challenge is that there is little in the way of <a href="https://doi.org/10.1017/pab.2016.51">statistically significant</a> analyses of the fossil record to support dimorphism. </p>
<h2>Statistical shifts</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/443057/original/file-20220127-6424-dz34sy.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A line graph showing two peaks." src="https://images.theconversation.com/files/443057/original/file-20220127-6424-dz34sy.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/443057/original/file-20220127-6424-dz34sy.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=315&fit=crop&dpr=1 600w, https://images.theconversation.com/files/443057/original/file-20220127-6424-dz34sy.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=315&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/443057/original/file-20220127-6424-dz34sy.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=315&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/443057/original/file-20220127-6424-dz34sy.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=396&fit=crop&dpr=1 754w, https://images.theconversation.com/files/443057/original/file-20220127-6424-dz34sy.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=396&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/443057/original/file-20220127-6424-dz34sy.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=396&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Very large sex differences can create a bimodal distribution that looks like two distinct groupings of a certain measurement.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Bimodal.png">Maksim via WikimediaCommons</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>There are a couple of ways paleontologists could test for sexual dimorphism. They could look to see if there are statistically significant differences between fossils from presumed males and females, but there are very few specimens where researchers <a href="https://doi.org/10.1073/pnas.0708903105">know the sex</a>. Another method is to see whether there are two distinct groupings of a trait, called a bimodal distribution, which could suggest a difference between males and females.</p>
<p>To tell whether a perceived difference between two groups is true, scientists have traditionally used a tool called the p-value. P-values quantify the probability of a result being due to random chance. If a p-value is low enough, the result is deemed “statistically significant” and considered unlikely to have happened by chance.</p>
<p>But p-values can be heavily influenced by sample size and the design of the study, in addition to the actual degree of sexual dimorphism. Because of the very small sample size of fossils, relying on this statistical technique makes it exceedingly difficult to categorically proclaim what dinosaur species were dimorphic. </p>
<p>The weakness of the black-or-white approach that focuses solely on whether a result is statistically significant has led to hundreds of scientists <a href="https://doi.org/10.1038/d41586-019-00857-9">calling to abandon significance testing with p-values</a> in favor of something called <a href="https://doi.org/10.1111/j.1469-185X.2007.00027.x">effect size statistics</a>. Using this approach, researchers would simply report the measured difference between two groups and the uncertainty in that measurement.</p>
<h2>Effect size statistics</h2>
<p>I have begun to apply effect size statistics in <a href="https://doi.org/10.1093/biolinnean/blaa105">my research on dinosaurs</a>. My colleagues and I compared sexual dimorphism in body size between three different dinosaurs: the duck-billed <em>Maiasaura</em>, <em>Tyrannosaurus rex</em> and <em>Psittacosaurus</em>, a small relative of <em>Triceratops</em>. None of these species would be expected to show statistically significant size differences between males and females according to p-values. But that approach does not capture the nature of the variation within these species. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/443567/original/file-20220131-15-3xspkd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A cast of a duck billed dinosaur fossil skeleton." src="https://images.theconversation.com/files/443567/original/file-20220131-15-3xspkd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/443567/original/file-20220131-15-3xspkd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/443567/original/file-20220131-15-3xspkd.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/443567/original/file-20220131-15-3xspkd.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/443567/original/file-20220131-15-3xspkd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/443567/original/file-20220131-15-3xspkd.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/443567/original/file-20220131-15-3xspkd.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Using effect size statistics, researchers were able to determine that the duck-billed dinosaur <em>Maiasaura</em> showed a larger amount of dimorphism with the least uncertainty in that estimate compared to other dinosaurs.</span>
<span class="attribution"><a class="source" href="https://en.wikipedia.org/wiki/Maiasaura#/media/File:Maiasaura_peeblesorum_cast_-_University_of_California_Museum_of_Paleontology_-_Berkeley,_CA_-_DSC04688.JPG">Daderot via WikimediaCommons</a></span>
</figcaption>
</figure>
<p>When we instead used effect size statistics, we were able to estimate that male and female <em>Maiasaura</em> demonstrate a greater difference in body mass compared to the other two species and that we had a higher confidence in this estimate as well. A few of the characteristics within the data helped reduce the uncertainty. First, we had a large number of <em>Maiasaura</em> fossils, from individuals of various ages. These bones very nicely fit with trajectories of how size changes as an individual grows from juvenile to adult, so we could control for differences due to age and instead focus on differences due to sex.</p>
<p>Additionally, the <em>Maiasaura</em> fossils all come from a <a href="https://doi.org/10.1017/pab.2015.19">single bone bed</a> of individuals that died in the same place at the same time. This means that variation between individuals is likely not due to them being different species from different regions or time periods. </p>
<p>If my colleagues and I had approached the problem expecting a yes or no answer on whether males and females differed in size, we would have completely missed all of these intricacies. Effect size statistics allow researchers to produce much more nuanced and, I think, informative results. It is almost as much a difference in the philosophical approach to science as it is a mathematical one.</p>
<p>Studying dinosaur dimorphism is not the only place p-values create issues. Many fields of science, including <a href="https://theconversation.com/the-replication-crisis-is-good-for-science-103736">medicine and psychology</a>, are having similar <a href="https://doi.org/10.1080/00031305.2018.1543137">debates about issues in statistics</a> and a worrying problem of <a href="https://doi.org/10.1371/journal.pmed.0020124">unrepeatable studies</a>.</p>
<p>Embracing uncertainty in data – rather than looking for black-or-white answers to questions like whether male and female dinosaurs were sexually dimorphic – can help elucidate dinosaur biology. But this shift in thinking may be felt far and wide across the sciences. A careful consideration of problems within statistics could have deep impacts across many fields.</p>
<p>[<em>Understand new developments in science, health and technology, each week.</em> <a href="https://memberservices.theconversation.com/newsletters/?nl=science&source=inline-science-understand">Subscribe to The Conversation’s science newsletter</a>.]</p><img src="https://counter.theconversation.com/content/173634/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Evan Thomas Saitta does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The lack of large numbers of fossils makes it hard to study sexual dimorphism in dinosaurs. But a new statistical approach offers insight into this question and others across science.Evan Thomas Saitta, Postdoctoral Scholar in Paleontology, University of ChicagoLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1627112021-06-17T15:37:28Z2021-06-17T15:37:28ZDarwin got sexual selection backwards, research suggests<figure><img src="https://images.theconversation.com/files/406739/original/file-20210616-13-330jub.jpeg?ixlib=rb-1.1.0&rect=46%2C7%2C5184%2C3391&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Males elephant seals dwarf their female counterparts. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/northern-elephant-seal-mirounga-angustirostris-male-440246053">Sean Lema/Shutterstock</a></span></figcaption></figure><p>Charles Darwin was a careful scientist. In the middle of the 19th century, while he was collecting evidence for his theory that species evolve by <a href="https://www.nationalgeographic.org/encyclopedia/natural-selection/">natural selection</a>, he noticed it didn’t explain the fancy tails of male peacocks, the antlers paraded by male deer, or why some the males of some species are far larger then their female counterparts.</p>
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<p>For these <a href="https://theconversation.com/these-useless-quirks-of-evolution-are-actually-evidence-for-the-theory-107395">quirks</a>, Darwin proposed a secondary theory: the <a href="http://darwin-online.org.uk/content/frameset?pageseq=1&itemID=F937.1&viewtype=text">sexual selection</a> of traits that increase an animal’s chance of securing a mate and reproducing. He carefully distinguished between weapons such as horns, spurs, fangs and sheer size that are used to subdue competing rivals, and ornaments that are aimed at charming the opposite sex.</p>
<p>Darwin thought that sexually selected traits could be explained by uneven sex ratios – when there are more males than females in a population, or vice versa. <a href="http://darwin-online.org.uk/content/frameset?pageseq=1&itemID=F937.1&viewtype=text">He reasoned</a> that a male with fewer available females would have to work harder to secure one of them as a mate, and that this competition would drive sexual selection. </p>
<p>In a <a href="https://onlinelibrary.wiley.com/doi/abs/10.1111/evo.14273">new study</a>, my colleagues and I have confirmed a link between sexual selection and sex ratios, as Darwin suspected. But surprisingly, our findings suggest Darwin got things the wrong way round. We found that sexual selection is most pronounced not when potential mates are scarce, but when they’re abundant – and this means looking again at the selection pressures at play in animal populations that feature uneven sex ratios. </p>
<p>Since Darwin’s time, we’ve learned a lot about uneven sex ratios, which are common in wild animal populations. For instance, in many <a href="https://www.nature.com/articles/6883570/">butterflies</a> and <a href="https://www.journals.uchicago.edu/doi/abs/10.1086/415033">mammals</a>, including humans, the number of adult females exceeds the number of adult males.</p>
<p>This skew is most extreme <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/jeb.12415">among marsupials</a>. In Australian antechinus, for instance, all males <a href="https://www.livescience.com/40229-marsupials-mate-to-death.html">abruptly die</a> after the mating season, so there are times when no adult males are alive and the entire adult population is made up of pregnant females. </p>
<p>In contrast, many birds parade more males than females in their populations. In some plovers, for example, the males <a href="https://www.onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2011.02305.x">outnumber females</a> by six to one.</p>
<p>So why do many birds species have more males, while mammals often have more females? The short answer is that we don’t know. But there are smoking guns. </p>
<h2>Explaining uneven sex ratios</h2>
<p>Some uneven sex ratios can be partially explained by <a href="https://royalsocietypublishing.org/doi/10.1098/rsbl.2019.0867">lifespan differences</a>. <a href="https://pubmed.ncbi.nlm.nih.gov/32205429/">Female mammals</a>, including humans, usually outlive their male counterparts by a wide margin. In humans, females live on average about <a href="https://www.bbc.com/future/article/20151001-why-women-live-longer-than-men">5% longer</a> than males. In <a href="https://press.uchicago.edu/ucp/books/book/chicago/R/bo3620491.html">African lions</a> and <a href="https://pubmed.ncbi.nlm.nih.gov/24317695/">killer whales</a>, the female lifespan is longer by up to 50%.</p>
<p>Predator preferences could also play a part. African lions kill approximately <a href="https://oxford.universitypressscholarship.com/view/10.1093/acprof:oso/9780199208784.001.0001/acprof-9780199208784">seven times more</a> male than female buffalo, because male buffalo tend to roam alone, whereas females are protected within herds. In contrast, cheetahs kill <a href="https://academic.oup.com/jmammal/article/80/4/1084/851830">many more female</a> Thompson’s gazelles than males, presumably because they can outrun female gazelles easier – especially the pregnant ones.</p>
<figure class="align-center ">
<img alt="A lion in front of a herd of buffalo" src="https://images.theconversation.com/files/406743/original/file-20210616-3862-g8e4ss.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406743/original/file-20210616-3862-g8e4ss.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=323&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406743/original/file-20210616-3862-g8e4ss.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=323&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406743/original/file-20210616-3862-g8e4ss.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=323&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406743/original/file-20210616-3862-g8e4ss.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=405&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406743/original/file-20210616-3862-g8e4ss.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=405&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406743/original/file-20210616-3862-g8e4ss.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=405&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Lions don’t appear to fancy their chances against a herd of female buffalo.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/african-buffalo-747035347">Seyms Brugger/Shutterstock</a></span>
</figcaption>
</figure>
<p>Finally, males and females often <a href="https://pubmed.ncbi.nlm.nih.gov/8982783/">suffer differently</a> from parasites and diseases. The COVID-19 pandemic is a striking example of this: the number of infected men and women is similar in most countries, but male patients have <a href="https://theconversation.com/coronavirus-the-puzzle-of-why-the-risk-of-death-is-greater-for-men-and-for-the-elderly-135176">higher odds of death</a> compared to female ones.</p>
<h2>Sex ratios and sexual selection</h2>
<p>Despite our growing knowledge of uneven sex ratios, Darwin’s insight linking sex ratios with sexual selection has received little attention from scientists. Our study sought to address this, pulling together these two strands of evolutionary theory in order to revisit Darwin’s argument.</p>
<p>We looked in particular at the evolution of <a href="https://onlinelibrary.wiley.com/doi/abs/10.1111/evo.14273">large males</a> in different species, which are often several times larger than their female counterparts. We see this in <a href="https://onlinelibrary.wiley.com/doi/abs/10.1002/ajpa.10011">male baboons</a>, <a href="https://link.springer.com/article/10.1007%2Fs00265-002-0507-x">elephant seals</a> and <a href="https://academic.oup.com/beheco/article/15/4/592/205890">migratory birds</a>, for example. </p>
<p>Sometimes, females are larger than males – as with some species of <a href="https://oxford.universitypressscholarship.com/view/10.1093/acprof:oso/9780199208784.001.0001/acprof-9780199208784-chapter-4">bird</a>, such as the African jacana. The scientific term for when one sex in a species is larger than the other is “<a href="https://www.oxfordbibliographies.com/view/document/obo-9780199941728/obo-9780199941728-0110.xml">sexual size dimorphism</a>”. </p>
<figure class="align-center ">
<img alt="A larger female African janaca and a smaller male" src="https://images.theconversation.com/files/406957/original/file-20210617-15-18mi7gu.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/406957/original/file-20210617-15-18mi7gu.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=360&fit=crop&dpr=1 600w, https://images.theconversation.com/files/406957/original/file-20210617-15-18mi7gu.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=360&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/406957/original/file-20210617-15-18mi7gu.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=360&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/406957/original/file-20210617-15-18mi7gu.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=452&fit=crop&dpr=1 754w, https://images.theconversation.com/files/406957/original/file-20210617-15-18mi7gu.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=452&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/406957/original/file-20210617-15-18mi7gu.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=452&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The female African jacana, on the left, is larger than the male.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/berniedup/6001942849/">Bernard DUPONT/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>It’s clear how <a href="https://academic.oup.com/condor/article/112/1/183/5152556">sexual selection</a> can sometimes create size dimorphism. Knocking out an enemy requires muscular power, while fight endurance requires stamina. So being bigger often means dominating rivals, thereby winning the evolutionary lottery of reproduction.</p>
<p>Analysing 462 different species of reptiles, mammals and birds, our study found a tight association between sexual size dimorphism and sex ratios, vindicating Darwin’s conjectures. </p>
<p>But the trend was the opposite to the one Darwin predicted with his limited evidence. It turns out the most intense sexual selection – indicated by larger males relative to females – occurred in species where there were plenty of females for males to choose from, rather than a scarcity of females as Darwin suggested.</p>
<h2>Implications for sexual selection</h2>
<p>This in no way invalidates Darwin’s theories of natural selection and sexual selection. Our finding simply shows that a different mechanism to the one Darwin proposed is driving mating competition for animals living in sex-skewed populations.</p>
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Read more:
<a href="https://theconversation.com/how-to-slam-dunk-creationists-when-it-comes-to-the-theory-of-evolution-81581">How to slam dunk creationists when it comes to the theory of evolution</a>
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<p>Darwin’s assumption was based on the idea that the most intense competition for mates should occur when there’s a shortage of mating partners. But <a href="https://www.pnas.org/content/106/Supplement_1/10001">more recent theories</a> suggest this logic may not be correct, and that sexual selection is actually a system in which the winner takes all. </p>
<p>That means that when there are many potential partners in the population, a top male – in our study, the largest and heaviest – enjoys a disproportionately high payout, fertilising a large number of females at the expense of smaller males, who may not reproduce at all.</p>
<p>We need further studies to help us understand how males and females seek out new partners in male-skewed and female-skewed populations, and in what circumstances ornaments, armaments and sheer size are particularly useful. Such studies could provide us with unprecedented new insights into how nature works, building on Darwin’s original theory of sexual selection.</p><img src="https://counter.theconversation.com/content/162711/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tamas Szekely receives funding from The Royal Society and theELVONAL programme of Hungarian Research Foundation (NKFIH).</span></em></p>The sexual selection of larger males may be driven by an abundance – not a scarcity – of females.Tamas Szekely, Professor of Biodiversity at The Milner Centre for Evolution, University of BathLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1276642020-01-28T17:52:18Z2020-01-28T17:52:18ZHow Darwin’s sexual selection theory co-stars in ‘The Handmaid’s Tale’<figure><img src="https://images.theconversation.com/files/311680/original/file-20200123-162194-13sgd8u.jpg?ixlib=rb-1.1.0&rect=29%2C119%2C4962%2C3128&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The role of women in futuristic drama TV series 'The Handmaid's Tale' makes references to Darwin's writings on evolution.</span> <span class="attribution"><span class="source">(Shutterstock)</span></span></figcaption></figure><p><em>The Handmaid’s Tale</em> is a TV series based on the <a href="https://www.penguinrandomhouse.com/books/6125/the-handmaids-tale-by-margaret-atwood/">1985 novel of the same name by Margaret Atwood</a> that presents a dystopian vision of a male-dominated society known as Gilead.</p>
<p>Widespread infertility means that the few fertile women who remain have been enslaved as handmaids and assigned to Gilead’s leaders to produce their future offspring. The series follows the struggles of June, who was separated from her family and forced to become a handmaid.</p>
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<figcaption><span class="caption">A trailer for the third season of The Handmaid’s Tale.</span></figcaption>
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<p>But just what is a reference to the evolutionist Charles Darwin doing in an episode of <em>The Handmaid’s Tale</em>?</p>
<p>As a historian of science currently writing a book on the Darwinian Revolution, I am intrigued by Darwin’s connection with this fictional society. </p>
<p>The reference may have something to do with Darwin’s chief evolutionary mechanism: <a href="https://plato.stanford.edu/entries/natural-selection/">natural selection or the survival of the fittest</a>. But we’ll see that Darwin’s inclusion in <em>The Handmaid’s Tale</em> actually relates to his less well-known and secondary evolutionary mechanism, <a href="https://www.darwinproject.ac.uk/commentary/evolution/sexual-selection">sexual selection</a>.</p>
<h2>‘Women can be useful’</h2>
<p>The third episode of season 3, titled “Useful,” finds June assigned to Joseph Lawrence, a high-ranking commander in Gilead. As Lawrence hosts a meeting of Gilead’s leaders in his drawing room, the men discuss what to do about recently captured female insurgents. </p>
<p>Should they be forced into hard labour in the Colonies or be publicly executed? Commander Lawrence, however, finds neither of these options appealing. The realities of the widespread infertility that spurred the regime’s existence suggests that some of the intransigent women might be useful after all.</p>
<p>Lawrence then tells June to get a book from his bookcase that will help the leadership group better understand “an individual’s value in the world as it pertains to gender.” The book in question is Charles Darwin’s <a href="https://www.bl.uk/collection-items/the-descent-of-man-by-darwin"><em>Descent of Man</em></a>, which Lawrence calls “an oldie but a goodie.” </p>
<p>After June hands the book to Lawrence, he then exclaims to his powerful guests: “See? Women can be useful.”</p>
<h2>Darwin’s theory of sexual selection</h2>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/311676/original/file-20200123-162185-1a7i68b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/311676/original/file-20200123-162185-1a7i68b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/311676/original/file-20200123-162185-1a7i68b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=988&fit=crop&dpr=1 600w, https://images.theconversation.com/files/311676/original/file-20200123-162185-1a7i68b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=988&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/311676/original/file-20200123-162185-1a7i68b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=988&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/311676/original/file-20200123-162185-1a7i68b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1242&fit=crop&dpr=1 754w, https://images.theconversation.com/files/311676/original/file-20200123-162185-1a7i68b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1242&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/311676/original/file-20200123-162185-1a7i68b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1242&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Title page of ‘The Descent of Man’ by Charles Darwin (London, John Murray, 1875).</span>
<span class="attribution"><span class="source">Wellcome Collection</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Far from being a throwaway remark, the reference to Darwin’s <em>Descent of Man</em> in this scene is highly suggestive. While Darwin’s 1859 <a href="http://darwin-online.org.uk/EditorialIntroductions/Freeman_OntheOriginofSpecies.html"><em>On the Origin of Species</em></a> is much more widely read and appreciated as establishing evolution as a science, Darwin did not write specifically about human evolution until 1871 when <a href="http://darwin-online.org.uk/contents.html#descent"><em>The Descent of Man and Selection in Relation to Sex</em></a> was published. In this book, Darwin put forward his theory of sexual selection.</p>
<p>Today, Darwin’s theory of sexual selection is often used to explain the bright and brilliant plumage typical of male birds of paradise, along with their often bizarre mating rituals meant to entice females. While Darwin spent several chapters discussing the esthetic senses of birds that led to such interesting evolutionary adaptations, he did not limit his application of sexual selection to birds alone.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/311678/original/file-20200123-162216-luienz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/311678/original/file-20200123-162216-luienz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/311678/original/file-20200123-162216-luienz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/311678/original/file-20200123-162216-luienz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/311678/original/file-20200123-162216-luienz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/311678/original/file-20200123-162216-luienz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/311678/original/file-20200123-162216-luienz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/311678/original/file-20200123-162216-luienz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Darwin explained the ostentatious displays of male birds - like this lesser bird-of-paradise - as a way to attract females and ensure genetic continuation.</span>
<span class="attribution"><span class="source">(Shutterstock)</span></span>
</figcaption>
</figure>
<p>Science historian Evelleen Richards explains that Darwin believed that his theory of sexual selection could <a href="https://www.press.uchicago.edu/ucp/books/book/chicago/D/bo25338514.html">account for important aspects of human evolution as well, such as the physical and mental distinctions between men and women</a>. </p>
<p>As Richards shows, when it came to issues of gender (and race for that matter), Darwin was very much a man of his time, and this fact shaped his evolutionary views. He argued that <a href="https://www.icr.org/article/darwins-teaching-womens-inferiority/">there was a vast distinction between the intellectual capabilities of men and women, believing that men were ultimately superior</a>:</p>
<blockquote>
<p>“The chief distinction in the intellectual powers of the two sexes is shewn by man attaining to a higher eminence, in whatever he takes up, than woman can attain — whether requiring deep thought, reason or imagination, or merely the use of the senses and hands.”</p>
</blockquote>
<p>This distinction could be explained, according to Darwin, by the fact that men had to struggle against one another in the contest for mates, whereas women were largely passive. Through this struggle, men acquired certain intellectual capabilities late in their development, capabilities that were then only passed on to male offspring. </p>
<p>Because of this, the mental state of women was arrested in time and “characteristic,” as Darwin put it, “of the lower races, and therefore of a past and lower state of civilization.”</p>
<h2>Embracing maternity</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/311940/original/file-20200126-81341-1u66mvy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/311940/original/file-20200126-81341-1u66mvy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/311940/original/file-20200126-81341-1u66mvy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=800&fit=crop&dpr=1 600w, https://images.theconversation.com/files/311940/original/file-20200126-81341-1u66mvy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=800&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/311940/original/file-20200126-81341-1u66mvy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=800&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/311940/original/file-20200126-81341-1u66mvy.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1005&fit=crop&dpr=1 754w, https://images.theconversation.com/files/311940/original/file-20200126-81341-1u66mvy.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1005&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/311940/original/file-20200126-81341-1u66mvy.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1005&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Commander Joseph Lawrence is played by the actor Bradley Whitford; in the show, Commander Lawrence is a high-ranking official.</span>
<span class="attribution"><a class="source" href="https://www.hulu.com/press/actor/bradley-whitford/?show_id=1132">(Hulu Press)</a></span>
</figcaption>
</figure>
<p>It followed for Darwin that it would be a waste of resources to bring women up to the intellectual level of men. Women should, therefore, fully embrace their maternal instincts, raise children and establish happy homes. </p>
<p>Darwin’s wife Emma did exactly that, creating a comfortable home for Darwin to pursue his scientific endeavours while giving birth to 10 children, <a href="https://www.darwinproject.ac.uk/emma-darwin">with the last one born when she was 48 years old</a>.</p>
<p>In many ways, <em>The Handmaid’s Tale</em> presents a society that is reduced to the gender stereotypes that were inscribed in Darwin’s theory of sexual selection. Women are useful only in so far as they can reproduce. And those who can, like June, are essentially made sexual slaves to the powerful. Men, meanwhile, are physically and intellectually in control of the levers of power. </p>
<p>When Commander Lawrence tells June to get <em>Descent of Man</em> from the bookshelf, he is reminding her that she can be useful, but only from within the narrow sphere defined by her biology. <em>The Handmaid’s Tale</em>, of course, is ultimately critical of such gender determinism, as the development of June herself makes clear. </p>
<p>But the politics of biology is not just a relic of the 19th century or the product of science fiction. Philosopher Cordelia Fine shows that <a href="https://www.wwnorton.co.uk/books/9780393082081-testosterone-rex">there are still many evolutionary scientists who hold that the inequality between the sexes is natural, not cultural</a>. </p>
<p>On the surface, therefore, Gilead may look like an unrealistic society that chose to embrace its most extreme views about gender in order to survive. But the brilliance of <em>The Handmaid’s Tale</em> is in showing that many of those views, in certain contexts, are actually not so radical, and can even be found in classic biology texts. </p>
<p>This all suggests that the reference to Darwin’s <em>Descent of Man</em> in <em>The Handmaid’s Tale</em> is not only relevant but even necessary.</p><img src="https://counter.theconversation.com/content/127664/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ian Hesketh holds an Australian Research Council Future Fellowship grant FT170100194, entitled "The Place of History in Science: Reassessing the Darwinian Revolution."</span></em></p>In the television show ‘The Handmaid’s Tale,’ Charles Darwin’s ‘Descent of Man’ makes a cameo — and its appearance makes a comment on how Gilead functions.Ian Hesketh, ARC Future Fellow, The University of QueenslandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1281232020-01-16T19:01:11Z2020-01-16T19:01:11ZHomosexuality may have evolved for social, not sexual reasons<figure><img src="https://images.theconversation.com/files/307834/original/file-20191219-11939-s5zoa8.jpg?ixlib=rb-1.1.0&rect=34%2C0%2C4558%2C3050&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">We propose same-sex attraction evolved to allow greater social integration and stronger same-sex social bonds. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/young-male-couple-holding-hands-standing-287829986">SHUTTERSTOCK</a></span></figcaption></figure><p>How did homosexuality in humans evolve? </p>
<p>Typically, <a href="https://www.ncbi.nlm.nih.gov/pubmed/19539396">this question</a> is posed as <a href="https://www.amazon.com.au/Goodness-Paradox-Evolution-Made-Violent/dp/1781255830">a paradox</a>. </p>
<p>The argument is this: gay sex alone can’t produce children, and for traits to evolve, they have to be passed onto children, who get some form of competitive advantage from them. </p>
<p>From this perspective, some argue homosexuality should not have evolved.</p>
<p><a href="https://www.frontiersin.org/articles/10.3389/fpsyg.2019.02955/full?utm_source=F-NTF&utm_medium=EMLX&utm_campaign=PRD_FEOPS_20170000_ARTICLE">In a paper published yesterday</a> by myself and Duke University professor Brian Hare, we propose human sexuality (including homosexuality) evolved as an outcome of the evolution of increased sociability in humans. </p>
<p>We argue many of the evolutionary forces that shaped human sexuality were social, rather than based on reproductive ability. </p>
<p>This is our “sociosexual hypothesis” for the evolution of gay sex and attraction. </p>
<h2>Sex for bonding</h2>
<p>For humans, and many other animals, sex is not just about reproduction. </p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/310400/original/file-20200116-181608-16veogf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/310400/original/file-20200116-181608-16veogf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/310400/original/file-20200116-181608-16veogf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=901&fit=crop&dpr=1 600w, https://images.theconversation.com/files/310400/original/file-20200116-181608-16veogf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=901&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/310400/original/file-20200116-181608-16veogf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=901&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/310400/original/file-20200116-181608-16veogf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1133&fit=crop&dpr=1 754w, https://images.theconversation.com/files/310400/original/file-20200116-181608-16veogf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1133&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/310400/original/file-20200116-181608-16veogf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1133&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Bonobos and chimpanzees share about 99.6% of their DNA with humans.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/bonobo-green-tropical-jungle-natural-background-1596689767">shutterstock</a></span>
</figcaption>
</figure>
<p><a href="https://global.oup.com/academic/product/primate-sexuality-9780199544646?cc=au&lang=en&">In our closest primate relative</a>, the bonobo, <a href="https://link.springer.com/article/10.1007%2Fs10329-010-0229-z">straight and gay sex have vital roles</a> in play, social transactions, barter of food, same-sex social bonding and bonding between mating pairs.</p>
<p>We shouldn’t limit our thinking about the evolution of sex to its reproductive functions. We must also consider its social functions.</p>
<p>Based on the social behaviour of primates (and other social mammals), we argue our species’ recent cognitive and behavioural evolution was driven by natural selection favouring traits that allowed better social integration. This is called prosociality.</p>
<p>Early humans that could quickly and easily access the benefits of group living had a strong selective advantage. We believe this led to the evolution of a whole range of traits including reduced aggression, increased communication, understanding, social play and affiliation.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/gay-gene-search-reveals-not-one-but-many-and-no-way-to-predict-sexuality-122459">'Gay gene' search reveals not one but many – and no way to predict sexuality</a>
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<p>Species such as the bonobo, that evolved for high prosociality, evolved to use sexual behaviour in many social contexts. This results in an increase of sex in general, greater diversity in the contexts of sex, and an increase in gay sex.</p>
<p>We believe something similar happened in recent human evolution. Gay sex and attraction may have evolved because individuals with a degree of same-sex attraction benefited from greater social mobility, integration and stronger same-sex social bonds. </p>
<p>This may sound counterintuitive, given <a href="https://journals.sagepub.com/doi/full/10.1177/1529100616637616?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dpubmed">gay people are socially marginalised, ostracised and even criminalised in many societies.</a> </p>
<p>However, our argument addresses the early evolution of human sexuality, not how relatively recent phenomena like religion and religion-based legal structures have responded to sexual minorities.</p>
<h2>Supporting facts</h2>
<p>Many studies since the <a href="https://www.jstor.org/stable/j.ctt173zmgn">pioneering</a> <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1447861/">research</a> of Alfred Kinsey and colleagues have emphasised that <a href="https://journals.sagepub.com/doi/full/10.1177/1529100616637616?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dpubmed">sexual minorities occur across all cultures</a>, and the levels of gay and bisexual people in populations have been quite stable over time.</p>
<p>Our hypothesis predicts that bisexuality and people who identify as “mostly straight” should be more common than people who identify as exclusively gay, <a href="https://journals.sagepub.com/doi/full/10.1177/1529100616637616?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub%3Dpubmed">and this is the case</a>. </p>
<p>Recent genetic analyses confirm <a href="https://science.sciencemag.org/content/365/6456/eaat7693">hundreds of genes influence sexuality</a> in complex ways. </p>
<p>We quite randomly inherit half our genes from each parent. Each person’s genetic makeup is unique, so it would be highly unlikely to find two people with exactly the same set of genes influencing their sexuality. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/born-this-way-an-evolutionary-view-of-gay-genes-26051">Born this way? An evolutionary view of 'gay genes'</a>
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</em>
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<hr>
<p>Thus, variation is expected, and individuals fall along a spectrum ranging from a majority who are straight, to a minority who identify as gay.</p>
<p>Our hypothesis for the evolution of homosexuality would predict this kind of variation in human sexuality, and can help explain why it is generally stable across cultures. </p>
<p>We believe sexuality is a highly complex trait, interwoven with sociality. Attraction, sexual behaviour, social bonds and desire all contribute to its complexity.</p>
<h2>Asking the right questions</h2>
<p>Height is another feature influenced by hundreds of genes, many of which interact with our external environments in complex ways. </p>
<p>We see a continuous variation in human height – some very tall and very short people exist. </p>
<p>We might draw on nutritional ecology to explore the evolution of human height, but would not feel the need to introduce special evolutionary arguments to explain the existence of tall or short people. </p>
<p>No special explanation is necessary. They are simply exhibiting natural, genetically influenced variations in height.</p>
<p>Similarly, we think asking how gay sex and attraction evolved is the wrong question. </p>
<p>A more useful question to ask is: how did human sexuality evolve in all its forms?</p>
<p>In doing do, we acknowledge homosexuality does not present a paradox needing a special explanation. It is simply a result of our species’ recent sociosexual evolution.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/gay-gene-testing-apps-arent-just-misleading-theyre-dangerous-126522">'Gay gene' testing apps aren't just misleading – they're dangerous</a>
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<img src="https://counter.theconversation.com/content/128123/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andrew Barron currently receives funding from Macquarie University, The Australian Research Council and the Templeton World Charity Foundation</span></em></p>Scientists don’t ask how some people evolved to be tall. In the same way, asking how homosexuality evolved is the wrong question. We need to ask how human sexuality evolved in all its forms.Andrew Barron, Professor, Macquarie UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1282842019-12-11T12:44:19Z2019-12-11T12:44:19ZLeopard slugs mate in the most beautifully bizarre way – and nobody knows why<figure><img src="https://images.theconversation.com/files/306088/original/file-20191210-95125-sj2zwu.jpg?ixlib=rb-1.1.0&rect=0%2C7%2C4710%2C2987&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Beautifully bizarre.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/mating-leopard-slugs-limax-maximus-australia-1435271348">Ken Griffiths/Shutterstock</a></span></figcaption></figure><p>Under the cover of night, two large leopard slugs begin to court, circling each other, before climbing single-file up a tree or onto a rock. They lower themselves on a mucus rope, while entwining their bodies in a strictly anti-clockwise fashion.</p>
<p>Both slugs then push out and entwine two overly-sized penises from openings on the side of their head, before exchanging sperm that may later fertilise each of their eggs. Or, perhaps be eaten. Eventually, one slug crawls off and the other follows, eating the mucus trapeze as it goes.</p>
<p>The astonishing sex lives of leopard slugs, or <em>Limax maximus</em>, have long been recognised by naturalists and frequently feature in <a href="https://youtu.be/wG9qpZ89qzc">wildlife documentaries</a>. But while their carnal dance has mesmerised millions, nobody knows why they mate in this most bizarre way.</p>
<p>This is because slug sex science has rarely attracted anything other than observational study. Fortunately for our curiosity, there are a few <a href="http://www.scholarpedia.org/article/Gastropod_reproductive_behavior">noble individuals</a> who have taken time to understand the mating habits of snails and slugs, and <a href="http://www.joriskoene.com/">whose research</a> can give us some valuable clues.</p>
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<p>It’s well known that leopard slugs, like the majority of land-based snails and slugs, are hermaphrodites – meaning that both sexual organs are contained in the same individual. Yet, self-fertilisation is generally not the <a href="https://zslpublications.onlinelibrary.wiley.com/doi/abs/10.1017/S0952836905007648">preferred option</a>. This is likely because natural selection favours mating with another individual to avoid the loss of health, fertility and fitness associated with <a href="https://www.sciencedirect.com/science/article/pii/S0169534716301586">inbreeding</a>.</p>
<p>Even though they can choose whether to mate as male or female, most slugs and snails mate as male and female at the same time. They can also <a href="http://www.scholarpedia.org/article/Gastropod_reproductive_behavior">store sperm</a> for months and even years, and so don’t always need to receive sperm if they have previously mated with a better partner. They can have the best of both worlds by choosing to eat and digest most of the sperm, while retaining just enough to fertilise their eggs.</p>
<p>We also know why leopard slugs turn anti-clockwise when mating. Just like human hearts are <a href="https://www.theguardian.com/science/blog/2016/sep/08/situs-inversus-and-my-through-the-looking-glass-body">nearly always</a> to the left hand side in our bodies, a slug’s body is also asymmetric. This is most obvious during mating, when the genitals emerge from the right side of the head. This asymmetry makes leopard slugs turn anti-clockwise in synchrony during courting and mating – and is also what made rare left-coiling snail Jeremy <a href="https://web.archive.org/web/20171020222103/http://www.bbc.co.uk/bbcthree/item/249e54d9-7c5c-451e-940c-7826f6dd2a14">a media sensation</a> in his/her <a href="https://www.npr.org/sections/thetwo-way/2017/05/17/528796939/tragic-love-triangle-is-sad-for-lonely-rare-snail-still-good-for-science">search for love</a>.</p>
<h2>The rest is mystery</h2>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/306207/original/file-20191210-95138-q0ny8k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/306207/original/file-20191210-95138-q0ny8k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=903&fit=crop&dpr=1 600w, https://images.theconversation.com/files/306207/original/file-20191210-95138-q0ny8k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=903&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/306207/original/file-20191210-95138-q0ny8k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=903&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/306207/original/file-20191210-95138-q0ny8k.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1135&fit=crop&dpr=1 754w, https://images.theconversation.com/files/306207/original/file-20191210-95138-q0ny8k.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1135&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/306207/original/file-20191210-95138-q0ny8k.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1135&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">It’s a long way up from there.</span>
<span class="attribution"><a class="source" href="https://flickr.com/photos/kongniffe/48904576193/">Inge Knoff/flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>The rest of their elaborate mating behaviour is less well understood. It might be that that <a href="https://www.youtube.com/watch?v=UOcLaI44TXA">communication and cooperation</a> are important aspects of sexual behaviour in the mollusc world, beginning with the head-to-tail <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/brv.12023">trail-following</a>. The long mucus trapeze could be an example of sexual evolution going into overdrive to signify commitment to what’s to come, making sure that any mating efforts won’t be wasted. The spiral entwinement between mating slugs may also facilitate close physical contact and commitment, minimising the risk of sudden withdrawal.</p>
<p>But this behaviour is also more sinister than it first appears. Some slugs and snails engage in hormone warfare or sexual conflict to increase their chances of fertilising their mate. For example, as artistically interpreted by <a href="https://www.youtube.com/watch?v=BckqviVaWl0">Isabella Rossellini</a>, many snails (including the common garden variety) <a href="https://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-5-25">stab each</a> with <a href="https://en.wikipedia.org/wiki/Love_dart">love-darts</a>, transferring hormones to improve the chances that sperm are used for fertilisation. The field slug <em>Deroceras</em>, seen below, <a href="https://youtu.be/b70CGCdeP3I?t=180">flicks and strokes</a> its partner with what looks like a <a href="https://bioone.org/journals/american-malacological-bulletin/volume-23/issue-1/0740-2783-23.1.137/A-review-of-mating-behavior-in-slugs-of-the-genus/10.4003/0740-2783-23.1.137.short">sticky slug blanket</a> for the same reason.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/b70CGCdeP3I?wmode=transparent&start=210" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>The leopard slug’s beautiful entwinement could be another manifestation of this sexual coercion, maximising surface area for hormone transfer. The long penises – which can be <a href="http://www.scholarpedia.org/article/Gastropod_reproductive_behavior">60 to 90cm</a> long in one <a href="http://www.scholarpedia.org/article/File:Limax_Long_Penes.jpg">Italian version</a> of the leopard slug – may also be another extreme result of an evolutionary arms race to improve the prospects of fertilisation.</p>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/306350/original/file-20191211-95159-bxkp7.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/306350/original/file-20191211-95159-bxkp7.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1067&fit=crop&dpr=1 600w, https://images.theconversation.com/files/306350/original/file-20191211-95159-bxkp7.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1067&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/306350/original/file-20191211-95159-bxkp7.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1067&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/306350/original/file-20191211-95159-bxkp7.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1341&fit=crop&dpr=1 754w, https://images.theconversation.com/files/306350/original/file-20191211-95159-bxkp7.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1341&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/306350/original/file-20191211-95159-bxkp7.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1341&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Yes, that’s all penis.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Accoppiamento_fra_lumache_2.JPG">Viktor Volkov/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Why they intertwine so intricately is another matter. It could be that the beautiful complexity makes it more difficult for one slug to “cheat” by giving sperm and then not receiving some in return.</p>
<p>In the absence of direct study, the above explanations can only be considered speculation. The truth is that science doesn’t yet have a firm handle on the fascinating sex rituals of leopard slugs.</p>
<h2>More than voyeurism</h2>
<p>Scientists are not just being voyeuristic when we say we’d like to unravel the mysteries of slug sex. Aside from just understanding the wonder and beauty of the behaviour, there are potential benefits.</p>
<p>Some species of slugs are <a href="https://www.rhs.org.uk/advice/profile?pid=228">farm and garden pests</a>, eating holes in leaves, stems, flowers, tubers and bulbs and causing particular damage to new growth. With the pending ban of key pesticides for agricultural use in some countries, including the active ingredient in <a href="https://www.gardenorganic.org.uk/news/metaldehyde-slug-pellets-ban-overturned">slug pellets</a>, there is growing pressure to find other ways to control their spread. One way could be to identify otherwise harmless chemicals that interfere with their sex lives. A contraceptive sheath for slugs, so to speak.</p>
<p>Another approach could be to question why some of the slugs that cause the most agricultural nuisance forgo sex completely, especially in northern latitudes. Lack of sex reduces genetic variation, which causes crops such as potatoes and bananas to suffer from disease outbreaks. Studying the self-contained reproductive habits of slugs may reveal a similar vulnerability that could be exploited to control their numbers.</p>
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</figure>
<p>There may of course also be benefits which we can’t anticipate. So just as people champion trees, bees and butterflies, we need more slug enthusiasts of <a href="https://www.bbc.co.uk/news/entertainment-arts-46399187">all kinds</a> to help unravel their mucosal mysteries, including backyard explorers who can contribute to <a href="https://www.rhs.org.uk/slugssurvey">citizen science</a> studies. </p>
<p>Of course, if you are already a convert, then how about a leopard slug sex ornament for the <a href="http://www.furaffinity.net/view/25744884/">Christmas tree</a>?</p><img src="https://counter.theconversation.com/content/128284/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Angus Davison received funding from the Biotechnology and Biological Sciences Research Council</span></em></p>Scientists don’t just want to unravel the mysteries of slug sex for voyeurism.Angus Davison, Associate Professor and Reader in Evolutionary Genetics, University of NottinghamLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1258052019-12-03T18:29:59Z2019-12-03T18:29:59ZPunjabi ideas of honour can lead to girl-shaming and prenatal sex selection<figure><img src="https://images.theconversation.com/files/303883/original/file-20191127-112526-1o2nshl.jpg?ixlib=rb-1.1.0&rect=229%2C229%2C3639%2C2516&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A woman takes a selfie with three others while attending the Vaisakhi Parade in Surrey, B.C., on April 22, 2017. </span> <span class="attribution"><span class="source">THE CANADIAN PRESS/Darryl Dyck</span></span></figcaption></figure><p>Studies suggest a significant proportion of Indian-origin families in Canada are practising female feticide, sex-selective abortion. A disproportionately higher number of male children were born to mothers of Indian origin in Canada, <a href="https://www.theglobeandmail.com/life/health-and-fitness/health/some-couples-in-canada-practising-prenatal-sex-selection-in-favour-of-male-fetuses-studies-show/article29583670/">according to research</a> published in the <em>Canadian Medical Association Journal</em>. This imbalance was starker for couples who already had two daughters. </p>
<p>The findings <a href="https://www.theglobeandmail.com/opinion/sex-selective-abortions-strive-for-cultural-understanding-over-outrage/article29641641/">stirred much debate</a> <a href="https://www.thestar.com/news/canada/2016/12/11/its-time-to-debate-sex-selective-abortion-tory-leadership-hopeful-pierre-lemieux-says.html">and discussion</a> in the Canadian media. </p>
<p>Since Punjabis constitute a significant <a href="https://www12.statcan.gc.ca/nhs-enm/2011/as-sa/99-010-x/99-010-x2011001-eng.cfm">portion of Indian immigrants</a> as well as <a href="https://vancouversun.com/opinion/columnists/douglas-todd-indo-canadians-in-uproar-over-surge-of-foreign-students">foreign students</a> in Canada, it is crucial to understand the manifestation of gender inequality that may cause this sex selection and prejudice against girl-children within the traditional patriarchal Punjabi culture.</p>
<h2>Origins of gender inequality</h2>
<p>Researchers <a href="https://www.nber.org/papers/w17098">in economics</a> <a href="https://news.virginia.edu/content/patriarchy-and-plow">and sociology</a> have traced the origins of persistent gender inequalities to the development of plow agriculture, which devalued women’s labour, rendered their status inferior to that of men and led to a <a href="https://www.tandfonline.com/doi/full/10.1080/02255189.2018.1450737?src=recsys">strong preference for sons</a>. </p>
<p>In Punjab, an agrarian, male-dominated society, women are expected to preserve their family’s <a href="https://www.tandfonline.com/doi/full/10.1080/02255189.2018.1450736"><em>izzat</em></a>, or honour. This means refraining from doing anything that can disgrace the men of their family. </p>
<p>In this context, where a woman’s identity is defined by her male counterparts, being unmarried invites social sanctions for women as well as for their families. </p>
<h2>Unmarried daughters cause shame, disgrace</h2>
<p>Almost every aspect of Punjabi culture is rife with the notion of women being the bearers of the <em>izzat</em> of their fathers, brothers and all other male relatives. Even the songs, called <em>suhag</em>, sung on the eve of a girl’s wedding day emphasize why getting married is important. Being unmarried brings her father extreme shame. </p>
<p>Consider, for example, the lyrics of a popular <em>suhag</em> <a href="https://www.punjabi-kavita.com/PunjabiSuhag.php"><em>“Kotha kyon niveya'n”</em> (“Why is the house collapsing”)</a>:</p>
<blockquote>
<p>Why has the father bowed down (shamefaced)?</p>
<p>Why is the righteous father feeling humiliated?</p>
<p>The daughter of this father is still unmarried,</p>
<p>That’s why he is feeling humiliated.</p>
</blockquote>
<p>In a patriarchal culture dominated by notions of hypermasculinity, where bowing down is mortifying for a man, a daughter’s singlehood can bring a father to his knees. </p>
<p>Women’s bodies and sexualities are controlled and regulated by men through the cultural constructs of <a href="https://tspace.library.utoronto.ca/bitstream/1807/69458/3/Mucina_Mandeep_K_201501_PhD_thesis.pdf">honour and shame</a>. An unmarried daughter always remains a “threat” to the men’s <em>izzat</em> and any expression of her sexuality has the potential to destroy the family honour. It is not uncommon for men to use violence to reinstate that honour. </p>
<p>An extreme manifestation of that violence is “honour killing,” instances of which <a href="https://www.justice.gc.ca/eng/rp-pr/cj-jp/fv-vf/hk-ch/p2.html">aren’t unheard of in Canada</a>.</p>
<p>It is considered tragic if the daughter remains single for an extended period of time as the parental home is not considered <em>apna ghar</em> (own home) for the daughter. She is somebody’s <em>amaanat</em> (precious possession), and the parents are simply temporary caretakers.</p>
<h2>Personal stories</h2>
<p>To further unpack the devaluation of women in Punjabi society and by Punjabi-Canadian society, we use a collaborative <a href="https://qualpage.com/2018/11/15/what-is-autoethnography/">autoethnography</a> technique. To do this, we discuss our lived experiences of being single, Punjabi and upper-caste women. We were both born and brought up in Punjab and we came to Canada to pursue higher education.</p>
<h2><em>Sumeet’s story</em></h2>
<p>My sister was 23 years old, and a newly minted dentist in Amritsar, a prominent city in Punjab, when my parents started looking for a “suitable match” for her. Suitability is defined in terms of social status, age and education. Social status, in turn, is constituted by caste, wealth and land ownership. </p>
<p>All of 17 years of age at the time, I can recall a distinct sense of foreboding as I watched my parents trying to arrange a marriage for my sister. They used several words to describe their feelings about her marriage: duty, responsibility, burden.</p>
<p>My sister and I were quite used to the idea of being perceived as a burden. We had grown up hearing from family friends and members of the extended family that it was unfortunate for our father to have two daughters. We heard things like: “Your poor father — he has to marry off two girls.” </p>
<p>In addition to the strong preference for sons, note that all the sympathy was reserved for my father and not for both my parents. </p>
<p>With the passage of time, my parents became increasingly desperate to marry off my sister as she was becoming, ostensibly, less desirable with age. They cajoled, begged, and even threatened my sister to agree to wed any man whose family showed even the slightest interest in her regardless of her own feelings about the men in question. </p>
<p>The general state of anxiety in our household did not end until a suitable match was arranged for my sister. She was 25 years old.</p>
<p>Clearly, an unmarried daughter is a looming threat for the family’s <em>izzat</em>.</p>
<h2><em>Navjot’s account</em></h2>
<p>Rural Punjab. It was 4:30 in the morning. There was an unease caused by some hushed whispers that I could sense in my partially sleepy state. I went back to sleep to wake up a few hours later for school (I was a 24-year old grad student), unaware that the worst trauma of my life was going to present itself in a matter of minutes. </p>
<p>One of my male cousins snatched my laptop bag from me and I was told to keep quiet. I looked at them in bewilderment as they proceeded to confiscate my phone and took the battery out. I, somehow, gathered up my courage to ask what was happening once the Earth stopped slipping from under my feet. “We are not letting you go to the university anymore.” </p>
<p>“Just be thankful that we didn’t kill him,” was one of the sentences being uttered by someone in the bunch of male relatives of my joint family surrounding me. It was only after a couple of hours that I realized that this bunch had gone out that fateful foggy morning of November and had beaten the daylights out of my alleged “boyfriend.” </p>
<p>I realized that I was constantly under surveillance — if not by my family, then by “sympathizers” of my family concerned about my family’s honour. </p>
<p>The intensity and urgency of my reprimand might have been caused and compounded by the fact that just a few months earlier, one of my cousins had the courage to go against family wishes and marry a lower-caste man. She was then excommunicated and remains that way to this day.</p>
<h2>Understanding <em>izzat</em>: Honour</h2>
<p>We hope this piece helps Canadian readers understand the ways in which the cultural construct of <em>izzat</em>, combined with ideas of hypermasculinity, leads to the devaluation of women, as well as a strong preference for sons, among Punjabi immigrant families. </p>
<p>Given that many women in Canadian society have significant autonomy over their lives, Punjabi immigrant families likely impose harsh sanctions on young women upon their arrival in Canada to protect them against such freedoms. </p>
<p>We can begin to address these issues by bringing them them out in the public. Special efforts should be made to include Punjabi men in this discourse. </p>
<p>We need Canadian academics to research the idea of <em>izzat</em>, as well as ideas of hypermasculinity and agrarian patriarchy as practised by the Punjabi community in Canada.</p>
<p>[<em>Deep knowledge, daily.</em> <a href="https://theconversation.com/ca/newsletters?utm_source=TCCA&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=deepknowledge">Sign up for The Conversation’s newsletter</a>]</p><img src="https://counter.theconversation.com/content/125805/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Studies suggest a significant proportion of Indian-origin families in Canada are practising female feticide. It is crucial to understand how gender inequality may lead to sex selection.Sumeet Sekhon, Postdoctoral Research Fellow, Development and Gender Studies, University of British ColumbiaNavjotpal Kaur, PhD Candidate, Sociology, Memorial University of NewfoundlandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1219742019-08-21T19:54:13Z2019-08-21T19:54:13ZHow the US right-to-life movement is influencing the abortion debate in Australia<figure><img src="https://images.theconversation.com/files/288832/original/file-20190821-170918-168octv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The recent decriminalisation debates in Australia have included a new focus on partial restrictions on abortions – a strategy that has been very successful in many US states.</span> <span class="attribution"><span class="source">Joel Carrett/AAP</span></span></figcaption></figure><p>As the abortion decriminalisation bill <a href="https://www.theguardian.com/australia-news/2019/aug/20/nsw-abortion-bill-delayed-until-september-amid-liberal-division">gradually</a> makes its way through the NSW parliament, opponents have been increasingly drawing on their long relationship with the right-to-life movement in the United States to lobby against the measure and try to push for more restrictive amendments. </p>
<p>This has been a trend in the anti-abortion movement in Australia for a while now. Activists have adopted some of the most successful elements of the US movement’s rhetoric and tactics in recent years in an effort to influence the debate in Australia.</p>
<p>The Australian public is strongly pro-choice. In a <a href="https://onlinelibrary.wiley.com/doi/full/10.1111/1753-6405.12825">2018 survey</a> of NSW residents, 73% supported full decriminalisation of abortion. A <a href="https://www.smh.com.au/national/nsw/polling-shows-overwhelming-support-for-a-womans-right-to-choose-20150926-gjvfyu.html">2015 poll</a>, also conducted in NSW, indicated that 87% believe a woman should be able to have an abortion, with only 6% opposing abortion in all circumstances. </p>
<p>The Australian right-to-life movement is <a href="https://journals.sagepub.com/doi/pdf/10.1177/1440783309335646">tiny</a> compared to the US, but their views have an outsized place in the abortion debate because of their vocal <a href="https://www.tandfonline.com/doi/full/10.1080/10361146.2011.595387?src=recsys">political</a> and <a href="https://www.theguardian.com/world/2019/aug/04/nsw-abortion-law-reform-united-church-breaks-religious-ranks-to-back-decriminalisation">religious</a> allies. </p>
<p>Less understood is their successful borrowing from the examples and experiences of international right-to-life movements, particularly in the US. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/288839/original/file-20190821-170906-17v4jd0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/288839/original/file-20190821-170906-17v4jd0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=397&fit=crop&dpr=1 600w, https://images.theconversation.com/files/288839/original/file-20190821-170906-17v4jd0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=397&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/288839/original/file-20190821-170906-17v4jd0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=397&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/288839/original/file-20190821-170906-17v4jd0.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=499&fit=crop&dpr=1 754w, https://images.theconversation.com/files/288839/original/file-20190821-170906-17v4jd0.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=499&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/288839/original/file-20190821-170906-17v4jd0.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=499&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Anti-abortion activists protesting a bill to decriminalise the procedure in Queensland last year.</span>
<span class="attribution"><span class="source">Dan Peled/AAP</span></span>
</figcaption>
</figure>
<h2>Adopting rhetoric and lobbying techniques</h2>
<p>American abortion opponents have long positioned themselves as leaders in a global cause. They pursue both a national and international agenda, seeking to sharply limit access to abortion around the world, with the ultimate aim of banning the procedure. </p>
<p>At an international level, they advance this goal via measures such as the “<a href="https://theconversation.com/trumps-global-gag-rule-will-cause-more-abortions-not-fewer-71881">global gag rule</a>”, which prohibits international NGOs that receive American aid from providing advice, counselling, or information about abortion. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/heres-why-there-should-be-no-gestational-limits-for-abortion-121500">Here's why there should be no gestational limits for abortion</a>
</strong>
</em>
</p>
<hr>
<p>Australia is a part of this transnational network. Protest groups that target local abortion clinics, such as <a href="https://www.abc.net.au/triplej/programs/hack/abortion-article/9035338">Helpers of God’s Precious Infants</a> and <a href="https://indaily.com.au/news/2019/05/31/adelaide-abortion-clinic-calls-for-safe-access-from-protesters/">40 Days for Life</a>, are chapters of US organisations. And for decades, even groups without direct ties to the US have hosted prominent American <a href="https://prolifeaction.org/joe/">opponents</a> of <a href="https://researchonline.nd.edu.au/cgi/viewcontent.cgi?article=1153&context=theses">abortion</a>, seeking to learn from their example. </p>
<p>In 2015, Right to Life Australia made the controversial choice to invite prominent anti-abortion activist Troy Newman of Operation Rescue for a national lecture tour. He ultimately <a href="https://www.abc.net.au/news/2015-10-01/troy-newman-detained-after-flying-to-melbourne/6819410">had his visa cancelled and was deported</a>, in part because his writings have questioned why abortion doctors are not executed. </p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"649859702960947201"}"></div></p>
<p>There have been some efforts to foster the polarised and divisive US style of abortion politics here, too. Religious historian Marion Maddox says that under Prime Minister John Howard, <a href="https://www.allenandunwin.com/browse/books/academic-professional/politics-government/God-Under-Howard-Marion-Maddox-9781741145687">conservative politicians</a> tried to import the “family values” rhetoric and policies of the US Republican Party on issues like abortion and gay rights. </p>
<p>As decriminalisation efforts have progressed in Australia, these overseas connections have become quite explicit. </p>
<p>After South Australia Greens MP Tammy Franks <a href="http://hansardpublic.parliament.sa.gov.au/Pages/HansardResult.aspx#/docid/HANSARD-10-25583">introduced a decriminalisation bill</a> in that state’s Legislative Council last year, her staffers received verbally abusive phone calls and threats of rape from Canadian abortion opponents. According to her staff, all members of the Legislative Council were then spammed with over 4,000 right-to-life emails, which they suspected came from the US.</p>
<p>Earlier this year, <a href="https://thesoutherncross.org.au/news/2019/04/10/timely-visit-by-pro-lifers/">anti-abortion activists from the US</a>, including the chair of 40 Days for Life, also brought their lobbying efforts directly to Adelaide. They met with several MPs to discuss ways they might assist in the fight against a bill to fully decriminalise abortion in the state.</p>
<h2>Late termination of pregnancies</h2>
<p>The current decriminalisation debates in Australia have included a new focus on specific measures aimed at partial restrictions on abortions – a strategy that has been very successful in many US states.</p>
<p>In recent weeks, several high-profile politicians, including former <a href="https://www.sbs.com.au/news/nsw-abortion-bill-death-on-demand-tony-abbott-tells-cpac-s-conservative-faithful">Prime Minister Tony Abbott</a> and <a href="https://www.news.com.au/lifestyle/health/barnaby-joyce-blasted-for-antiabortion-robocalls/news-story/f50ddafa6dabfafd3ab13e95608d34e7">Nationals MP Barnaby Joyce</a>, have erroneously claimed that NSW is making abortion legal up to birth. <a href="https://www.sbs.com.au/news/dutton-comes-out-against-nsw-abortion-decriminalisation-bill-saying-22-weeks-is-too-late">Liberal MP Peter Dutton</a> and former Liberal candidate <a href="https://www.theaustralian.com.au/nation/politics/crazy-lunatic-bill-mundine-lashes-nsw-liberals-on-abortion/news-story/cc74e59721f075ac281fd09df0cf9f6e">Warren Mundine</a> have further suggested that the proposed gestation limit is “too late”. </p>
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<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/after-119-years-nsw-is-set-to-decriminalise-abortion-why-has-reform-taken-so-long-121112">After 119 years, NSW is set to decriminalise abortion. Why has reform taken so long?</a>
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</em>
</p>
<hr>
<p>In reality, the <a href="https://www.theguardian.com/australia-news/2019/aug/19/nsw-abortion-law-the-decriminalisation-reform-bill-explained">gestation limit</a> in the NSW bill is modelled on <a href="https://www.theguardian.com/australia-news/2018/oct/17/queensland-parliament-votes-to-legalise-abortion">legislation passed in Queensland</a> in 2018. It allows abortion up to 22 weeks, and after that with approval from two doctors. This is a lower cut-off than the laws in Victoria and the ACT.</p>
<p>The NSW bill is also more restrictive than the law in comparable Western countries. The <a href="https://www.rcog.org.uk/en/news/campaigns-and-opinions/human-fertilisation-and-embryology-bill/qa-the-abortion-time-limit/">gestation limit in Britain is 24 weeks</a>, with access available after that point for reasons of severe foetal anomaly or life-threatening risk to the mother. In the US, <a href="https://www.plannedparenthoodaction.org/issues/abortion/roe-v-wade">Roe v Wade</a> prevents outright bans on abortion before foetal viability (interpreted as 24 to 28 weeks), while <a href="https://www.cbc.ca/news/health/abortion-access-canada-us-bans-1.5140345">Canada</a> has no upper gestation limit.</p>
<p>This focus on “late-term abortions” emerged as a strategy in the US almost three decades ago. </p>
<p>By the early 1990s, American right-to-lifers had failed in their primary goal of overturning Roe v Wade. So, they <a href="https://global.oup.com/academic/product/ourselves-unborn-9780195323436?cc=au&lang=en&">refocused</a> their energies, seeking first to outlaw a comparatively rare technique they dubbed “<a href="https://www.nytimes.com/2003/11/06/us/bush-signs-ban-on-a-procedure-for-abortions.html">partial-birth abortions</a>”. More recently, they have sought to <a href="https://www.nytimes.com/2018/01/29/us/politics/senate-abortion-ban-20-weeks.html">prevent terminations after 20 weeks</a> using the medically contested notion of foetal pain. </p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1162445809339621377"}"></div></p>
<p>Focusing on later terminations allows opponents of abortion to insist that the foetus possesses unique rights and subjectivity that override those of the pregnant person. Further, the gestational point at which an abortion becomes “late” is contested and ever-shifting terrain, allowing for opponents to push for a lower and lower limit. </p>
<p>When discussing these types of terminations, opponents also rely on <a href="https://www.northernstar.com.au/news/osullivan-hits-out-barbaric-abortion-bill/3077028/">emotive, inaccurate, and often disturbing</a> language and imagery. </p>
<p>Yet, only <a href="https://theconversation.com/heres-why-there-should-be-no-gestational-limits-for-abortion-121500">1% to 3% of abortions</a> in Australia occur after 20 weeks, the majority of which are performed for reasons of severe or fatal foetal anomalies.</p>
<h2>Sex-selective abortions</h2>
<p>In <a href="https://www.catholicweekly.com.au/qld-abortion-law-will-allow-targeting-girls/">Queensland</a>, <a href="https://emilysvoice.com/abortion-for-any-reason-sa/">South Australia</a>, and <a href="https://www.smh.com.au/politics/nsw/finance-minister-to-move-amendments-on-sex-selection-abortions-20190815-p52hio.html">NSW</a>, opponents of abortion have also raised the spectre of sex-selective abortions. </p>
<p>This concern also originates from outside Australia. In the 2000s, the US right-to-life movement began framing <a href="https://www.journals.uchicago.edu/doi/10.1086/684239?mobileUi=0">abortion as a form of discrimination</a>, warning of abortions conducted on the grounds of sex, race, or disability. Eight states subsequently banned sex-selective abortions. </p>
<p>In the 2010s, this approach received further amplification after The Telegraph published a series of articles insisting the practice was <a href="https://www.telegraph.co.uk/news/health/news/9099925/Abortion-investigation-Available-on-demand-an-abortion-if-its-a-boy-you-wanted.html">rife</a> in the UK and that pro-choice feminists had turned a blind eye to a generation of “<a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5367570/">lost girls</a>”. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/a-concise-history-of-the-us-abortion-debate-118157">A concise history of the US abortion debate</a>
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</em>
</p>
<hr>
<p>In an attempt to appeal to critics in her own party, NSW Premier Gladys Berejiklian has expressed a willingness to ban “<a href="https://www.smh.com.au/politics/nsw/berejiklian-backs-strengthening-of-abortion-bill-amid-pressure-from-conservative-mps-20190814-p52gub.html">gender selection</a>” abortions in NSW, though it <a href="https://www.theguardian.com/world/2019/aug/15/nsw-abortion-law-backers-unlikely-to-support-calls-for-sex-selection-ban">appears unlikely</a> the bill’s supporters will agree to an amendment.</p>
<p>Studies in the <a href="https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/456642/sex_selection_doc.pdf">UK</a>, <a href="https://scholarship.law.cornell.edu/cgi/viewcontent.cgi?article=2536&context=facpub">US</a>, and <a href="https://www.ncbi.nlm.nih.gov/pubmed/30052991">Australia</a> have found no conclusive evidence that such abortions are occurring. </p>
<p>Rather, these campaigns work to stigmatise and vilify abortion care providers by accusing them of committing gendered acts of violence. And they suggest that medical professionals need to subject certain immigrant communities to more stringent forms of monitoring and surveillance. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/288842/original/file-20190821-170922-brlk49.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/288842/original/file-20190821-170922-brlk49.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=380&fit=crop&dpr=1 600w, https://images.theconversation.com/files/288842/original/file-20190821-170922-brlk49.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=380&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/288842/original/file-20190821-170922-brlk49.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=380&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/288842/original/file-20190821-170922-brlk49.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=477&fit=crop&dpr=1 754w, https://images.theconversation.com/files/288842/original/file-20190821-170922-brlk49.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=477&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/288842/original/file-20190821-170922-brlk49.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=477&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Abortion rights supporters demonstrating earlier this year in New York against extreme anti-abortion laws passed in Alabama, Georgia, and Missouri.</span>
<span class="attribution"><span class="source">Justin Lane/EPA</span></span>
</figcaption>
</figure>
<h2>The subtle impact of these tactics</h2>
<p>Collectively, these more covert right-to-life strategies have been part of a massive erosion of abortion rights in the United States. </p>
<p>In an Australian context, they work in more subtle ways. They prevent abortion being treated as just another type of health care, one of the explicit goals of the current decriminalisation campaigns. </p>
<p>They require doctors to assess and judge a pregnant person to see if they really want an abortion. And they inject uncertainty and attach further stigma to the work performed by abortion care providers. </p>
<p>Unable to wind back the clock on decriminalisation, Australian activists still insist abortion is a problematic and exceptional procedure. And they are drawing from the US right-to-life movement to shape how it is culturally, medically, and legally understood in Australia.</p><img src="https://counter.theconversation.com/content/121974/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Prudence Flowers receives funding from the South Australian Department of Human Services. She is a member of the South Australian Abortion Action Coalition. </span></em></p>The Australian right-to-life movement is tiny compared to the US, but its recent adoption of US-style campaign strategies has given it an outsize voice in the debate here.Prudence Flowers, Lecturer in US History, College of Humanities, Arts, and Social Sciences, Flinders UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1210722019-07-29T15:48:33Z2019-07-29T15:48:33ZBelligerent beetles show that fighting for mates could help animals survive habitat loss<figure><img src="https://images.theconversation.com/files/286026/original/file-20190729-43145-1p8sarc.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">En garde!</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/rhinoceros-beetle-on-wood-forest-243404500?src=x3wJuQTOXK6isXUv-_Gwog-1-1&studio=1">BaLL LunLa/Shutterstock</a></span></figcaption></figure><p>Animals around the world are seeing their environments change. <a href="https://theconversation.com/animals-will-struggle-to-adapt-fast-enough-to-cope-with-climate-change-study-finds-120857">Climate change</a> is causing heating and changes to weather patterns, the oceans are becoming <a href="https://theconversation.com/the-worlds-shellfish-are-under-threat-as-our-oceans-become-more-acidic-103868">more acidic</a>, and previously undisturbed habitats are being <a href="https://theconversation.com/habitat-loss-doesnt-just-affect-species-it-impacts-networks-of-ecological-relationships-117687">altered and degraded</a> by human activities.</p>
<p>If we want to understand how these changes will affect animals around the world, we need a better understanding of how their biology might determine how well they survive these changes. My colleagues and I have just <a href="https://doi.org/10.1111/ele.13358">published research</a> that demonstrates how important an animal’s mating system is to this. We found that species whose males compete for mates are more likely to survive damaging changes to their environment.</p>
<p>In many species, males try to woo females <a href="https://theconversation.com/strut-your-stuff-how-rockstars-and-peacocks-attract-the-ladies-29045">with signals</a> like calls, colouration or long tails, or they try to monopolise access to females by fighting other males with weaponry like horns or antlers. This competition for mates helps drive the evolution of these species, in a process called <a href="https://theconversation.com/why-men-are-not-biologically-useless-after-all-42012">sexual selection</a>. The most attractive or most aggressive mates are more likely to pass on their genes to the next generation and produce more offspring with their attractive features or aggressive nature. </p>
<p>There are many reasons to think competitive mating could affect the resilience of a species to environmental change. First, the signals and weapons that often evolve in those species where competition is more intensive are costly to grow and to carry. They can make animals more conspicuous to predators, and both contests with rival males and extravagant displays to females can use enormous amounts of energy. So these strongly sexually selected species could be less able to cope with environmental change because of these costs.</p>
<p>On the flip side, strong competition between males for mates means that only a few particularly strong, healthy or energetic males “win” and father the majority of the next generation. If the environment is changing, then males that are genetically best suited to the new environment are likely to be in the best condition. If these males end up as the winners in the competition for mating then their well-adapted genes will spread very rapidly. So strong sexual selection could make animal populations <a href="https://mast.queensu.ca/%7Etday/pdf/Lorchetal03.pdf">adapt faster to new environments</a>, making them more resilient in the face of it changing.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/286032/original/file-20190729-43136-166fkp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/286032/original/file-20190729-43136-166fkp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/286032/original/file-20190729-43136-166fkp8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/286032/original/file-20190729-43136-166fkp8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/286032/original/file-20190729-43136-166fkp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/286032/original/file-20190729-43136-166fkp8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/286032/original/file-20190729-43136-166fkp8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">Peacocking is a costly mating strategy.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/indian-male-peacock-1147396844?src=8jEkBHVkTbTf6jxsFJXBGw-1-4&studio=1">Kandarp/Shutterstock</a></span>
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</figure>
<p>So which process is more important in influencing species survival? A <a href="https://www.nature.com/articles/s41467-019-10074-7">series of lab studies</a> have consistently found that strong sexual selection improves outcomes for animal species when the environment shifts from their optimum. But studies of animals in the field have often found either no effect of sexual selection or the opposite. For example, when birds have been introduced to islands <a href="https://besjournals.onlinelibrary.wiley.com/doi/10.1046/j.1365-2656.1998.00199.x">such as New Zealand</a>, then the species that are more sexually selected are less likely to become established.</p>
<p>One possible reason for this disparity is that the field studies have often concentrated on very small populations of animals. It’s possible that the effects of sexual selection on a population’s resilience vary with its size. </p>
<p>Very small populations living in a certain location might not have enough genetic variety to produce individuals that are very well adapted to its particular environment. In which case, the costs of sexual selection could make them more likely to go extinct. Whereas large populations are more likely to have the genetic variety that will produce “winning” males even when the environment is unfriendly.</p>
<p>What was needed was a field study of sexual selection and persistence in larger populations. To that end, my colleagues and I conducted a study, published in <a href="https://doi.org/10.1111/ele.13358">Ecology Letters</a>, of how dung beetles respond to environmental change in the rainforest of Sabah in Malaysian Borneo.</p>
<h2>Horny beetles</h2>
<p>Dung beetles are fascinating animals for many reasons, one of which is the diversity of their sex lives. Males from the familiar ball-rolling species do compete for matings. But there are also many species of dung beetle that don’t roll, instead burying dung directly under where they find it, and these species show much greater variability. Some species have <a href="https://www.sciencenewsforstudents.org/article/little-beetle-big-horns">males with horns</a>, which they use in <a href="http://hs.umt.edu/dbs/labs/emlen/documents/Emlen%20Publications/BRCLososEmlenFINALlowres.pdf">fights with other males</a>, whereas others are less strongly sexually selected, with hornless males who are less aggressive in their pursuit of mates.</p>
<p>Using an existing large-scale study called the <a href="https://www.safeproject.net/">SAFE Project</a>, we followed 34 species of beetle found in untouched “old growth” forest. We looked at how they fared in lightly-logged and heavily logged forest and then oilpalm plantation where the original forest was largely removed. </p>
<p>We found that those species with horns were more likely to survive in all these cases. Strikingly, all 11 remaining species in the most disturbed plantation environment carried horns. </p>
<p>We also compared species with relatively small horns against those with big horns for their size. We found that beetle species with big horns are not only more likely to survive in disturbed environments, but they also tend to have larger remaining population sizes.</p>
<p>This tells us that – in some particular cases at least – we should think about sexual selection as well as other aspects of an animal’s biology if we want to predict or to manage population sizes in the face of environmental change. Sexual selection is a ubiquitous and powerful force driving evolution in the animal kingdom and has been intensively studied by behavioural and evolutionary biologists. Maybe now it’s time ecologists and wildlife management specialists started to think about it as well.</p><img src="https://counter.theconversation.com/content/121072/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rob Knell does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>New evidence boosts the idea that species with males who compete for mates adapt faster to changing circumstances.Rob Knell, Reader in Evolutionary Ecology, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1111492019-02-22T00:07:07Z2019-02-22T00:07:07ZSexual selection in action: Birds that attract multiple mates change their songs more quickly<figure><img src="https://images.theconversation.com/files/260221/original/file-20190221-195883-15xmtnf.jpg?ixlib=rb-1.1.0&rect=984%2C342%2C3020%2C1987&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Male collared flycatcher, singing for multiple females.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/singing-beautiful-male-collared-flycatcher-ficedula-1113698822">Kennerth Kullman/Shutterstock.com</a></span></figcaption></figure><p>How do individuals choose their mates? Why are some more successful at attracting mates than others?</p>
<p>These age-old questions are broadly relevant to all animals, including human beings. <a href="http://darwin-online.org.uk/content/frameset?pageseq=1&itemID=F376&viewtype=side">Darwin’s theory of natural selection</a> offers one way to answer them. Sometimes phrased as “survival of the fittest,” the theory can also apply to mate choice, predicting that it’s <a href="https://www.amnh.org/exhibitions/darwin/evolution-today/how-does-natural-selection-work">beneficial to choose the mate who’s best adapted</a> to surviving in its environment — the fastest runner, the best hunter, the farmer with the highest yields.</p>
<p>That’s a bit simplistic as a summary of human sexuality, of course, since people pair up in the context of complex social norms and gender roles that are uniquely human. Researchers like us do think, though, that mate choice in other animals is influenced by these kinds of perceived adaptations. It fits with scientists’ understanding of evolution: If females choose to mate with well-adapted males, their offspring might have a better chance of surviving as well. Advantageous traits wind up <a href="https://evolution.berkeley.edu/evolibrary/article/evo_25">passed down and preserved in future generations</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/260223/original/file-20190221-195879-1sg1s5j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260223/original/file-20190221-195879-1sg1s5j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260223/original/file-20190221-195879-1sg1s5j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260223/original/file-20190221-195879-1sg1s5j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260223/original/file-20190221-195879-1sg1s5j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260223/original/file-20190221-195879-1sg1s5j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260223/original/file-20190221-195879-1sg1s5j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260223/original/file-20190221-195879-1sg1s5j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A peacock’s tail’s only advantage is that females love it.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/portrait-beautiful-peacock-feathers-out-304866848">PrimePhoto/Shutterstock.com</a></span>
</figcaption>
</figure>
<p>But in many species, males try to attract mates by displaying characteristics that <a href="https://evolution.berkeley.edu/evolibrary/article/side_0_0/runawayselec_01">seem to be decidedly non-adaptive</a>. These signals – such as a dazzling tail on a peacock or a beautiful tune from a songbird – were originally a big wrench thrown into Darwin’s theory of natural selection. Traits like these seem to do the opposite of making an animal more likely to survive in its environment. A flashy tail display or a showy melody is cumbersome, and it announces you to predators as well as love interests. Darwin got so upset by this inconsistency that he said “The sight of a feather in a peacock’s tail, whenever I gaze at it, <a href="https://www.darwinproject.ac.uk/letter/DCP-LETT-2743.xml">makes me sick</a>.” </p>
<p>Thinking about this conundrum <a href="http://darwin-online.org.uk/content/frameset?pageseq=1&itemID=F937.1&viewtype=side">led Darwin to another major theory</a>: <a href="https://evolution.berkeley.edu/evolibrary/article/evo_28">sexual selection</a>. Instead of directly displaying adaptations, males might need to produce costly, non-adaptive signals if females prefer those features when choosing mates. For the females, these signals might indirectly communicate that a male would be a good mate because he’s able to survive and succeed — in spite of the ornament, not because of it. Under this model, the costliest traits are the most attractive.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/260008/original/file-20190220-148520-1nycy7f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260008/original/file-20190220-148520-1nycy7f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/260008/original/file-20190220-148520-1nycy7f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=729&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260008/original/file-20190220-148520-1nycy7f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=729&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260008/original/file-20190220-148520-1nycy7f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=729&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260008/original/file-20190220-148520-1nycy7f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=916&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260008/original/file-20190220-148520-1nycy7f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=916&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260008/original/file-20190220-148520-1nycy7f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=916&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Red-winged blackbirds are polygynous, with males angling to mate with multiple females.</span>
<span class="attribution"><a class="source" href="https://www.audubon.org/birds-of-america/red-winged-starling-or-marsh-blackbird">John James Audubon Center at Mill Grove in Audubon, Pennsylvania and the Montgomery County Audubon Collection</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>But what if the stakes are raised, as in species that are polygynous, with males trying to attract and form bonds with multiple females? A logical next step to this theory might predict that the pressure to produce beautiful signals would skyrocket, compounding the rewards for individuals with elaborate ornaments. If the most successful males have the most extraordinary traits, an ensuing arms race over many generations could shift the population toward more extreme characteristics. <a href="http://darwin-online.org.uk/content/frameset?pageseq=282&itemID=F937.1&viewtype=side">This is an intuitive theory</a> – increased competition for mates would lead to increasingly elaborate sexually selected traits – but it hasn’t been tested across the tree of life.</p>
<p>Do non-monogamous mating systems truly increase sexual selection in real animals? As the strength of sexual selection increases, do sexually selected characteristics become more extreme? Do tails get longer? Songs, more beautiful? <a href="http://creanzalab.com">As two biologists</a> with <a href="https://scholar.google.com/citations?user=vwQdgAYAAAAJ&hl=en&oi=ao">expertise in computational methods</a>, <a href="https://scholar.google.com/citations?user=hScDPxkAAAAJ&hl=en">the evolution of behaviors</a> and songbirds, <a href="https://doi.org/10.1038/s41467-019-08621-3">we decided to investigate</a>.</p>
<h2>Building up the bird database</h2>
<p>Evolution is as complex as life itself. New computational abilities allow researchers like us to go beyond testing whether certain traits simply tend to occur together. Instead, we can delve into the past and try to discern the path that species have traveled through history to arrive where they are today.</p>
<p><a href="https://doi.org/10.1038/s41467-019-08621-3">To test the theory</a> that males trying to attract multiple mates would amplify sexual selection and drive the evolution of increasingly elaborate displays, we needed both a new dataset and innovative methods.</p>
<p>Songbirds are an excellent system with which to study this question. First, many species are socially (though not necessarily sexually) monogamous, <a href="http://crosstalk.cell.com/blog/10-examples-of-monogamy-in-the-animal-kingdom">which is otherwise</a> <a href="https://www.audubon.org/news/monogamy-rare-wild">exceedingly rare</a> in the animal kingdom, but there have been numerous independent transitions to polygyny over the course of their history. That makes it easy for us to compare the songs of birds searching for a single partner to the songs of those looking for multiple mates. Songbirds also have an incredible diversity of song, from the <a href="https://www.allaboutbirds.org/guide/house_sparrow">simple tweets of the house sparrow</a> to the <a href="https://www.allaboutbirds.org/guide/Northern_Mockingbird/">elaborate cadenzas of the mockingbird</a>.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/260086/original/file-20190221-148513-1nkx9sl.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260086/original/file-20190221-148513-1nkx9sl.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/260086/original/file-20190221-148513-1nkx9sl.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=801&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260086/original/file-20190221-148513-1nkx9sl.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=801&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260086/original/file-20190221-148513-1nkx9sl.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=801&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260086/original/file-20190221-148513-1nkx9sl.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1007&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260086/original/file-20190221-148513-1nkx9sl.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1007&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260086/original/file-20190221-148513-1nkx9sl.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1007&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A family tree of birds shows their evolutionary connections.</span>
<span class="attribution"><a class="source" href="https://doi.org/10.1038/s41467-019-08621-3">Kate T. Snyder & Nicole Creanza, Nature Communications, volume 10, Article number: 884 (2019)</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>By searching published literature and field guides, we gathered mating system data on almost 700 species and song data for over 350 species, <a href="https://www.nature.com/articles/s41467-019-08621-3#MOESM1">the largest database of its kind to date</a>. We obtained a <a href="http://birdtree.org">recently published phylogeny</a> – essentially a “family tree” that stretches all the way back to the ancestor of all birds – that covered all of avian evolutionary history. This would serve as our map through the songbird lineages.</p>
<p>We merged our trait data with the phylogeny to trace backwards in time, estimating how the ancestors of each group of songbirds might have sounded and behaved.</p>
<p>This approach is kind of like if we dropped in on a human family reunion and noticed that the vast majority of family members have blonde hair and were speaking Swedish – we’d guess that a long-gone matriarch of the family probably also had blonde hair and likely spoke Swedish. Then, we could visit another family reunion, distant relatives of the first, to find blonde people speaking mostly Norwegian. At yet another gathering, perhaps we’d see brown-haired people speaking Spanish. By doing this hundreds of times, researchers could figure out whether there was any association between hair color and language in these families’ histories.</p>
<p>Using similar methods with <a href="http://www.onezoom.org/OZtree/static/OZLegacy/EDGE_birds.htm">the bird family tree</a>, we were able to test not only how mating behavior correlates with the songs of living species, but also how these behaviors affected one another over thousands and even millions of years of songbird evolutionary history. By estimating the likely behaviors of the ancestors of modern-day songbirds, we could calculate the rate of evolution of these traits, including how rates of song evolution might be influenced by mating behavior, or vice versa.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/260224/original/file-20190221-195864-y2w5eg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260224/original/file-20190221-195864-y2w5eg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/260224/original/file-20190221-195864-y2w5eg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260224/original/file-20190221-195864-y2w5eg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260224/original/file-20190221-195864-y2w5eg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260224/original/file-20190221-195864-y2w5eg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260224/original/file-20190221-195864-y2w5eg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260224/original/file-20190221-195864-y2w5eg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Male house sparrows have simple songs, despite the fact that they are looking for multiple mates.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/house-sparrow-sitting-outside-urban-birds-1245300223">ViktoriaIvanets/Shutterstock.com</a></span>
</figcaption>
</figure>
<h2>Sexual selection, but not in one direction</h2>
<p>When we performed this deep analysis, the results surprised us. We did not find the expected relationship that songs became more elaborate in species where males were seeking multiple mates. Instead, we found an interesting evolutionary pattern: Songs seemed to be evolving faster polygynous lineages, but not in any particular direction.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/260043/original/file-20190220-148530-12huz5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/260043/original/file-20190220-148530-12huz5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/260043/original/file-20190220-148530-12huz5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=729&fit=crop&dpr=1 600w, https://images.theconversation.com/files/260043/original/file-20190220-148530-12huz5f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=729&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/260043/original/file-20190220-148530-12huz5f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=729&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/260043/original/file-20190220-148530-12huz5f.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=916&fit=crop&dpr=1 754w, https://images.theconversation.com/files/260043/original/file-20190220-148530-12huz5f.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=916&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/260043/original/file-20190220-148530-12huz5f.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=916&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Mockingbirds sing the complex songs the researchers expected would go along with polygynous mating strategies, but are generally socially monogamous.</span>
<span class="attribution"><a class="source" href="https://www.audubon.org/birds-of-america/mocking-bird">John James Audubon Center at Mill Grove in Audubon, Pennsylvania and the Montgomery County Audubon Collection</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Instead of these ancestral males trying to outcompete one another with more elaborate songs, songs seemed to oscillate between simple and complex like a swinging pendulum over the generations – changing quickly in the moment, but not in a consistent direction over the long term. If these polygynous species’ songs got too simple or too elaborate, they started moving back towards the middle.</p>
<p>These results challenge our initial broad intuitions about reproductive success and evolutionary pressures. By studying the songs of many monogamous and polygynous bird species across the evolutionary tree, we found results that stood in contrast to the prevailing wisdom: Species that attract multiple mates did not have more complex songs overall, but their songs were evolving faster. This is a new piece of evidence that may alter classical hypotheses on non-monogamy and sexual selection in evolution. </p>
<p>Our work shows that when scientists study sexual selection in the future, we need to think not only about the magnitude of the traits being studied, but also how fast they change.</p><img src="https://counter.theconversation.com/content/111149/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Nicole Creanza has received funding from Vanderbilt University, the Ruth Landes Memorial Research Fund, the John Templeton Foundation, and the Stanford Center for Computational, Evolutionary, and Human Genomics.</span></em></p><p class="fine-print"><em><span>Kate Snyder receives funding from Vanderbilt University Department of Biological Sciences and the Vanderbilt University Graduate School. </span></em></p>Biologists investigated whether birds that search for multiple mates would evolve ever more elaborate songs to attract them. What they found might have surprised Darwin.Nicole Creanza, Assistant Professor of Biological Sciences, Vanderbilt UniversityKate Snyder, Ph.D. Candidate in Biological Sciences, Vanderbilt UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1073952018-12-11T12:20:38Z2018-12-11T12:20:38ZThese ‘useless’ quirks of evolution are actually evidence for the theory<figure><img src="https://images.theconversation.com/files/249754/original/file-20181210-76968-opdxh7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/pavana-dam-park-lonavala-maharastra-india-782708224?src=JMSf6-7XjBmhlcHU3B8N3A-1-96">Chirag Nimavat/Shutterstock</a></span></figcaption></figure><p>Evolution is a fascinating field but can be rife with misunderstanding. One misconception is that evolution has some innate sense of direction or purpose. In reality, evolution is a mindless, plan-free phenomenon, driven into endless possibilities by random mutations, the most successful of which win out.</p>
<p>People also often think that every aspect of every living creature has a function, that it helps the organism survive in some small way. But there are some areas of evolutionary biology where benefits are murkier and, in some instances, where traits seem to make no sense at all. This is the realm of sexual selection, vestigial traits and evolutionary spandrels.</p>
<p>As important as the concept of <a href="https://www.bbc.com/bitesize/guides/zt4f8mn/revision/3">survival of the fittest</a> is to evolution, there are many examples that seem to undermine this idea. In fact, various aspects of evolutionary biology may seem counterintuitive and could even be seen as a reason to reject evolution as a whole. In fact, they strengthen our understanding rather than diminish it. Here’s how.</p>
<h2>Sexual selection</h2>
<p>Many species invest heavily in camouflage and other means of <a href="http://rstb.royalsocietypublishing.org/content/319/1196/505.short">blending into the surroundings</a> to avoid predators. So the physically heavy and downright ostentatious plumage of birds of paradise, peacocks and many other birds seems like a clear invitation to be eaten. But crucially they help these birds pass on their genes because they <a href="https://www.theguardian.com/science/2017/may/19/a-peacocks-tail-how-darwin-arrived-at-his-theory-of-sexual-selection">increase their chances</a> of attracting a mate.</p>
<p>This is what’s known as <a href="https://www.sciencedirect.com/science/article/pii/0169534791900553">sexual selection</a> at its finest. It strengthens the theory of evolution in that these seemingly weaker individuals are actually showing how well they can do in the face of adversity. It’s the evolutionary equivalent of using a pretty over-confident dating profile to impress potential partners.</p>
<h2>Vestigial traits</h2>
<p>When an anatomical structure appears frankly inept, it is probably a vestigial trait. This is a feature that no longer does whatever made it advantageous enough to evolve in the first place. If we could embody evolution as a person, then he or she would be creative but inherently lazy. If something is not being used then why bother maintaining it? It’s hard to say why they haven’t disappeared altogether but give it another million years and perhaps they will.</p>
<p>Some snakes, for example, still show vestigial traits harking back to their four-legged ancestry. Male pythons have little <a href="https://www.amnh.org/exhibitions/darwin/evolution-today/how-do-we-know-living-things-are-related/vestigial-organs">claw-like structures</a> towards the tail, which, although they aid courtship, are all that remain of their hindlimbs. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/249761/original/file-20181210-76965-1jjwgdg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/249761/original/file-20181210-76965-1jjwgdg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/249761/original/file-20181210-76965-1jjwgdg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/249761/original/file-20181210-76965-1jjwgdg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/249761/original/file-20181210-76965-1jjwgdg.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/249761/original/file-20181210-76965-1jjwgdg.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/249761/original/file-20181210-76965-1jjwgdg.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Cave fish have lost most of their eyes.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/mexican-tetra-astyanax-mexicanus-known-blind-414095764?src=buaGPamDktDdBnZr_mjPGA-1-12">Vladimir Wrangel/Shutterstock</a></span>
</figcaption>
</figure>
<p>Some <a href="http://advances.sciencemag.org/content/1/8/e1500363">cave fish</a> have, over generations, lost most of the components of their eyes because sight uses up a lot of energy and isn’t helpful when you live in complete darkness. Many flightless birds, such as penguins and Galapagos flightless cormorants, have wings so small that they are effectively redundant in terms of flying.</p>
<p>Closer to home, the human appendix is a good example of a vestigial trait (although there’s now <a href="https://www.sciencedaily.com/releases/2017/01/170109162333.htm">some evidence</a> it may not be useless after all). But there is a weirder one, the <a href="https://www.crcpress.com/Ophthalmic-Disease-in-Veterinary-Medicine/Martin/p/book/9781840761184">plica semilunaris</a>. The next time you look into the eyes of a loved one (it’s more awkward with a stranger on the bus), look at that little pink, triangular bit on the inside of each eye.</p>
<p>It’s not completely vestigial, as it helps ensure that tears drain properly and gives a slightly greater range of movement, but that’s not its original function. Long ago, when we shared a recent ancestry with birds and other reptiles, this little structure would have formed a nictating membrane, or “third eyelid”, to provide further protection to our eyes. So, although we have lost this clear, extra eyelid, evolution has upcycled it for another use. </p>
<h2>Spandrels</h2>
<p><a href="http://www.pnas.org/content/94/20/10750.full">Spandrels</a> are in many ways, the rarest and hardest to see “weird” evolutionary quirks. The word comes from an architectural term for the triangular sections between arches in older, usually fancy, buildings. These zones were often ornately decorated but incidental to the real function of the structure of the building.</p>
<p>An evolutionary spandrel is a physical structure or behavioural characteristic that is a by-product from some other functional adaptation. But despite some apparent examples, truly useless spandrels are hard to find within evolutionary biology.</p>
<p>One well-studied example is seen in an island-dwelling population of Italian wall lizards (<em>Podarcis sicula</em>), which spend less time basking in the sun than their mainland cousins. This behaviour can be seen as a spandrel because there’s no obvious advantage to it.</p>
<p>Scientists <a href="https://bmcevolbiol.biomedcentral.com/articles/10.1186/1471-2148-10-289">have proposed</a> it’s a by-product of the lizards’ evolution of increased levels of aggression, sexual activity and food intake. This has also led to more active melanocortin receptors, part of the hormone system that works in response to sunlight, and so the lizards don’t need to bask so much.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/249764/original/file-20181210-76983-11gn8tz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/249764/original/file-20181210-76983-11gn8tz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=439&fit=crop&dpr=1 600w, https://images.theconversation.com/files/249764/original/file-20181210-76983-11gn8tz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=439&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/249764/original/file-20181210-76983-11gn8tz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=439&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/249764/original/file-20181210-76983-11gn8tz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=552&fit=crop&dpr=1 754w, https://images.theconversation.com/files/249764/original/file-20181210-76983-11gn8tz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=552&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/249764/original/file-20181210-76983-11gn8tz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=552&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">What separates us from the beasts.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/big-man-photo-18747406?src=GY8PXwwUbhGg2SPd5GLbPQ-1-14">Zastolskiy Victor/Shutterstock</a></span>
</figcaption>
</figure>
<p>One genuine exception is something that defines our species as modern human beings: the chin. No other animals, or even extinct human relatives like Neanderthals, have one. As human diets changed, the bones and muscles in our jaws became smaller so we didn’t waste energy on them but we were left with a protruding bone at the bottom of the face. And no one has come up with a <a href="http://www.bbc.com/earth/story/20160204-why-do-humans-have-chins">wholly convincing reason</a> why.</p>
<p>Although the chin throws a spandrel in the works, there is nearly always a reason or, at least, an explanation for the myriad traits we see across biology. A better understanding of these evolutionary obscurities paves the way for a deeper understanding of the complex factors and drivers which influence the natural world around us.</p><img src="https://counter.theconversation.com/content/107395/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ben Garrod does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Why are humans the only animals with chins?Ben Garrod, Fellow, Animal and Environmental Biology, Anglia Ruskin UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1038062018-09-25T14:52:59Z2018-09-25T14:52:59ZThe ethical case against sex-selective abortion isn’t simple<figure><img src="https://images.theconversation.com/files/237799/original/file-20180924-85752-1vjq1id.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/cropped-image-pregnant-woman-holding-ultrasound-257359048?src=1SVIjoGcjROpKYPsR-5Tiw-1-7">Dragon Images/Shutterstock.com</a></span></figcaption></figure><p>A key theme in public debate over abortion in many countries over the last few years has been the morality and legality of sex-selective terminations. Now the use of an early prenatal testing technique in the UK has led to further concerns.</p>
<p>The Non-Invasive Prenatal Test (NIPT) is <a href="https://www.bbc.co.uk/news/uk-wales-43919301">being fully introduced</a> on the NHS this year, as a safe method of detecting Down’s Syndrome and other genetic conditions. But it has been internationally available from private providers for a number of years, and, as a 2017 report <a href="http://nuffieldbioethics.org/project/non-invasive-prenatal-testing">noted</a>, is often offered as a sex-determination test. This has <a href="https://www.independent.co.uk/news/health/selective-abortions-gender-tests-girls-uk-labour-a8540851.html">raised concerns</a> that the test may be used to facilitate sex-selective abortion – particularly within communities where women can be subject to strong cultural and familial pressure not to have girls. The current legal status of this practice in the UK is a matter of some <a href="http://discovery.ucl.ac.uk/1529386/1/sex-selective%20abortion%20manuscript.pdf">controversy</a>. </p>
<p>The BBC’s <a href="https://www.independent.co.uk/news/health/selective-abortions-gender-tests-girls-uk-labour-a8540851.html">Victoria Derbyshire</a> recently reported online discussion among British women about using NIPT to abort female foetuses. In response, the Labour Party <a href="https://www.bbc.co.uk/news/health-45497454">announced</a> a policy of banning the use of NIPT for sex determination. Labour’s equality and women’s spokesperson, Naz Shah, <a href="https://www.pressreader.com/uk/scottish-daily-mail/20180918/282076277777105">described</a> sex-selective abortion as “incredibly unethical”.</p>
<p>Shah’s view – that sex-selective abortion is morally wrong, and that the law ought therefore to prevent it – is widely shared, including among those who otherwise identify as pro-choice. <a href="https://www.bbc.co.uk/news/health-17136213">Similar sentiments</a> were expressed by politicians in 2012, when an <a href="https://www.telegraph.co.uk/news/health/news/9099511/Abortion-investigation-doctors-filmed-agreeing-illegal-abortions-no-questions-asked.html">undercover investigation</a> by the Telegraph found a number of doctors that appeared willing to perform sex-selective terminations. </p>
<p>But if you propose to deprive women of facts about their pregnancies, or interfere with choices they might make about their own bodies and futures, you must do more than allege wrongdoing. Articulation of a clear and compelling moral objection is required. </p>
<p>So: what kind of objections can be made in this case?</p>
<h2>Sex discrimination</h2>
<p>Sex-selective abortion is often dismissed as an abortion chosen due to a trivial preference, like a preference for one consumer product over another. And it would be natural to infer that it is therefore a classic case of unjust discrimination. What could be more obvious than that it is wrong to treat someone – in this case a foetus - less favourably simply on the arbitrary grounds of sex?</p>
<p>This characterisation of the practice is, however, dubious. Take first the thought that sex-selective abortion cannot be motivated by serious reasons. In fact, women who seek such abortions can have purposes that are just as weighty as those of women seeking non-selective terminations. They may be justifiably afraid, in particular, that the gender of their child may lead to the failure of their marriage, or their being left destitute. Cases <a href="https://www.bbc.co.uk/news/health-45497454">described</a> by the organisation <a href="https://jeena.org.uk">Jeena International</a>, for instance, show that it would be a mistake to assume that women in countries like the UK are necessarily immune to these risks. Yet, while Jeena International advocates the prohibition of sex selection, it is precisely when women face precarious circumstances like these that the option of abortion seems most crucial.</p>
<p>Consider next the suggestion that sex-selective abortion constitutes unjust discrimination against the foetus. This casts the foetus as having a right to be treated as our equal. But that idea undermines the case for abortion rights in general, not just sex-selective abortion. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/237909/original/file-20180925-149955-iezvnj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/237909/original/file-20180925-149955-iezvnj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/237909/original/file-20180925-149955-iezvnj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/237909/original/file-20180925-149955-iezvnj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/237909/original/file-20180925-149955-iezvnj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/237909/original/file-20180925-149955-iezvnj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/237909/original/file-20180925-149955-iezvnj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">General pro-choice arguments seem to apply naturally to sex selection as well.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/warsawpoland-23-march-2018-thousands-people-1052978201?src=rW6VzV8AUDJeDA6XD3KRwA-1-4">Grand Warszawski/Shutterstock.com</a></span>
</figcaption>
</figure>
<h2>Social impact</h2>
<p>In order to avoid providing tacit support for a pro-life position, ethical critics of sex selection often argue that the practice’s victims are not foetuses, but existing persons in society. There are number of versions of this position, pointing to a variety of anticipated bad social consequences. The question is whether any are strong enough to justify abridging choice in an area of such deep personal importance as procreation.</p>
<p>One immediate fear is that, if sex-selective abortion is available, it could lead to harmful unbalancing of the sex ratio. This seems a fairly remote possibility in a country like the UK, at least, where strong son-preference is not widespread.</p>
<p>But another common rationale for prohibiting sex-selective abortion may be more applicable. It <a href="https://www.telegraph.co.uk/women/womens-life/11362379/Gender-abortion-Its-time-for-urgent-action.html">appears</a>, for instance, to have been part of the motivation behind an (unsuccessful) attempt to change the law on sex selection by Fiona Bruce MP in 2015. On this view, a ban is needed to protect women from being coerced by their families or spouses into having the procedure. </p>
<p>Domestic coercion is undoubtedly a matter of grave concern. But it is unclear that prohibiting sex-selective abortion is the right remedy. This is because vulnerable women are at risk of being coerced into unwanted abortions generally. In all such cases, the appropriate policy is arguably instead to tackle domestic oppression directly, giving women meaningful exit options, while leaving abortion open to those who need it.</p>
<h2>Tackling inequality</h2>
<p>Yet another argument focuses on the message that allowing sex-selective abortion allegedly transmits. As one columnist evocatively <a href="https://www.telegraph.co.uk/women/womens-politics/11429377/Sex-selective-abortion-is-simply-indefensible.html">puts the point</a>: “I don’t want my daughter to learn that a girl’s life is worthless.” The suggestion here is that the existence of the practice vividly expresses the inferior status of women, making the struggle for gender equality still more difficult. But this case for prohibition can also be questioned on an ethical level. </p>
<p>One reason is that it is analogous to an argument that some <a href="http://prochoicealliance.org/files/asch.disabiilty.equality.prenatal.testing.florida.state.law.review.pdf">disability rights advocates</a> have made, to the effect that prenatal testing likewise broadcasts a hurtful, disrespectful message about the value of disabled lives. If someone were to argue, on these grounds, that selective abortion for disability is not only ethically problematic but ought to be banned, pro-choice people would no doubt reply (a) that there is no intention to express any such message, and (b) that in any case preventing women from obtaining needed abortions is not an equitable way of pursuing justice and equality for the disabled. These replies seem applicable in the case of sex selection too.</p>
<p>All this indicates that the ethical case for prohibition is less straightforward than one might expect. We can agree, of course, that much more progress needs to be made towards a world without the pervasive sex inequalities that lead some women to choose sex-selective abortion in the first place. But our problem is what to do here and now, while those inequalities persist. The dilemma is that, while such abortion can plausibly be seen as reinforcing relevant inequalities, prohibiting it arguably involves a perverse shift of the burdens of achieving gender justice onto vulnerable women themselves.</p><img src="https://counter.theconversation.com/content/103806/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jeremy Williams does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The view that sex-selective abortion is unethical is widely shared. But those who propose to use the law to prevent it need to supply a clear, compelling moral justification.Jeremy Williams, Lecturer in Global Ethics, Department of Philosophy, University of BirminghamLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/896892018-01-09T16:01:26Z2018-01-09T16:01:26ZSuper-black feathers can absorb virtually every photon of light that hits them<figure><img src="https://images.theconversation.com/files/201228/original/file-20180108-142334-1h044en.jpg?ixlib=rb-1.1.0&rect=0%2C54%2C925%2C708&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Super-black feathers on these guys are like looking into a dark cave.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/sdnatasha/4514108926">Natasha Baucas</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>What do birds and aerospace engineers have in common? Both have invented incredibly dark, “super-black” surfaces that absorb almost every last bit of light that strikes them. </p>
<p>Of course scientists worked intentionally to devise these materials. It’s evolution that brought this amazing trait about in birds. My co-lead author <a href="http://vertebrates.si.edu/birds/birds_staff_pages/TeresaFeo_staffpage.html">Teresa Feo</a>, our colleagues <a href="http://www.graphics.cornell.edu/%7Etodd/pcg/Home.html">Todd A. Harvey</a> and <a href="https://prumlab.yale.edu/">Rick Prum</a> and I <a href="http://nature.com/articles/doi:10.1038/s41467-017-02088-w">investigated the super-black feathers</a> in some of the most outlandish animals on earth: <a href="http://www.birdsofparadiseproject.org/">the Birds of Paradise</a>.</p>
<p>These are resplendent birds native to Papua New Guinea and surrounding areas. Males are brilliantly colored, with complicated mating dances. Females, who are drab and brown in comparison, carefully inspect the ornaments and dances of males before choosing their mate.</p>
<p>We wanted to know more about these birds’ super-black plumage and how it works. What mechanism do these feathers employ to be so effective at absorbing light?</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/201237/original/file-20180108-83581-d443ug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/201237/original/file-20180108-83581-d443ug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/201237/original/file-20180108-83581-d443ug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/201237/original/file-20180108-83581-d443ug.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/201237/original/file-20180108-83581-d443ug.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/201237/original/file-20180108-83581-d443ug.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/201237/original/file-20180108-83581-d443ug.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/201237/original/file-20180108-83581-d443ug.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A male Superb Bird of Paradise displays his super-black and brilliant blue plumage to an onlooking female.</span>
<span class="attribution"><span class="source">Ed Scholes</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Fanciest feathers, under the microscope</h2>
<p>The Birds of Paradise have evolved many remarkable traits, but none are more mysterious than the males’ velvety black plumage.</p>
<p>This black is so dark that your eyes cannot focus on its surface; it looks like a cave, or a fuzzy black hole in space. Using optical measurements, we found that these feather patches <a href="http://nature.com/articles/doi:10.1038/s41467-017-02088-w">absorb up to 99.95 percent of directly incident light</a>. That’s comparable to human-made very black materials such as solar panels, the lining of space telescopes, and even the “blackest black” material: <a href="http://www.cnn.com/2017/11/15/world/vantablack-blackest-black-material/index.html">Vantablack</a>, which absorbs 99.96 percent of light.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/201235/original/file-20180108-83567-ish8tm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/201235/original/file-20180108-83567-ish8tm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/201235/original/file-20180108-83567-ish8tm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=298&fit=crop&dpr=1 600w, https://images.theconversation.com/files/201235/original/file-20180108-83567-ish8tm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=298&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/201235/original/file-20180108-83567-ish8tm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=298&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/201235/original/file-20180108-83567-ish8tm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=374&fit=crop&dpr=1 754w, https://images.theconversation.com/files/201235/original/file-20180108-83567-ish8tm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=374&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/201235/original/file-20180108-83567-ish8tm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=374&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">On the left, a normal black feather from a Lesser Melampitta. On the right, a super-black feather from the Paradise Riflebird.</span>
<span class="attribution"><span class="source">Dakota McCoy</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Normal feathers are flat, and look like fractals; when you zoom in using a microscope, each branch of the feather looks like a tiny, flat feather. Under a powerful scanning electron microscope, we were surprised to see that the super-black feathers look like miniature coral reefs, bottle brushes or trees with tightly packed leaves.</p>
<p>These tiny, specially shaped bits stick up to form a jagged, complex surface; together they act as microscopic light traps. When light rays strike these surface microstructures, they repeatedly scatter around the shapes and are absorbed, rather than being reflected back to an observer. It’s an iterative process: Each time a scattering event occurs, a portion of the light is absorbed until it’s almost completely absorbed.</p>
<p>Human-made super-black materials such as “<a href="https://www.pv-tech.org/guest-blog/black-silicon-theres-more-than-meets-the-eye">black silicon</a>” also rely on what materials scientists call structural absorption. Like the super-black feathers, their microscopic “<a href="https://doi.org/10.1063/1.4719108">light traps</a>” are due to a rough surface that scatters light repeatedly, but the actual surface shapes they use are different. Rather than the feathers’ bottle brush shapes, human engineers designed regularly spaced microscopic cones and pits. With almost no exposed flat surface, these structurally black materials are the opposite of a mirror.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/201236/original/file-20180108-83556-1i5x62g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/201236/original/file-20180108-83556-1i5x62g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/201236/original/file-20180108-83556-1i5x62g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=301&fit=crop&dpr=1 600w, https://images.theconversation.com/files/201236/original/file-20180108-83556-1i5x62g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=301&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/201236/original/file-20180108-83556-1i5x62g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=301&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/201236/original/file-20180108-83556-1i5x62g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=378&fit=crop&dpr=1 754w, https://images.theconversation.com/files/201236/original/file-20180108-83556-1i5x62g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=378&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/201236/original/file-20180108-83556-1i5x62g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=378&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Due to its unusual microstructure, the feather from the Paradise Riflebird (on the right) still appears super-black when coated with gold, as compared to a regular black feather (on the left).</span>
<span class="attribution"><span class="source">Dakota McCoy</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The Birds of Paradise’s super-black feathers are so good at absorbing light that even when we coated them in gold, a shiny metal, they still looked black. That’s because it’s not the inside of the feather making the color via pigment or ordered nanostructures; instead, just as with human-made <a href="https://doi.org/10.1039/C4EE01152J">black silicon</a>, the super black comes from the physical surface structure. Evolution and human ingenuity arrived at the same solution.</p>
<h2>Advantages of super-black feathers</h2>
<p>But why do these birds have such incredibly dark black patches? What selective advantage caused this trait to evolve? It’s tempting to think that super black somehow helps with camouflage, to keep predators away. In fact, some <a href="https://doi.org/10.1038/srep01846">snakes have super-black scales</a> that mimic shadows between leaves, helping them blend into the forest floor. The snake example illustrates evolution by natural selection – “survival of the fittest.”</p>
<p>But other factors can also influence evolution’s course, including random chance or sexual selection. As my colleague Rick Prum points out in his new book “<a href="https://www.penguinrandomhouse.com/books/224257/the-evolution-of-beauty-by-richard-o-prum/9780385537216/">The Evolution of Beauty: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World – and Us</a>,” mate choice is a powerful force driving evolution. In Birds of Paradise, super-black feathers help male birds look more beautiful to a female’s eye.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/UYbn9R11Rrs?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">A Superb Bird of Paradise displays his best plumage to potential mate.</span></figcaption>
</figure>
<p>To understand how, it helps to look at Bird of Paradise mating dances. Males vigorously display their super-black patches to females, making sure that females can’t get a view from the side. This is because these feathers are highly directional, and they look darkest from straight ahead. </p>
<p>And super-black patches always sit around or next to brilliant color patches. A super-black, anti-reflective frame makes nearby colors appear brighter, almost glow. In other words, super black is an <a href="https://en.wikipedia.org/wiki/Checker_shadow_illusion">evolved optical illusion</a> that relies on the way animal eyes and brains adjust our perceptions based on ambient light.</p>
<p>In the high-stakes game of choosing a mate, a single feather that isn’t quite blue enough <a href="https://www.penguinrandomhouse.com/books/224257/the-evolution-of-beauty-by-richard-o-prum/9780385537216/">could be enough to turn off</a> a female Bird of Paradise. Clearly, female Birds of Paradise prefer males with super-black plumage. As females <a href="https://doi.org/10.1111/evo.13196">pick the most impressive males to mate with</a>, those dazzling feather genes are passed on to future generations while the genes of less splendid males, overlooked by females, are not. Sexual selection drove evolution toward super-black plumage.</p>
<p>Evolution is not an orderly, coherent process; evolutionary arms races can produce great innovation. Perhaps these super-black feathers with their unique microscopic structure could eventually inspire better solar panels, or new textiles; super-black butterfly wings <a href="https://link.springer.com/article/10.1186/s11671-015-1052-7">already have</a>. Evolution has had millions of years to tinker; we still have much to learn from its solutions.</p><img src="https://counter.theconversation.com/content/89689/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>This research was
funded by the W. R. Coe Fund of Yale University, by a Sigma XI student research
fellowship to D.E.M., and by a Mind, Brain, and Behavior Graduate Student Award
to D.E.M. D.E.M. was supported by the Department of Defense (DoD) through the
National Defense Science and Engineering Graduate Fellowship (NDSEG)
Program. Tomography data collections at the Advanced Photon Source beamline 2-
BM, Argonne National Laboratory were supported by the U.S. Department of
Energy Office of Science (Proposal ID 41887). T.J.F. was supported by a NSF
Postdoctoral Fellowship in Biology (#1523857). Richard Pfisterer of Photon
Engineering graciously licensed FRED to T.A.H. for this research. This work was
performed in part at the Harvard University Center for Nanoscale Systems (CNS), a
member of the National Nanotechnology Coordinated Infrastructure Network
(NNCI), which is supported by the National Science Foundation under NSF ECCS
award no. 1541959.</span></em></p>Male Birds of Paradise have patches of super-black plumage that absorb 99.95 percent of light. New research identified their feathers’ microscopic structures that make them look so very dark.Dakota McCoy, PhD Student in Organismic and Evolutionary Biology, Harvard UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/596652017-01-20T10:59:31Z2017-01-20T10:59:31ZData should smash the biological myth of promiscuous males and sexually coy females<figure><img src="https://images.theconversation.com/files/153319/original/image-20170118-26573-1kkbpx1.jpg?ixlib=rb-1.1.0&rect=104%2C0%2C1850%2C1092&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Victorian mores influenced ideas not just about men and women but animals too.</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/eoskins/10092139186">Joseph Christian Leyendecker</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>That males are naturally promiscuous while females are coy and choosy is <a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674002357">a widely</a> <a href="https://global.oup.com/academic/product/the-selfish-gene-9780198788607?q=the%20selfish%20gene&lang=en&cc=us">held belief</a>. Even many scientists – including some biologists, psychologists and anthropologists – tout this notion when <a href="http://people.com/archive/why-are-men-more-promiscuous-its-in-the-genes-says-a-psychologist-vol-12-no-17/">interviewed</a> by the media about almost any aspect of male-female <a href="http://canadiancrc.com/newspaper_articles/Time_Magazine_infidelity_in_genes_15AUG94.aspx">differences</a>, <a href="http://www.cbsnews.com/news/a-controversial-book-on-rape/">including</a> <a href="http://www.npr.org/templates/story/story.php?storyId=1069570">in human beings</a>. In fact, certain human behaviors such as rape, marital infidelity and some forms of domestic abuse have been portrayed as <a href="https://mitpress.mit.edu/books/natural-history-rape">adaptive traits that evolved</a> because males are promiscuous while females are sexually reluctant.</p>
<p>These ideas, which are pervasive in Western culture, also have served as the cornerstone for the evolutionary study of sexual selection, sex differences and sex roles among animals. Only recently have some scientists – fortified with modern data – begun to question their underlying assumptions and the resulting paradigm.</p>
<h2>It all comes down to sperm and eggs?</h2>
<p>These simple assumptions are based, in part, on the differences in size and presumed energy cost of producing sperm versus eggs – a contrast that we <a href="http://dx.doi.org/10.1016/0022-5193(72)90007-0">biologists call anisogamy</a>. <a href="http://darwin-online.org.uk/EditorialIntroductions/Freeman_TheDescentofMan.html">Charles Darwin was the first to allude</a> to anisogamy as a possible explanation for male-female differences in sexual behavior.</p>
<p>His brief mention was ultimately expanded by others into the idea that because males produce millions of cheap sperm, they can mate with many different females without incurring a biological cost. Conversely, females produce relatively few “expensive,” nutrient-containing eggs; they should be highly selective and mate only with one “best male.” He, of course, would provide more than enough sperm to fertilize all a female’s eggs.</p>
<p>In 1948, Angus Bateman – a botanist who never again published in this area – was the first to test Darwin’s predictions about sexual selection and male-female sexual behavior. He set up a series of breeding experiments using several inbred strains of fruit flies with different mutations as markers. He placed equal numbers of males and females in laboratory flasks and allowed them to mate for several days. Then he counted their adult offspring, using inherited mutation markers to infer how many individuals each fly had mated with and how much variation there was in mating success. </p>
<p>One of Bateman’s most important conclusions was that male reproductive success – as measured by offspring produced – increases linearly with his number of mates. But female reproductive success peaks after she mates with only one male. Moreover, <a href="http://doi.org/10.1038/hdy.1948.21">Bateman alleged this was a near-universal characteristic</a> of all sexually reproducing species.</p>
<p>In 1972, theoretical biologist Robert Trivers highlighted Bateman’s work when he formulated the <a href="http://joelvelasco.net/teaching/3330/trivers72-parentalinvestment.pdf">theory of “parental investment.”</a> He argued that sperm are so cheap (low investment) that males evolved to abandon their mate and indiscriminately seek other females for mating. Female investment is so much greater (expensive eggs) that females guardedly mate monogamously and stay behind to take care of the young.</p>
<p>In other words, females evolved to choose males prudently and mate with only one superior male; males evolved to mate indiscriminately with as many females as possible. Trivers believed that this pattern is true for the great majority of sexual species.</p>
<p>The problem is, modern data simply don’t support most of Bateman’s and Trivers’ predictions and assumptions. But that didn’t stop “Bateman’s Principle” from influencing evolutionary thought for decades.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/153445/original/image-20170119-26555-db89el.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/153445/original/image-20170119-26555-db89el.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/153445/original/image-20170119-26555-db89el.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=336&fit=crop&dpr=1 600w, https://images.theconversation.com/files/153445/original/image-20170119-26555-db89el.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=336&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/153445/original/image-20170119-26555-db89el.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=336&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/153445/original/image-20170119-26555-db89el.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=422&fit=crop&dpr=1 754w, https://images.theconversation.com/files/153445/original/image-20170119-26555-db89el.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=422&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/153445/original/image-20170119-26555-db89el.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=422&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A single sperm versus a single egg isn’t an apt comparison.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/3d-render-representing-human-sperm-egg-362907557">Gametes image via www.shutterstock.com.</a></span>
</figcaption>
</figure>
<h2>Examining the assumptions about males</h2>
<p>In reality, it makes little sense to compare the cost of one egg to one sperm. As comparative psychologist <a href="http://www.jstor.org/stable/2461181">Don Dewsbury pointed out</a>, a male produces millions of sperm to fertilize even one egg. The relevant comparison is the cost of millions of sperm versus that of one egg. </p>
<p>In addition, males produce semen which, in most species, contains critical bioactive compounds that presumably are <a href="http://dx.doi.org/10.1080/00224499.2016.1150938">very expensive to produce</a>. As is now also well-documented, sperm production is limited and males can run out of sperm – what researchers term “sperm depletion.”</p>
<p>Consequently, we now know <a href="http://dx.doi.org/10.1016/S0169-5347(02)02533-8">males may allocate more or less sperm to any given female</a>, depending on her age, health or previous mated status. Such differential treatment among preferred and nonpreferred females is a form of male mate choice. In some species, males may even <a href="http://dx.doi.org/10.1126/science.213.4509.779">refuse to copulate</a> <a href="http://dx.doi.org/10.1006/anbe.1996.0432">with certain females</a>. Indeed, <a href="http://dx.doi.org/10.1080/00224499.2016.1150938">male mate choice</a> is now a particularly active field of study. </p>
<p>If sperm were as inexpensive and unlimited as Bateman and Trivers proposed, one would not expect sperm depletion, sperm allocation or male mate choice.</p>
<h2>Assumptions about females don’t match reality</h2>
<p><a href="https://global.oup.com/academic/product/evolutionary-ecology-of-birds-9780198510888?q=Evolutionary%20Ecology%20of%20Birds:%20Life%20Histories,%20Mating%20Systems%20and%20Extinction&lang=en&cc=us">Birds have played a critical role</a> in dispelling the myth that females evolved to mate with a single male. In the 1980s, approximately 90 percent of all songbird species were believed to be “monogamous” – that is, one male and one female mated exclusively with one another and raised their young together. At present, only about 7 percent are classified as monogamous.</p>
<p>Modern molecular techniques that allow for paternity analysis revealed <a href="http://dx.doi.org/10.1073/pnas.96.18.10236">both males and females</a> often <a href="https://books.google.com/books?id=3i4Q8SvohfEC">mate and produce offspring with multiple partners</a>. That is, they engage in what researchers call “extra-pair copulations” (EPCs) and “extra pair fertilizations” (EPFs).</p>
<p>Because of the assumption that reluctant females mate with only one male, many scientists initially assumed promiscuous males coerced reluctant females into engaging in sexual activity outside their home territory. But behavioral observations quickly determined that <a href="http://joelvelasco.net/teaching/3330/hrdy-mythofcoy.pdf">females play an active role</a> in <a href="http://dx.doi.org/10.1007/978-1-4615-5985-6_12">searching for nonpair males and soliciting</a> extra-pair copulations.</p>
<p>Rates of EPCs and EPFs vary greatly from species to species, but the superb fairy wren is one socially monogamous bird that <a href="http://dx.doi.org/10.1098/rspb.1994.0032">provides an extreme example</a>: 95 percent of clutches contain young sired by extra-pair males and 75 percent of young have extra-pair fathers.</p>
<p>This situation is not limited to birds – across the animal kingdom, females frequently mate with multiple males and produce broods with multiple fathers. In fact, Tim Birkhead, a well-known behavioral ecologist, concluded in his 2000 book <a href="https://books.google.com/books?id=3i4Q8SvohfEC">“Promiscuity: An Evolutionary History of Sperm Competition</a>,” “Generations of reproductive biologists assumed females to be sexually monogamous but it is now clear that this is wrong.”</p>
<p>Ironically, Bateman’s own study demonstrated the idea that female reproductive success peaks after mating with only one male is not correct. When Bateman presented his data, he did so in two different graphs; only one graph (which represented fewer experiments) led to the conclusion that female reproductive success peaks after one mating. The other graph – largely ignored in subsequent treatises – showed that the number of offspring produced by a female increases with the number of males she mates with. That finding runs directly counter to the theory there is no benefit for a “promiscuous” female.</p>
<p>Modern studies have demonstrated this is true in a broad <a href="http://doi.org10.1093/beheco/ars077">range of species</a> – <a href="http://dx.doi.org/10.1080/00224499.2016.1150938">females that mate with more than one male produce more young</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/153322/original/image-20170118-26539-1i8w62u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/153322/original/image-20170118-26539-1i8w62u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/153322/original/image-20170118-26539-1i8w62u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=456&fit=crop&dpr=1 600w, https://images.theconversation.com/files/153322/original/image-20170118-26539-1i8w62u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=456&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/153322/original/image-20170118-26539-1i8w62u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=456&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/153322/original/image-20170118-26539-1i8w62u.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=573&fit=crop&dpr=1 754w, https://images.theconversation.com/files/153322/original/image-20170118-26539-1i8w62u.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=573&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/153322/original/image-20170118-26539-1i8w62u.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=573&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">What’s happening in society outside the lab can influence what you see inside it.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/nlireland/5863096328">National Library of Ireland on The Commons</a></span>
</figcaption>
</figure>
<h2>Seeing what society leads you to expect</h2>
<p>So if closer observation would have disproved this promiscuous male/sexually coy female myth, in the animal world at least, why didn’t scientists see what was in front of their eyes?</p>
<p>Bateman’s and Trivers’ ideas had their origins in Darwin’s writings, which were greatly <a href="https://doi.org/10.1093/icb/45.5.831">influenced by the cultural beliefs of the Victorian era</a>. Victorian social attitudes and science were closely intertwined. The common belief was that males and females were radically different. Moreover, attitudes about Victorian women influenced beliefs about nonhuman females. Males were considered to be active, combative, more variable, and more evolved and complex. Females were deemed to be passive, nurturing; less variable, with <a href="http://www.hup.harvard.edu/catalog.php?isbn=9780674802919">arrested development equivalent to that of a child</a>. “True women” were expected to be pure, submissive to men, <a href="http://www.jstor.org/stable/3173022">sexually restrained and uninterested in sex</a> – and this representation was also seamlessly applied to female animals.</p>
<p>Although <a href="http://dx.doi.org/10.1080/00224499.2016.1150938">these ideas may now seem quaint</a>, most scholars of the time embraced them as scientific truths. These stereotypes of men and women survived through the 20th century and influenced research on male-female sexual differences in animal behavior.</p>
<p>Unconscious biases and expectations can influence the <a href="https://www.scientificamerican.com/article/point-of-view-affects-how-science-is-done/">questions scientists ask and also their interpretations of data</a>. Behavioral biologist Marcy Lawton and colleagues <a href="http://dx.doi.org/10.1007/978-1-4615-5985-6_4">describe a fascinating example</a>. In 1992, eminent male scientists studying a species of bird wrote an excellent book on the species – but were mystified by the lack of aggression in males. They did report violent and frequent clashes among females, but dismissed their importance. These scientists <a href="http://dx.doi.org/10.1007/978-1-4615-5985-6_12">expected males to be combative and females to be passive</a> – when observations failed to meet their expectations, they were unable to envision alternative possibilities, or realize the potential significance of what they were seeing.</p>
<p>The same likely happened with regard to sexual behavior: Many scientists saw promiscuity in males and coyness in females because <a href="http://dx.doi.org/10.1007/978-1-4615-5985-6_11">that is what they expected to see</a> and what theory – and societal attitudes – told them they should see.</p>
<p>In fairness, prior to the advent of molecular paternity analysis, it was extremely difficult to accurately ascertain how many mates an individual actually had. Likewise, only in modern times has it been possible to accurately measure sperm counts, which led to the realization that sperm competition, sperm allocation and sperm depletion are important phenomena in nature. Thus, these <a href="http://dx.doi.org/10.1046/j.1365-294x.2000.00964.x">modern techniques also contributed to overturning stereotypes</a> of male and female sexual behavior that had been accepted for more than a century. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/153456/original/image-20170119-26577-15nkg6g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/153456/original/image-20170119-26577-15nkg6g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/153456/original/image-20170119-26577-15nkg6g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=395&fit=crop&dpr=1 600w, https://images.theconversation.com/files/153456/original/image-20170119-26577-15nkg6g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=395&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/153456/original/image-20170119-26577-15nkg6g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=395&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/153456/original/image-20170119-26577-15nkg6g.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=496&fit=crop&dpr=1 754w, https://images.theconversation.com/files/153456/original/image-20170119-26577-15nkg6g.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=496&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/153456/original/image-20170119-26577-15nkg6g.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=496&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">What looks like monogamy at first glance very often isn’t.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/couple-waved-albatross-galapagos-111927389">Waved Albatross image via www.shutterstock.com.</a></span>
</figcaption>
</figure>
<h2>Bateman’s research has not been replicated</h2>
<p>Besides the data summarized above, there is the question of whether Bateman’s experiments are replicable. Given that <a href="https://theconversation.com/half-of-biomedical-research-studies-dont-stand-up-to-scrutiny-and-what-we-need-to-do-about-that-45149">replication is an essential criterion of science</a>, and that Bateman’s ideas became an unquestioned tenet of behavioral and evolutionary science, it is shocking that more than 50 years passed before an attempt to replicate the study was published.</p>
<p>Behavioral ecologist Patricia Gowaty and collaborators had found numerous methodological and statistical problems with Bateman’s experiments; when they <a href="http://dx.doi.org/10.1111/j.1558-5646.2007.00212.x">reanalyzed his data, they were unable to support his conclusions</a>. Subsequently, they reran Bateman’s critical experiments, using the exact same fly strains and methodology – and <a href="http://dx.doi.org/10.1073/pnas.1207851109">couldn’t replicate his results or conclusions</a>.</p>
<p>Counterevidence, evolving social attitudes, recognitions of flaws in the studies that started it all – Bateman’s Principle, with its widely accepted preconception about male-female sexual behavior, is currently undergoing serious scientific debate. The scientific study of sexual behavior may be experiencing a paradigm shift. Facile explanations and assertions about male-female sexual behaviors and roles just don’t hold up.</p><img src="https://counter.theconversation.com/content/59665/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Zuleyma Tang-Martinez received funding from NSF in the past - but did not have funding at the time that this article was written </span></em></p>Victorian attitudes influenced what scientists thought they were observing about sexual behaviors in the animal world. But modern techniques reveal the myth for what it is.Zuleyma Tang-Martinez, Professor Emerita of Biology, University of Missouri-St. LouisLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/695672016-12-09T03:37:24Z2016-12-09T03:37:24ZSex lives of reptiles could leave them vulnerable to climate change<figure><img src="https://images.theconversation.com/files/148357/original/image-20161202-25645-2ddus1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Bearded dragons can be genetically male but look like and function as females. </span> <span class="attribution"><span class="source">Arthur Georges, Institute for Applied Ecology, University of Canberra</span>, <span class="license">Author provided</span></span></figcaption></figure><p>We are only just starting to appreciate the full sexual diversity of animals. What we are learning is helping us understand evolution and how animals will cope with a changing world. </p>
<p>In humans and other mammals, sex chromosomes (the Xs and Ys) determine physical sex. But in reptiles, sometimes sex chromosomes do not match physical sex. We call this “sex reversal”.</p>
<p>Environmental factors such as temperature can trigger sex reversal in reptiles. In our recent study, we investigated <a href="http://www.karger.com/Article/Abstract/450972">how common sex reversal is in reptiles</a>. We concluded that it is widespread and a powerful evolutionary force. </p>
<p>This raises important questions about how reptiles will survive in a warming world. </p>
<h2>Xs and Ys, Ws and Zs</h2>
<p>In humans, sex chromosomes determine if an embryo’s physical sex is either male (XY) or female (XX). </p>
<p>Reptile sex determination is more complicated. Some species, including snakes, use sex chromosomes like humans do. But in other species, such as crocodiles and marine turtles, sex is determined by the temperature the eggs are raised in.</p>
<p>We’ve recently come to realise that many species use a combination of both. When the temperature sends opposite signals to the embryo’s sex chromosomes, sex reversal is the result. For these lizards, the sex chromosomes don’t match their physical appearance and reproductive function.</p>
<p>The <a href="https://en.wikipedia.org/wiki/Central_bearded_dragon">central bearded dragon</a> (<em>Pogona vitticeps</em>) is probably the best-known example of reptile sex reversal. Its sex chromosomes are named Z and W. </p>
<p>Male dragons have two Z chromosomes and females have a Z and W. Female dragons normally produce roughly equal numbers of male (ZZ) and female (ZW) offspring. But when the eggs are incubated in a hot environment (greater than 32°C), more females than males hatch. Some of these females from hot nests are sex-reversed.</p>
<p>Sex-reversed females are fully functional. In fact they produce <a href="http://www.nature.com/nature/journal/v523/n7558/abs/nature14574.html">twice as many eggs</a> as females with female sex chromosomes. This suggests that sex reversal might actually be an advantage in this species.</p>
<p>Another fairly well-understood example from Australia is the <a href="https://en.wikipedia.org/wiki/Eastern_three-lined_skink">eastern three-lined skink</a> (<em>Bassiana duperreyi</em>). </p>
<p>In this species males are XY and females are XX. Although these chromosomes share the same name, they aren’t the same as those found in humans. They have arisen independently and use different genes to trigger male and female development.</p>
<p>In this skink, females (XX) can reverse to males, but at cool incubation temperatures, a phenomenon we’ve observed both <a href="http://rsbl.royalsocietypublishing.org/content/4/2/176">in the lab</a> and in <a href="http://www.karger.com/Article/Abstract/450972">a wild alpine population</a>.</p>
<p>In both species, the sex with matching sex chromosomes (ZZ males in the dragon and XX females in the skink) is the one that reverses. In dragons it happens at high temperatures, and in the skink at low temperatures.</p>
<h2>Why reverse sex?</h2>
<p>Sex reversal can have major effects on the behaviour of an individual. <a href="http://rspb.royalsocietypublishing.org/content/283/1832/20160217.abstract">Male-to-female central bearded dragons are bolder than males and females with matching sex chromosomes</a>. This may help them find food and mates, but at the same time exposes them to predators.</p>
<p>Not all lizards lay eggs. Sex reversal caused by temperature is also thought to occur in species that give birth to live young, such as <a href="http://www.nature.com/nature/journal/v468/n7322/abs/nature09512.html">Tasmania’s snow skink</a> (<em>Niveoscincus ocellatus</em>). In live bearers, sex reversal is caused by the environmental temperatures that a mother experiences during pregnancy.</p>
<p>We believe that sex reversal is widespread in reptiles. Emerging evidence suggests that environmentally induced sex reversal may also be common in <a href="http://www.karger.com/Article/FullText/449297">fish</a> and <a href="http://www.karger.com/Article/FullText/448797">amphibians</a>, playing a role in evolution of new species and having serious implications in rapidly changing environments.</p>
<p>We suspect the reason no one has yet fully appreciated the role of sex reversal in reptiles is because much research has focused on <a href="http://www.karger.com/Article/Abstract/448361">mammals</a> and <a href="http://www.karger.com/Article/Abstract/448365">birds</a>, where sex reversal is usually caused by mutations that affect gene expression during embryonic development. This has created the false impression that sex reversal is harmful to an individual. </p>
<p>Another reason is that many reptile species have sex chromosomes that are very difficult to tell apart. That makes instances of sex reversal very difficult to spot.</p>
<p>An obvious question of deep concern is whether climate change could cause extinction by reversing the sex of entire populations. For temperature-sensitive species like the bearded dragon, crocodiles and marine turtles, is the future a warmer world without males? </p>
<p>The answer will be different for each species. Reptile survival under climate change depends on the answer to several questions. </p>
<p>Can the species control when and where they nest? How quickly are environmental conditions changing? Can the temperature at which sex reversal occurs change? </p>
<p>Each species will face a unique path as we experience an uncertain and changing environment. Some paths will undoubtedly lead to extinction, but others may utilise flexibility in sex-determination strategies to survive.</p>
<hr>
<p><em>This research was conducted at the <a href="http://www.csiro.au/en/Research/Collections/ANWC">Australian National Wildlife Collection CSIRO</a>, in partnership with the <a href="http://appliedecology.edu.au/">Institute for Applied Ecology</a> at the <a href="http://www.canberra.edu.au/">University of Canberra</a> and the <a href="https://www.usc.edu.au/">University of the Sunshine Coast</a>.</em></p><img src="https://counter.theconversation.com/content/69567/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Clare Holleley receives funding from the Australian Research Council and the Commonwealth Scientific and Industrial Research Organisation (CSIRO).</span></em></p>We are only just starting to appreciate the full sexual diversity of animals.Clare Holleley, Senior Research Scientist, Australian National Wildlife Collection, CSIROLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/656632016-09-21T18:59:51Z2016-09-21T18:59:51ZYou can thank our pre-mammalian ancestors for your sexy teeth<figure><img src="https://images.theconversation.com/files/138412/original/image-20160920-11127-lx5rs3.jpg?ixlib=rb-1.1.0&rect=0%2C49%2C650%2C406&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">This skull belongs to the carnivorous gorgonopsian therapsid Smilesaurus ferox which lived 255 million years ago</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Smilesaurus_skull.jpg">Cradle of Humankind/Flickr/Wikimedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Next time you’re getting ready for a hot date and pause to flash a toothy grin at yourself in the mirror, thank your ancestors.</p>
<p>Mammals have a dentition divided into three distinct types of teeth.</p>
<p>There are large, sharp canines and next to them incisors. Behind them, in our cheeks, are teeth known as post-canine dentition. This separation has been traced back more than 300 million years when our ancestors still looked like huge reptiles. </p>
<p>These were the pre-mammalian <a href="http://www.newworldencyclopedia.org/entry/Therapsid">therapsids</a>. They had long, sometimes sabre-like canines. Scientists long thought that these sharp teeth were deadly hunting devices. But there was a problem: even herbivorous species of therapsid had sabre-like canines. Their chompers clearly weren’t for hunting prey. Some speculated that the canines in question might be for defence from predators.</p>
<p>Or were they actually used for sexual display? </p>
<p>Today, sabre-tooth mammals such as the walrus or the deer-like <a href="http://www.iucnredlist.org/details/42190/0">muntiac</a>, have their canines constantly on display. This allows them to seduce mates or intimidate their kin. That’s the modern situation. My colleagues and I <a href="http://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0161457">wanted to know</a> whether sexual selection was also an important phenomenon among our pre-mammalian ancestors.</p>
<p>The answer, <a href="http://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0161457">uncovered</a> by cutting edge technology and careful study, is “yes”.</p>
<h2>Putting therapsids under the microscope</h2>
<p>Our research involved a team of palaeontologists from the University of the Witwatersrand’s <a href="https://www.wits.ac.za/esi/">Evolutionary Studies Unit</a>; a group from the university’s <a href="https://www.wits.ac.za/anatomicalsciences/">School of Anatomical Sciences</a> and scientists from the <a href="http://www.esrf.eu/">European Synchrotron Radiation Facility</a> in Grenoble, France.</p>
<p>Our subject was a mysterious fossil therapsid, Choerosaurus dejageri. It is part of the Eutheriodontia family. Little is known about this mammal-like reptile that lived 259 million years ago and belonged to the lineage that gave birth to mammals. Choerosaurus is unique, as it’s the only Eutheriodont to have two symmetrical bosses: horn-like structures on its upper and lower jaws, the maxilla and mandible.</p>
<p>We wanted to figure out what these cranial bosses were for: combat or sexual display.</p>
<p>Only one Choerosaurus fossil has been found, on a farm near Beaufort West in South Africa: a delicate skull. We used X-ray computerised micro-tomographic, or microCT, on this fossil. We compared the scans with those from another therapsid, the monstruous dinocephalian Moschops. The Moschops is known to have head butted its enemies, so its skull and cranial bosses were obviously developed for high energy combat. </p>
<p>But the Choereosaurus’ skull and cranial bosses were found to be too weak for such combat. In addition, the Choereosaurus’ maxillary boss was packed full of nerves and veins. This isn’t ideal for fighting, since any combat would cause a lot of pain and bleeding.</p>
<p>The maxillary boss is far more suited to supporting a colourful, sensitive cornified pad – a keratinous covering, like a horn. This suggests a bias towards display behaviour, and away from combat.</p>
<h2>Sexual selection</h2>
<p>This is the first evidence of structures dedicated solely to competition between males for mates and territory. These structures would have been used either for low energy fighting and/or sexual display in Eutheriodontia. Since this group was the direct ancestors of modern mammals, revealing their toothy secrets gives us a better understanding of our own mouths and those of other mammals.</p>
<p>The Choereosaurus fossil shows that sexual competition and the associated complex, ritualised behaviour like sexual display and ceremonies of intimidation were an important component of therapsid evolution. This finding suggests that sexual selection may have played a more important role in the origin of mammals than originally thought. </p>
<p>It’s a vital step to reshaping our understanding of humans’ deep evolutionary roots – right down to our canine teeth.</p><img src="https://counter.theconversation.com/content/65663/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Julien Benoit receives funding from PAST and its Scatterlings projects; the National Research Foundation of South Africa; and the DST-NRF Centre of Excellence in Palaeosciences (CoE in Palaeosciences). </span></em></p>Modern sabre-tooth mammals have their canines constantly on display. This allows them to seduce mates. But was sexual selection also an important phenomenon among our pre-mammalian ancestors?Julien Benoit, Postdoc in Vertebrate Palaeontology, University of the WitwatersrandLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/531482016-01-14T14:12:46Z2016-01-14T14:12:46ZWhy frills put female dinosaurs in the mood for love<figure><img src="https://images.theconversation.com/files/108161/original/image-20160114-2365-ofy2n2.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Rebecca Gelerenter/QMUL</span></span></figcaption></figure><p>Part of dinosaurs’ popularity has to be their fascinating, bizarre appearance. There’s the <em>Stegosaurus</em> with the famous row of plates down its back, the <em>Triceratops</em> with its giant frilled skull, and the “duck-billed” hadrosaurs with their peculiar and diverse array of crests.</p>
<p>Palaeontologists have been trying to decipher the function of these extravagant traits for many years. The <em>Stegosaurus</em> plates have been described as a way to regulate body temperature and it’s even been suggested that the hadrosaur crest might have been a kind of snorkel.</p>
<p>More recently, it has become increasingly popular to see these kind of features in the same way as the enlarged fins of male Siamese fighting fish or the plumes of birds of paradise. These are best explained as traits that evolved not because they improve survival but because they <a href="https://theconversation.com/five-lessons-in-seduction-from-the-males-of-the-animal-kingdom-52118">improve the mating success</a> of the bearer. My colleagues and I at Queen Mary University of London have now found what we believe to be some of <a href="http://palaeo-electronica.org/content/2016/1369-sexual-selection-in-ceratopsia">the best evidence</a> of unusual dinosaur features that were primarily used in this way.</p>
<h2>Sexual selection</h2>
<p>“<a href="http://www.nature.com/scitable/knowledge/library/sexual-selection-13255240">Sexual selection</a>” explains how animals can evolve features that may even reduce the bearer’s survival. For example, some male birds <a href="http://www.nature.com/nature/journal/v299/n5886/abs/299818a0.html">such as widowbirds</a> or pheasants have extraordinarily long tail feathers that require a lot of protein to grow and reduce the male bird’s ability to fly. Because the females of these species choose the males with the longest tails to father their chicks, those males have the highest evolutionary fitness despite being effectively handicapped by their ornaments.</p>
<p>We now know that sexual selection is the driving force behind the great majority of the extravagant, ornamental and showy traits that we find in the animal kingdom. This has led more and more <a href="http://www.nature.com/ncomms/journal/v4/n1/full/ncomms2377.html">palaeontologists to ask</a> whether sexual selection might also be behind the apparently ornamental traits that we find in many extinct species.</p>
<p>The problem is that it’s extremely difficult to tell if a particular feature of an extinct, prehistoric animal gave it an advantage in <a href="https://theconversation.com/mysterious-footprint-fossils-point-to-dancing-dinosaur-mating-ritual-52911">the mating game</a>. What’s more, there are examples of apparently pointless features of animals that have turned out to have had “normal” functional roles, such as the protruding <a href="http://bioscience.oxfordjournals.org/content/57/5/399.full">snouts of paddlefish</a> used as sensory organs. If we just decide that anything that seems a bit strange and inexplicable on a dinosaur fossil must have arisen by sexual selection, we run the risk of misinterpreting odd-looking but functional traits in these ancient animals, and palaeontologists have avoided using sexual selection as an explanation for the evolution of extravagant traits for this reason.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/108163/original/image-20160114-2345-1876wd7.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/108163/original/image-20160114-2345-1876wd7.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/108163/original/image-20160114-2345-1876wd7.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/108163/original/image-20160114-2345-1876wd7.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/108163/original/image-20160114-2345-1876wd7.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/108163/original/image-20160114-2345-1876wd7.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/108163/original/image-20160114-2345-1876wd7.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Your horns are showing.</span>
<span class="attribution"><span class="source">Daderot/Wikimedia Commons</span></span>
</figcaption>
</figure>
<p>One thing we can do to try to work out if a feature has evolved through sexual selection is study its size compared to the rest of the animal’s body. When something gets proportionally bigger as an animal gets bigger we say that it has <a href="http://www.ncbi.nlm.nih.gov/pubmed/20565262">positive allometry</a>. Although it’s by no means a universal rule, there seems to be a strong tendency for sexually selected traits, especially those that function as signals for attracting females or intimidating rivals, to be positively allometric. Positive allometry is also quite rare among traits that are not sexually selected.</p>
<p>Measuring the allometry of these extravagant traits in dinosaurs isn’t possible for many species because most extinct animals are only known from one or a few fossils. Complete specimens of dinosaurs are the exception and it’s difficult to even tell <a href="http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0126464">males from females</a>. One of the few species we have decent numbers of specimens of is the dinosaur <a href="http://www.bioone.org/doi/abs/10.1666/11-008.1"><em>Protoceratops andrewsi</em></a>. This smaller relative of the famous <em>Triceratops</em> had a skull that elongated into a large frill.</p>
<p><a href="http://palaeo-electronica.org/content/2016/1369-sexual-selection-in-ceratopsia">My colleagues and I</a> were able to put together a set of 37 <em>Protoceratops</em> fossils ranging in size from tiny juveniles up to the largest adults. By measuring the specimens’ skulls, we found that bigger <em>Protoceratops</em> had proportionally longer and wider frills – positive allometry. What’s more, larger animals seemed to have frills that spread more widely around the head whereas those of smaller animals were almost flat against the neck.</p>
<h2>Colourful animals</h2>
<p>This information strongly supports the idea that the frills of these animals, and by extension those of other related dinosaurs, were primarily a signal for other members of their species. Most likely they were associated with mating, although we can’t rule out other functions such as establishing <a href="http://beheco.oxfordjournals.org/content/20/4/844.full">social hierarchies</a> (the bigger the frill, the more important the dinosaur in the group).</p>
<p>Dinosaurs are often reconstructed as plodding, grey giants. If we start to think about them as active animals with complex social lives then perhaps we should move away from this picture. If these frills were primarily used as signals, maybe they were as colourful as similar traits found on modern birds.</p>
<p><em>Protoceratops</em> had several other unusual traits that might also have been involved in signalling and either attracting mates or establishing social position. They had some surprisingly long teeth and their tail vertebrae have long processes on the top, meaning that they were high and probably conspicuous. Considering these features, we can start to imagine these dinosaurs as social, active and brightly coloured rather than the dull plodders that we are used to thinking about.</p><img src="https://counter.theconversation.com/content/53148/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rob Knell has received funding from The Leverhulme Trust, the Wellcoem Trust and NERC for research related to that described here. </span></em></p>Researchers have found the first evidence that dinosaurs grew unusual features like crests and horns as a way of attracting a mate.Rob Knell, Senior Lecturer, Queen Mary University of LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/458362015-08-23T19:52:52Z2015-08-23T19:52:52ZChoosing children’s sex is an exercise in sexism<figure><img src="https://images.theconversation.com/files/92679/original/image-20150821-8346-14feriq.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The idea of 'family balancing' is based on the belief that children come in two genders that have essentially different traits. </span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/jasonpratt/2347428960/">Jason Pratt/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p><em><a href="http://theconversation.com/why-we-should-consider-whether-its-time-to-allow-sex-selection-in-ivf-nhmrc-46399">Click here to read</a> the National Health and Medical Research Council’s view of why it’s time to consider allowing sex selection with IVF.</em></p>
<hr>
<p>Australian guidelines for the ethical use of IVF allow <a href="https://www.nhmrc.gov.au/_files_nhmrc/publications/attachments/e78.pdf">selecting a child’s sex for medical reasons</a>. But <a href="http://consultations.nhmrc.gov.au/files/consultations/drafts/artdraftethicalguidelines150722.pdf">draft guidelines that are now open for public submissions</a> raise the possibility of extending this and allowing the choice for social reasons. </p>
<p>The draft guidelines recognise that sex selection is a controversial practice; it’s banned in several states of the United States, in Europe, New Zealand and in parts of Asia. It acknowledges that it can reinforce gender stereotyping and that legalising the selection of a child’s sex could open up the way for choosing a range of other non-disease traits. </p>
<p>But it also recognises the importance of respecting reproductive autonomy and choice, and that some parents travel abroad to access sex selection at overseas clinics. </p>
<h2>Examining choice</h2>
<p>To stimulate public discussion, the draft offers five case studies that involve issues around “family balancing”, selection to “replace” a dead child, reproductive tourism, parental autonomy, and “slippery-slope” claims. </p>
<p>These case studies provide examples of sex selection that suggest two arguments in its favour: first, that sex selection for family balancing is ethically more permissible than selection based on a strong gender preference. And second, that parents have the right to select their child’s sex based on respect for reproductive autonomy. </p>
<p>But are these arguments convincing? </p>
<p><strong>1. Family balancing</strong></p>
<p>The argument for family balancing suggests the practice is ethically permissible because it entails no harm from sexism; the concept is based on the idea that a family is “balanced” when it includes children of both sexes. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/92683/original/image-20150821-8350-jztjz5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/92683/original/image-20150821-8350-jztjz5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/92683/original/image-20150821-8350-jztjz5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/92683/original/image-20150821-8350-jztjz5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/92683/original/image-20150821-8350-jztjz5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/92683/original/image-20150821-8350-jztjz5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/92683/original/image-20150821-8350-jztjz5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The argument for family balancing suggests the practice is ethically permissible because it entails no harm from sexism.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/stwn/18550359565/in/photolist-ugerBB-hz9Egk-5JcFKv-bvW5ap-4GV9Rj-6uJvsx-6uJvdP-4FMdFr-aCkXAB-pnAG1K-2Spscy-6uJuVe-tbf1U-5aTH9F-i1524-6JxCzW-38r8Ps-78zutY-38mzwz-3Qsi6A-8ziVMi-boPnzK-s7raP1-dFpRTw-dxbCJt-dZVzL2-4FKdCc-4FPnD5-dRhpPG-dRbRgg-5QL8X2-5cbYiG-8WMqaG-r7tHfx-suzirh-pzPHwp-bXx6Fd-34hgAo-4YxfLb-mraBX-5DPXgo-dRbRba-6TTARf-5EHaQi-qwfJ8W-sPuQJo-sPvzk5-h5xvKG-4FPTS3-3QnvTd">stwn/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Such families are presented as diverse and as offering more enriching experiences to all family members. And since parents seek children of both sexes, it’s plausible there’s no gender bias of the sort that might lead to skewed sex ratios. But is family balancing really gender egalitarian? </p>
<p>In 2013, I conducted a small study as part of my doctoral studies interviewing nine Australian women who selected or wanted to select their child’s sex. All participants had several sons and desired daughters because they longed for a strong mother-child relationship and were convinced that such a bond was only possible with a girl. </p>
<p>In the interviews, girls were described as necessarily more family-oriented and more inclined to emotional connection, while boys were associated with independence and adventure. These interviews showed family balancing is based on the belief that children come in two genders that have essentially different traits. </p>
<p>To the extent that family balancing is based on the selection of children to fulfil preconceived binary gender roles, it reinforces sexism. And, by reinforcing a gender binary, it denies rather than creates diversity within families. </p>
<p><strong>2. Reproductive autonomy</strong></p>
<p>The argument for reproductive autonomy holds parents have the right to choose their child’s sex and that should be respected. Advocates argue that some parents have strong preferences for their child’s sex, which justifies their pursuit of sex selection. </p>
<p>But parental autonomy is not the only thing at stake. Other important values include the autonomy and well-being of the future child. </p>
<p>The risk of harm in sex selection stems from the fact that parents don’t desire any child, they want a child of a particular sex. And the child is presumed to develop characteristics within the limits of binary gender roles. </p>
<p>This view doesn’t take a child’s individuality into account, and can hinder the development of her attributes and skills. What if a daughter is independent and a son wants a close bond with his mother? </p>
<p>And what about children who don’t fit traditional expectations about sex, such as those with <a href="https://oii.org.au/allies/">intersex variations</a>, and gender, such as transgender and gender-diverse children? </p>
<h2>Reducing diversity</h2>
<p>The paradox of preconception sex selection is that prenatal genetic diagnosis allows for selecting sex chromosomes – but it cannot unambiguously guarantee a child with a particular gender identity, traits or behaviour. </p>
<p>While some may argue that all parents have gender expectations, those who select their child’s sex are not just making assumptions, they’re acting on their preferences and seeking to create children to fit them. </p>
<p>Children who don’t fulfil selectors’ expectations might face parental disappointment or pressure to conform to stereotypical gender roles. This can limit their freedom to develop autonomous gender identities and reduce their well-being. </p>
<p>And let’s not forget that the harm from sexism associated with sex selection also manifests on a social level. The practice sends out the message that it’s justifiable to create children for particular gender roles. And it re-affirms the idea that sex is a trait of fundamental importance in a child, one that may even be viewed as a condition for parental appreciation. </p>
<p>What’s more, legalising sex selection would be a retrograde step if we consider recent legal and policy developments regarding gender. </p>
<p>In 2013, <a href="https://theconversation.com/why-norries-court-victory-is-a-leap-forward-for-everyone-25200">the High Court ruled</a> that a person can formally identify as having an unspecified gender. The ruling effectively institutionalises a third category of non-specific gender in Australia. </p>
<p>The court’s decision signifies a step away from the dominant binary understanding of gender insofar as it acknowledges the diversity of gender identities. But any deregulation of preconception sex selection would represent a move back towards the binary understanding of sex and gender. </p>
<p><em><a href="http://theconversation.com/why-we-should-consider-whether-its-time-to-allow-sex-selection-in-ivf-nhmrc-46399">Click here to read</a> the National Health and Medical Research Council’s view of why it’s time to consider allowing sex selection with IVF.</em></p><img src="https://counter.theconversation.com/content/45836/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Tereza Hendl's PhD research was funded by the International Postgraduate Research Scholarships provided by the Australian Government. </span></em></p>The risk of harm in sex selection stems from the fact that parents don’t desire any child, they want a child of a particular sex, who is to remain within the limits of binary gender roles.Tereza Hendl, Research assistant at the Faculty of Health Sciences, University of SydneyLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/463992015-08-23T19:52:27Z2015-08-23T19:52:27ZWhy we should consider whether it’s time to allow sex selection in IVF: NHMRC<figure><img src="https://images.theconversation.com/files/92678/original/image-20150821-8374-61dbeh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Should parents be allowed to select the sex of their child through IVF when there's no compelling medical reason to do so?</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/marcus_hansson/7758937784/in/photolist-cPCzTo-5oyxeG-KKWAX-55m4QT-aKFaHn-51gVns-6TyRFP-8YWkeN-622Jk-tmB1Y-aizoR9-oFRmGv-uT8huE-7DsjCB-6TCTFW-6vWn3c-pjxzdF-qvsN4Q-6u52om-8RkMcC-6c4oU4-9yewju-5U3D2W-tmAZE-dEGUtx-5KbN5k-xnxwV-s1RsA-5BvDj7-aKhJYz-ourHzZ-fPfypm-7gB3Vt-tmARJ-ch3U2q-2EDxGa-k4Y9Dg-7vQYh-7tsiXY-gBTUGg-jiUs23-6TyRxT-n7kbU8-b732Da-dWoL9g-c7MTZQ-j3dweT-9RmQvK-vUj8L-pRFZyx">Marcus Hansson/Fkickr</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p><em>Read <a href="http://theconversation.com/choosing-childrens-sex-is-an-exercise-in-sexism-45836">an argument against allowing sex selection</a> for IVF.</em></p>
<hr>
<p>As part of a periodic review, the National Health and Medical Research Council (NHMRC) is <a href="http://consultations.nhmrc.gov.au/public_consultations/assisted-reproductive-tech">calling for public submissions</a> on its <a href="http://consultations.nhmrc.gov.au/files/consultations/drafts/artdraftethicalguidelines150722.pdf">draft guidelines on the use of assisted reproductive technology in clinical practice and research</a>. This, in lay terms, is the practice of in vitro fertilisation (IVF).</p>
<p>Producing guidelines to advise the community on ethical issues relating to health is one of the NHMRC’s many tasks. Revisions aim to reflect changes in technology and social attitudes and are based on advice from the Australian Health Ethics Committee (AHEC), which I chair.</p>
<p>The suggested change to the ethical guidelines that’s getting most attention is the question of whether parents should be able to select the sex of their child when there’s no compelling medical reason for doing so.</p>
<h2>A changing world</h2>
<p>When the NHMRC first introduced the ethical guidelines for assisted reproductive technology in 1996, the technology was new and community attitudes were still developing. Not only has technology changed considerably since, IVF is now routine practice, and the public is much more aware of related issues.</p>
<p>Since the last revision of the guidelines in 2007, for instance, technology has allowed for eggs to be frozen and defrosted for use, as sperm and embryos could already be. This has raised the possibility of having egg banks, akin to sperm banks, and the draft guidelines are also seeking comment on whether women should be compensated for the invasive procedure of donating their eggs.</p>
<p>The <a href="http://consultations.nhmrc.gov.au/public_consultations/assisted_reproductive">initial public consultation</a> on existing guidelines in 2014, which sought, among other things, to identify gaps in the guidance, received mixed comments relating to sex selection. Some supported allowing sex selection for non-medical reasons while others wished to maintain the current guidance. </p>
<p>The committee is aware that some Australians are pursuing sex selection in overseas clinics. And because not all international clinics have the same standard of care that exists in Australia, this could be risky for both the woman and her child.</p>
<p>Although fertility and reproduction are often seen as personal matters, they have wider implications. Many issues around IVF have social and political implications, such as relationships within families where parents aren’t the biological parents and priorities for health-care resources, in addition to ethical considerations.</p>
<h2>A slippery slope?</h2>
<p>It’s important to note that no decision about allowing sex selection in IVF has been made yet. <a href="https://consultations.nhmrc.gov.au/home">We’re seeking community input</a>, which we’ll take into account, along with ethical and practical considerations, before any changes are made.</p>
<p>This is not a simple all-or-nothing decision, as selecting the sex of your child is already allowed for certain sex-specific medical conditions. It’s allowed for haemophilia, for instance, where the son of an unaffected woman has a 50% chance of inheriting the illness (it’s very rare for girls to inherit haemophilia).</p>
<p>It’s now a matter of determining whether non-medical reasons for selecting the sex of your child are considered acceptable and, if so, where the line should be drawn. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/92682/original/image-20150821-8344-hbwnwv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/92682/original/image-20150821-8344-hbwnwv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/92682/original/image-20150821-8344-hbwnwv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/92682/original/image-20150821-8344-hbwnwv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/92682/original/image-20150821-8344-hbwnwv.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/92682/original/image-20150821-8344-hbwnwv.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/92682/original/image-20150821-8344-hbwnwv.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">One common reason for wanting to choose the sex of a child is ‘family balancing’.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/plushoff/2575864684/">Jacki Gallagher/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>One argument against allowing sex selection for non-medical reasons is that it may be the thin edge of the wedge towards being able to select for other characteristics, such as height or intelligence. </p>
<p>Aside from such choices not yet being medically possible, the slippery slope argument may falter because there’s no natural progression between approving non-medical sex selection and approving being able to select other characteristics. Sex selection is a discrete choice around which a definite boundary can be drawn.</p>
<h2>Food for thought</h2>
<p>The committee has provided case studies to help the community explore the complexities of the issue. One common reason for wanting to choose the sex of a child, for instance, is “family balancing”. Parents who have several children of the same sex may want to choose the opposite sex for their next child. </p>
<p>Although this is often presented as a choice of parents, the child’s interests also have to be considered. Will they meet their parent’s gender expectations, and should they have to? Similar questions arise when parents seek to “replace” children who have died.</p>
<p>Other case studies explore whether cultural reasons for wanting to select the sex of a child are appropriate. And whether we should allow medical reasons for sex selection where a disease is not sex-specific but there’s a slight prevalence imbalance between the sexes. Couples who have an autistic child, for instance, somewhat reduce their chances of having a second autistic offspring by having a girl because the condition is less common in that sex. </p>
<p>Another case explores whether there could be adverse psychological consequences for a child who is told that her parents selected her sex. Australia has quite strict requirements about allowing donor-conceived children to discover who their unknown parent is, but countries that allow non-medical sex selection don’t necessarily have similar regulations.</p>
<p>And if sex selection is eventually allowed, where should we draw the boundary? In Israel, for instance, couples can apply to the government to be considered for sex selection after having four children of the same sex. Would two or three kids of the same sex be thought enough in Australia?</p>
<h2>International norms</h2>
<p>Although IVF providers cannot practise without national accreditation, which depends on their agreement to follow NHMRC guidelines, state and territory-based legislation also governs what they’re allowed to do. </p>
<p>Sex selection for non-medical reasons, such as gender balancing, is prohibited by law in New South Wales, Victoria, South Australia and Western Australia. And it would still be disallowed if the guidelines were changed, unless local laws are amended. </p>
<p>Although Australians travel to countries where sex selection is allowed, Australia’s current position is by no means out of step internationally. Sex selection for non-medical purposes is illegal in four states of America, in Europe, New Zealand and in parts of Asia. </p>
<p>Finally, there’s the question of whether sex selection for non-medical purposes is a good use of medical resources. With all the other demands on the medical system to treat disease and restore wellness, we need to decide as a community what priority should be given to funding the ability of couples to choose the sex of their children when there’s no compelling medical reason to do so.</p>
<p><em>Read <a href="http://theconversation.com/choosing-childrens-sex-is-an-exercise-in-sexism-45836">an argument against allowing sex selection</a> for IVF.</em></p><img src="https://counter.theconversation.com/content/46399/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ian Olver does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>The National Health and Medical Research Council call for public submissions on whether sex selection should be allowed without a medical reason recognises changing social attitudes.Ian Olver, Director, Sansom Institute for Health Research; Chair of Translational Cancer Research, University of South AustraliaLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/420122015-05-19T05:12:10Z2015-05-19T05:12:10ZWhy men are not biologically useless after all …<figure><img src="https://images.theconversation.com/files/82069/original/image-20150518-25444-bnqnmp.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Told you so!</span> <span class="attribution"><a class="source" href="https://www.flickr.com/photos/shehal/2259452943/in/photolist-"> Photo of Charles Darwin uploaded by Shehal Joseph/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Does the world really need men? It <a href="http://www.nytimes.com/2012/08/25/opinion/men-who-needs-them.html?_r=0">has been suggested</a> that, in the age of cloning – and with enough sperm banks around to populate several future generations – the question is legitimate. However, <a href="http://nature.com/articles/doi:10.1038/nature14419">new research</a> suggests that the reason that we need two sexes is because it improves the overall genetic quality of a species and reduces the risk of population extinction. </p>
<p>The question of why sex is so widespread across nature has intrigued and puzzled scientists for a very long time. From a biological perspective, the purpose of life is to pass on your genes to the next generation. Asexual organisms, such as bacteria, do this <a href="http://www.biomedcentral.com/1471-2164/11/588">simply by duplicating</a> themselves. The offspring is an identical copy of the parent, which passes on all its genes. </p>
<p>Sexual organisms, however, need a partner to reproduce. Sexual species have two sexes, but only one of these can bear young. This means that sexual populations can only grow half as fast as asexual populations. Sex is also inefficient for the reason that you need to find a mate, which takes time and energy. Overall, life would be a lot simpler if we could just split in two to reproduce. </p>
<p>So why did sex evolve? The explanation may lie in mutations. Mutations are typos that occur as DNA is copied. The result is that we all have new variants of DNA that our parents don’t have. Most mutations have no effect, but some can be useful, and help an organism to survive. Other mutations, however, result in a loss of function in the gene they affect. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/82086/original/image-20150518-25422-hym6x6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/82086/original/image-20150518-25422-hym6x6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/82086/original/image-20150518-25422-hym6x6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/82086/original/image-20150518-25422-hym6x6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/82086/original/image-20150518-25422-hym6x6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/82086/original/image-20150518-25422-hym6x6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/82086/original/image-20150518-25422-hym6x6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Need to get in the mood? Just think of all the possible mutations.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/franciscojgonzalez/16170665796/in/photolist-22rMwT-9EpGD2-9FGf8L-bA7Uq2-7KZrSc-ceRMKW-dqUnGC-a4473f-qCWTyG-fDdcNZ-6VaL4y-a41evn-nrzezW-nvvz6f-nefzf2-nvuRqv-nGKQd2-nxT84C-nxirsC-4DRhRh-nxyAJT-ne4RDC-nf8ywp-nxzqCb-nfP4hf-ne69Aj-nuW5Gt-yJGMn-nEGLKs-nxk4xH-nGY6jS">Francisco Gonzalez/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>This leads to a possible explanation for why sex is useful: having two sets of genes – one from each parent – allows for the possibility of inheriting new, beneficial forms of a gene, while protecting the offspring from dysfunctional genes, because if one copy of a gene is damaged, the other copy can kick in and mask the damaging effect. </p>
<p>Natural selection removes badly damaged mutations from the gene pool very quickly. However, those with smaller effects, or those that can hide behind a functional version inherited from the other parent, can persist. In fact, the average human family carries hundreds of mutations that cause loss of function. </p>
<p>Some individuals in sexually reproducing species are better at finding mates than others, either because they compete successfully, or because they are more attractive to the opposite sex. This process, called “<a href="http://darwin200.christs.cam.ac.uk/pages/index.php?page_id=d4">sexual selection</a>”, was first identified by Charles Darwin. </p>
<p>Most research in this area concentrates on the implications of this selection for individuals, but if sexual selection is strong it might also affect the overall genetic quality of populations. In other words, sexual selection might provide the answer to why we have sex in the first place. If only “high-quality” individuals – those without harmful mutations – get to pass their genes on to the next generation, then the genetic benefits derived from sexual selection offset the costs of sex. </p>
<h2>A 10-year breeding test</h2>
<p>But that was all theory. How can we actually test this idea? The answer lies in experimental evolution. Short-term experiments have provided confusing results, but a real test takes years. Humans have experimented with evolution, selectively breeding from chosen individuals and not others, ever since we began to domesticate other species. Similar selective breeding of small species that can easily be kept in the lab, and that have rapid generation times, allow us to observe evolution in progress, and to measure its effect. </p>
<p>An international team of researchers, led by the University of East Anglia, bred two lines of flour beetles – a common pantry pest and a popular laboratory organism. One line was made up of 90 beetles of one sex, and 10 of the other in each generation. These are the conditions of strong sexual selection, under which many members of the abundant sex will fail to reproduce. The other line underwent weak sexual selection, with one female to five males, or no sexual selection at all, with one male for each female. Under weak or no sexual selection, most individuals will pass on their genes to the next generation. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/82081/original/image-20150518-25407-1nft6cu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/82081/original/image-20150518-25407-1nft6cu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/82081/original/image-20150518-25407-1nft6cu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/82081/original/image-20150518-25407-1nft6cu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/82081/original/image-20150518-25407-1nft6cu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/82081/original/image-20150518-25407-1nft6cu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/82081/original/image-20150518-25407-1nft6cu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Such effort - I wish I could just duplicate.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/bareego/5754122729/in/photolist-9Ltodz-P8cbe-5eTsFD-4NhwFV-eigdgq-eiatBg-eiatvV-ck1ehG-nb9Dqi-eigdbS-knJ3HY-dkghem-bWPVKJ-eiatrP-eiattT-eiatoM-fANgPy-ah3ZJg-cCb3f9-bX94um-bX94QQ-eBoBhG-eBkrmx-dhWCnx-dMGuWS-ozhS3C-3zNfV-oPKnxG-8QtoCm-dAiGE-6Fgjww-6FgjyS-6Fcbsx-6FcbpF-6FgjvG-6ZphR5-6mQVuR-eUoXi8-oQ7Gj-oQ7CS-ozR98g-HyvgE-dAiGG-dtyp3o-hqbZr6-grJMmb-6jN9La-dAuNFB-6GGtvH-cDVCBm">James Niland/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>After seven years of evolution, the researchers matched brothers and sisters with one another for a further three years. Such inbreeding <a href="http://onlinelibrary.wiley.com/doi/10.1002/ajmg.a.10020/abstract;jsessionid=AED50D1B8C91862AC5ACE25B4205ECE7.f02t01">quickly reveals harmful mutations</a>, because siblings share 50% of their genes. If they both pass a mutation on to offspring, then it is no longer masked, and the offspring is unlikely to survive. This means we can use how long a family survives as a measure of the overall genetic quality of the two lines.</p>
<p>None of the 108 families from populations that had evolved under weak sexual selection tested survived beyond the 10th generation of inbreeding. However, 8 of the 108 families from the strong sexual selection histories were still going strong after 20 generations of inbreeding.</p>
<p>Perhaps we now have an answer to the question of “why sex”? Because sexual selection – and partner choice – improves the overall genetic quality of a species and reduces the risk of population extinction.</p><img src="https://counter.theconversation.com/content/42012/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jo Setchell does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>Sperm banks and human cloning may not be the future. A study in beetles reveals that having two sexes boosts genetic quality.Jo Setchell, Reader in the Department of Anthropology, Durham UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/290452014-07-20T20:26:20Z2014-07-20T20:26:20ZStrut your stuff: how rockstars and peacocks attract the ladies<p>What is it that makes rockstars so attractive to the opposite sex? Turns out Charles Darwin had it pegged hundreds of years ago – and it has a lot to do with peacocks.</p>
<p>In The Descent of Man, and Selection in Relation to Sex (1871), <a href="http://www.fulltextarchive.com/pdfs/The-Descent-of-Man-and-Selection-in-Relation.pdf">Darwin suggested</a> natural selection can occur by individuals out-producing others in a population, through enhanced ability to secure a mate. </p>
<p>Darwin used the ornate peacock plumage and bird songs to exemplify this concept. </p>
<p>Peacocks are best known for flaunting their brightly coloured and sizable tails. Yet the tail is cumbersome and expends energy. </p>
<p>In addition, its extravagance makes the peacock conspicuous to predators and less able to escape them, reducing its survival prospects. Why then has the tail not been <a href="http://www.adeline-loyau.net/publications/Loyau_etal_AnimBehav2008.pdf">bred out of existence</a>?</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/Q7FKViW9mpw?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Looks great, but hardly seems practical.</span></figcaption>
</figure>
<p>The existence of the tail reflects the fact that it plays a major role in attracting the peahen, allowing greater numbers of offspring that inherit the genes for a long colourful tail from their father.</p>
<p>Not only does the peacock enjoy <a href="http://jeb.biologists.org/content/216/16/3035.full">greater reproductive success</a>, his sons are likely to inherit a similar capacity for increased reproductive success.</p>
<h2>Shake your tail feathers</h2>
<figure class="align-right ">
<img alt="" src="https://images.theconversation.com/files/53520/original/jvd733mn-1404973552.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/53520/original/jvd733mn-1404973552.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=901&fit=crop&dpr=1 600w, https://images.theconversation.com/files/53520/original/jvd733mn-1404973552.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=901&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/53520/original/jvd733mn-1404973552.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=901&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/53520/original/jvd733mn-1404973552.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1132&fit=crop&dpr=1 754w, https://images.theconversation.com/files/53520/original/jvd733mn-1404973552.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1132&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/53520/original/jvd733mn-1404973552.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1132&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The way peacocks compete is called intersexual selection.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/photoverulam/138380352/in/photolist-enPNMd-4gZ1RS-aNREb6-dqBQM4-gNTxM5-6WTFe2-4HERje-69pWXX-ERfzv-eovQQG-gNTuuU-dF7zhy-bmZ6iy-LCDPw-6Pyn-cVaM1-6AtnwH-7xdT5Y-deeE9-76xyoC-D9uuL-nvFVrp-PEAxA-eaqv7v-cM3Ynu-nfzvm9-4ZxyH1-3aJiN-cbzDdj-55TdHP-cLuxSN-4w9vq2-8Anp85-gR3jA-nABRxM-bVVgzd-4SMedU-4D3NGj-ghLm6H-37dURK-zUejp-efFig8-e8DofA-zRbLr-hmpTmi-7GnRo2-fbYbXj-akCno-5Kmcqy-8ura1L">Richard Gillin/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Sexual selection in humans, as in peacocks, is predominantly at the discretion of females. </p>
<p>The prevailing explanation behind this is the <a href="http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3096780/">Bateman Principle</a> – male reproductive success increases with the number of mates, whereas female reproductive success does not. </p>
<p>Thus males compete with each other for female mates and females mate only with the males they prefer. This mechanism of sexual selection is termed intersexual selection. </p>
<p>The tail of a peacock is an example of intersexual selection. Conflict and physical violence between male members of a species is called <a href="http://www.nature.com/scitable/knowledge/library/sexual-selection-13255240">intrasexual selection</a>.</p>
<h2>Voulez-vous coucher avec moi?</h2>
<p>Studies conducted in the <a href="http://www.elainehatfield.com/79.pdf">United States</a> and <a href="http://www.ncbi.nlm.nih.gov/pubmed/21369773">France</a>, where male and female confederates approached members of the opposite sex and requested “Will you come over to my apartment?” and “Would you go to bed with me?”, found that the majority of male respondents were willing to comply with a sexual proposition from an averagely attractive female. </p>
<p>But zero of the female respondents in the American study and only one of the female respondents in the French study were willing to go to bed with a male they had just met. </p>
<p>The authors interpret the findings as evidence of men’s eagerness for sexual relationships, and women’s association of higher risk with having a sexual liaison.</p>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/53519/original/7rw3zxnb-1404972856.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/53519/original/7rw3zxnb-1404972856.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=840&fit=crop&dpr=1 600w, https://images.theconversation.com/files/53519/original/7rw3zxnb-1404972856.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=840&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/53519/original/7rw3zxnb-1404972856.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=840&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/53519/original/7rw3zxnb-1404972856.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1055&fit=crop&dpr=1 754w, https://images.theconversation.com/files/53519/original/7rw3zxnb-1404972856.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1055&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/53519/original/7rw3zxnb-1404972856.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1055&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Carrying a guitar can work wonders.</span>
<span class="attribution"><a class="source" href="https://www.flickr.com/photos/two-wrongs/2077694966/in/photolist-4aAJk1-aKJMy-9uSE5F-cUhciS-4NHLZ5-9Kj6wp-f1tZAB-cUhnmU-cUh9oh-7kL9WH-cUhhsG-cUhbof-cUgy6h-dhCaXC-cUhfrh-NtS8D-P26Gg-5usjkg-9aLvHF-bty2xm-5PG1uK-9EjmgJ-yUovG-cUgVcA-7v4U1g-FYTiD-cUgyMW-cUgvcE-iQxAfU-cUh9Tm-cUgSv3-gSgLH-giSufa-eRZ3t8-7dHjbo-bdzQ9Z-bBMEZb-3fWLGd-eRZ2hK-bjHZpz-cUgW93-g7frED-7bpBHY-5WVuzD-5PLio3-5PG3Dn-dKGu9g-8JGB3-agmqqJ-cUgDPE">Vicki & Chuck Rogers/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc-sa/4.0/">CC BY-NC-SA</a></span>
</figcaption>
</figure>
<p>But it seems men can increase their chances by holding a guitar.</p>
<p>A recently published <a href="http://pom.sagepub.com/content/42/4/545.abstract">study</a> had male confederates approach women in the street holding a guitar, a sports bag or nothing, and ask for their phone number. </p>
<p>Significantly greater compliance was elicited by men merely holding a guitar.</p>
<p>Additionally, when asked to rate men as potential partners for a short-term relationship, women at peak fertility <a href="http://www.sscnet.ucla.edu/comm/haselton/papers/downloads/Haselton_Miller_2006_Creative_intelligence_and_fertility.pdf">preferred creativity</a> (including musical creativity) over wealth in prospective partners. </p>
<p>A 2014 British <a href="http://rspb.royalsocietypublishing.org/content/281/1784/20140403.full">study</a> of 1,500 women with an average age of 28 replicated this finding, showing that women have sexual preferences for composers of complex music during peak conception times, but not outside this time.</p>
<p>Finally, of interest, studies suggest that higher levels of <a href="http://psp.sagepub.com/content/39/7/870.abstract">narcissism</a> in men correlated with improved prospects in courting a woman.</p>
<h2>Swapping the tail for a guitar</h2>
<p>Harvard cognitive psychologist <a href="http://www.nytimes.com/2011/11/29/science/pinkerisms.html?_r=1&">Steven Pinker</a> has referred to music as: “auditory cheesecake, an exquisite confection crafted to tickle […] our mental faculties”.</p>
<p><a href="http://www.nature.com/neuro/journal/v14/n2/abs/nn.2726.html">Evidence</a> suggests that this is the case, with music arousing feelings of euphoria and craving through dopamine release in the striatal system.</p>
<p><a href="http://rspb.royalsocietypublishing.org/content/281/1784/20140403.full">Researchers</a> suggest that men who can play music display specific adaptive qualities, demonstrating excellent physical coordination and learning capacity.</p>
<p>Akin to a peacock ostentatiously fanning its brilliant plumage or a songbird vocalising a pleasant harmony, a strutting male rockstar generates an aesthetically and aurally pleasing performance. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/9irsg1vBmq0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Jimi Hendrix performing in 1969.</span></figcaption>
</figure>
<p>Rather than demonstrating his capacity for survival, he is producing something that is mentally gratifying to others, and appealing to the opposite sex. </p>
<p><a href="http://www.huffingtonpost.com/2012/07/11/mick-jagger-sex-life-4000-women_n_1666176.html">Mick Jagger</a> and <a href="http://stringvisions.ovationpress.com/2011/08/daily-bow-musicians-attractive-mates/">Jimi Hendrix</a> were known for their musical ability, narcissism and sexual escapades. </p>
<p>Before the advent of birth control, these men would have fathered many offspring. Their genes would have multiplied in frequency through the power of attracting members of the opposite sex. </p>
<p>Charles Darwin’s words from 1871 appear to ring true for male rockstars: </p>
<blockquote>
<p>musical notes and rhythm were first acquired by the male or female progenitors of mankind for the sake of charming the opposite sex.</p>
</blockquote><img src="https://counter.theconversation.com/content/29045/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Malcolm Forbes receives funding from the National Health and Medical Research Council. </span></em></p><p class="fine-print"><em><span>Ryan Anderson does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>What is it that makes rockstars so attractive to the opposite sex? Turns out Charles Darwin had it pegged hundreds of years ago – and it has a lot to do with peacocks. In The Descent of Man, and Selection…Malcolm Forbes, Medical Registrar, Townsville Hospital and Health Service and Adjunct Lecturer, James Cook UniversityRyan Anderson, PhD candidate, James Cook UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/150792013-06-13T03:44:07Z2013-06-13T03:44:07ZSquid or swallow: the sexual tastes of a cephalopod<figure><img src="https://images.theconversation.com/files/25456/original/tjyqtmsn-1371088012.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The sexual activity of the southern bottletail squid involves choosy females eating losers' ejaculate.</span> <span class="attribution"><span class="source">Saspotato</span></span></figcaption></figure><p>In romantic circles, reproduction is viewed as a harmonious venture between the sexes. After all, if you aim to produce the best offspring possible, wouldn’t it also be best to cooperate with your partner?</p>
<p>In recent decades, however, evolutionary biologists have revealed this harmonious ideal could not be <a href="http://en.wikipedia.org/wiki/Sexual_conflict">further from the truth</a>. And, contrary to what you may think, males don’t always have the upper hand when it comes to sexual conflict.</p>
<p>In a paper <a href="http://rsbl.royalsocietypublishing.org/content/9/4/20130192">published in Biology Letters</a> last week, my colleagues and I showed female southern bottletail squid dictate which lucky male’s sperm they use to fertilise their eggs - and which unlucky sperm they eat instead.</p>
<h2>Can’t we all just get along?</h2>
<p>What is now clear is that fierce battles between the sexes are being fought almost everywhere you look in sexually-reproducing species.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/25455/original/dqpgpybb-1371087846.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/25455/original/dqpgpybb-1371087846.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/25455/original/dqpgpybb-1371087846.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/25455/original/dqpgpybb-1371087846.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/25455/original/dqpgpybb-1371087846.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/25455/original/dqpgpybb-1371087846.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/25455/original/dqpgpybb-1371087846.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/25455/original/dqpgpybb-1371087846.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=501&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">JD Hancock</span></span>
</figcaption>
</figure>
<p>The reasons underlying this “sexual conflict” are due to the different ways in which males and females maximise their reproductive success. </p>
<p>For males, reproduction is typically only limited by the number of mates he can find. Sperm is cheap to produce and they are plentiful, so males are expected to try to mate with as many females as possible.</p>
<p>For females, it’s more a case of quality rather than quantity. Compared to sperm, eggs are much more expensive to produce and there are fewer of them available to be fertilised so females are expected to maximise their reproductive success by mating with high quality suitors.</p>
<p>These differences in ideal mating strategies and reproductive costs are the key to sexual conflict.</p>
<p>With a mismatch as to what is an “optimal reproductive effort”, both males and females attempt to pay the lowest price for reproduction, while still getting the biggest “bang for their buck”.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/25460/original/gvpyhhvn-1371090401.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/25460/original/gvpyhhvn-1371090401.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/25460/original/gvpyhhvn-1371090401.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=521&fit=crop&dpr=1 600w, https://images.theconversation.com/files/25460/original/gvpyhhvn-1371090401.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=521&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/25460/original/gvpyhhvn-1371090401.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=521&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/25460/original/gvpyhhvn-1371090401.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=654&fit=crop&dpr=1 754w, https://images.theconversation.com/files/25460/original/gvpyhhvn-1371090401.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=654&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/25460/original/gvpyhhvn-1371090401.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=654&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Ouch - a seed beetle’s penis.</span>
<span class="attribution"><span class="source">Wikimedia Commons</span></span>
</figcaption>
</figure>
<p>Several studies have shown how such sexual conflict has resulted in some quite bizarre, and at times brutal, reproductive strategies.</p>
<p>Seed beetles, for example, have evolved almost <a href="http://www.wired.co.uk/news/archive/2012-10/26/beetle-penis-spikes">barbaric looking penises</a>, aimed specifically at damaging the internal reproductive tract of the female. Some male flies have even <a href="http://www.sciencedirect.com/science/article/pii/S0965174896000847?np=y">evolved ejaculates</a> that not only stimulate the female to lay eggs, but also decrease her life span.</p>
<p>In such instances, males have evolved reproductive strategies aimed specifically at harming the female. By making the act of copulation even more costly for females, males lessen the likelihood of her mating again, thereby increasing his own chances of fertilising her eggs.</p>
<p>But are the scales of sexual conflict always tipped so heavily in the male’s favour?</p>
<h2>Ejaculate on the menu</h2>
<p>In the southern bottletail squid (<em><a href="http://australianmuseum.net.au/Southern-Bottletail-Squid-Sepiadarium-austrinum-Berry-1921">Sepiadarium austrinum</a></em>) females appear to engage in some reproductive manipulation of their own.</p>
<p>During copulation, males pass numerous sperm packages (<a href="http://en.wikipedia.org/wiki/Spermatophore">spermatophores</a>) to the female. These evert to form small, balloon-like structures (<a href="http://www.merriam-webster.com/dictionary/spermatangium">spermatangia</a>) that glue to the membrane around her mouth on an area known as the buccal cavity. Here, they are stored until she is ready to fertilise her eggs.</p>
<p>Importantly, the male’s sperm do not enter the reproductive tract, instead remaining housed inside their balloon-like casings. Although these spermatangia do offer some protection, the external nature of the buccal cavity means females are only able to store them for about three weeks before they, and the numerous sperm within, are lost.</p>
<p>When she is ready to produce a clutch, she extracts an egg from her body with her arms, passes it across the stored sperm bulbs, and then places it on seaweed or in crevices on the sea floor. </p>
<p>But recently it has been found these females will also eat the male’s spermatangia after copulation. Furthermore, she uses the nutrients from this behaviour for both tissue growth and the development of her unfertilised eggs. You can see this behaviour in the video below.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/eFdt0aKuKGk?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>As sperm storage is only short-term, if females do not lay eggs soon after mating, males could completely miss out on fertilising her eggs, and instead act as a contributor for the next male that comes along.</p>
<p>But are males entirely at the mercy of the females’ culinary whims?</p>
<h2>Not all squid are equal</h2>
<p>It appears that this exposure to ejaculate consumption has driven males to develop some counter-strategies of their own.</p>
<p>When assessing a female as a potential partner, males show a clear preference for larger females, often refusing to mate with sexually mature females that are too small. When we examined this closer, we actually found smaller females are the ones that will eat the most of the male’s ejaculate.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/25457/original/h9wqq5rm-1371089404.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/25457/original/h9wqq5rm-1371089404.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/25457/original/h9wqq5rm-1371089404.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/25457/original/h9wqq5rm-1371089404.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/25457/original/h9wqq5rm-1371089404.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/25457/original/h9wqq5rm-1371089404.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/25457/original/h9wqq5rm-1371089404.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/25457/original/h9wqq5rm-1371089404.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">Ben Clifford</span></span>
</figcaption>
</figure>
<p>So males appear to be using female size as a way to minimise their exposure to ejaculate consumption.</p>
<p>By looking closely at this inconspicuous squid, we have uncovered a range of questions about cephalopod reproductive strategies. </p>
<p>Do females view males as a possible food source? Are they even assessing the quality of their mate, saving the sperm of the best males for egg fertilisation whilst eating the rest? Is it possible that males have even evolved manipulative strategies of their own to ensure their sperm is used in offspring production?</p>
<p>Regardless of what is around the corner, this research highlights how the evolutionary implications of a single adaptation can dramatically affect the life history of a species. </p>
<p>Such studies not only give us insight into the mysterious world of cephalopods, but also provide an understanding as to how the costs of reproduction have worked to shape the evolution of all sexually-reproducing species.</p><img src="https://counter.theconversation.com/content/15079/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ben Wegener received funding from Monash University, the Victorian Marine Science Consortium, the Linnean Society of New South Wales, the Ecological Society of Australia and the Holsworth Wildlife Research Endowment.</span></em></p>In romantic circles, reproduction is viewed as a harmonious venture between the sexes. After all, if you aim to produce the best offspring possible, wouldn’t it also be best to cooperate with your partner…Ben Wegener, PhD candidate, Monash UniversityLicensed as Creative Commons – attribution, no derivatives.