tag:theconversation.com,2011:/us/topics/sexual-dimorphism-36914/articlesSexual dimorphism – The Conversation2022-02-01T13:14:21Ztag: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>
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<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>
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<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>
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<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>
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<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>
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<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>
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<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">
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<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>
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<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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<strong>
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/1580052021-04-22T12:25:25Z2021-04-22T12:25:25ZYou don’t have a male or female brain – the more brains scientists study, the weaker the evidence for sex differences<figure><img src="https://images.theconversation.com/files/395082/original/file-20210414-21-x0wfsg.jpg?ixlib=rb-1.1.0&rect=0%2C16%2C5654%2C3732&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Brain sex isn't a thing.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/human-brain-royalty-free-image/520456984">Sunny/Stone via Getty Images</a></span></figcaption></figure><p><a href="https://doi.org/10.1371/journal.pone.0110830">Everyone knows the difference between male and female brains.</a> One is chatty and a little nervous, but never forgets and takes good care of others. The other is calmer, albeit more impulsive, but can tune out gossip to get the job done. </p>
<p>These are stereotypes, of course, but they <a href="https://doi.org/10.1007/s12152-012-9169-1">hold surprising sway</a> over the way actual brain science is designed and interpreted. Since the <a href="https://en.wikipedia.org/wiki/Magnetic_resonance_imaging_of_the_brain">dawn of MRI</a>, neuroscientists have <a href="https://www.theguardian.com/books/2019/mar/05/the-gendered-brain-gina-rippon-review">worked ceaselessly</a> to find differences between men’s and women’s brains. This research attracts lots of attention because it’s just so easy to try to link any particular brain finding to some gender difference in behavior.</p>
<p>But as <a href="https://scholar.google.com/citations?user=tHcqut8AAAAJ&hl=en&oi=ao">a neuroscientist long experienced in the field</a>, I recently completed a painstaking <a href="https://doi.org/10.1016/j.neubiorev.2021.02.026">analysis of 30 years of research on human brain sex differences</a>. And what I found, with the help of excellent collaborators, is that virtually none of these claims has proven reliable. </p>
<p>Except for the simple difference in size, there are no meaningful differences between men’s and women’s brain structure or activity that hold up across diverse populations. Nor do any of the alleged brain differences actually explain the <a href="https://doi.org/10.1037/a0038208">familiar but modest differences</a> in personality and abilities between men and women.</p>
<h2>More alike than not</h2>
<p>My colleagues and I titled our study “Dump the Dimorphism” to debunk the idea that human brains are “sexually dimorphic.” That’s a very science-y term biologists use to describe a structure that comes in two distinct forms in males and females, such as antlers on deer or the genitalia of men and women. </p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/395097/original/file-20210414-19-m4hmqf.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A brightly colored male zebra finch is perched next to a more drab colored female." src="https://images.theconversation.com/files/395097/original/file-20210414-19-m4hmqf.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/395097/original/file-20210414-19-m4hmqf.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=480&fit=crop&dpr=1 600w, https://images.theconversation.com/files/395097/original/file-20210414-19-m4hmqf.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=480&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/395097/original/file-20210414-19-m4hmqf.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=480&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/395097/original/file-20210414-19-m4hmqf.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=603&fit=crop&dpr=1 754w, https://images.theconversation.com/files/395097/original/file-20210414-19-m4hmqf.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=603&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/395097/original/file-20210414-19-m4hmqf.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=603&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">A pair of wild zebra finches (<em>Taeniopygia guttata</em>) perch in South Australia. The male is in the foreground, the female behind.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/zebra-finch-pair-australia-royalty-free-image/584502287">Whitworth Images/Moment via Getty Images</a></span>
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<p>When it comes to the brain, some animals do indeed exhibit sexual dimorphism, such as certain birds whose brains contain a <a href="https://doi.org/10.1126/science.959852">song-control nucleus that is six times larger</a> in males and is responsible for male-only courtship singing. But as we demonstrate in our exhaustive survey, nothing in human brains comes remotely close to this. </p>
<p>Yes, men’s overall brain size is about 11% bigger than women’s, but unlike some songbirds, no specific brain areas are disproportionately larger in men or women. Brain size is proportional to body size, and the brain difference between sexes is actually smaller than other internal organs, such as the heart, lungs and kidneys, which <a href="https://doi.org/10.1016/s0379-0738(00)00401-1">range from 17% to 25% larger in men</a>.</p>
<p>When overall size is properly controlled, no individual brain region varies by more than <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2782778/">about 1%</a> between men and women, and even these tiny differences are <a href="https://doi.org/10.1016/j.neubiorev.2021.02.026">not found consistently across geographically or ethnically diverse populations</a>.</p>
<p>Other highly touted brain sex differences are also a product of size, not sex. These include the ratio of gray matter to white matter and the ratio of connections between, versus within, the two hemispheres of the brain. Both of these ratios are larger in people with smaller brains, <a href="https://doi.org/10.1016/j.neuroimage.2017.04.029">whether male or female</a>.</p>
<p>What’s more, <a href="https://doi.org/10.1080/1357650X.2018.1497044">recent research has utterly rejected</a> the idea that the tiny difference in connectivity between left and right hemispheres actually explains any behavioral difference between men and women. </p>
<h2>A zombie concept</h2>
<p>Still, “sexual dimorphism” won’t die. It’s a zombie concept, with the latest revival using artificial intelligence to predict whether a given brain scan comes from a man or woman. </p>
<p>Computers can do this with 80% to 90% accuracy except, once again, this accuracy falls to 60% (or not much better than a coin flip) when you <a href="https://theconversation.com/are-there-male-and-female-brains-computers-can-see-a-distinction-but-they-rely-strongly-on-differences-in-head-size-143972">properly control for head size</a>. More troublesome is that these algorithms don’t translate across populations, such as <a href="https://doi.org/10.3389/fnhum.2018.00399">European versus Chinese</a>. Such inconsistency shows there are no universal features that discriminate male and female brains in humans – unlike those deer antlers. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/395103/original/file-20210414-21-pu5nwu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Brain MRI images showing multiple views" src="https://images.theconversation.com/files/395103/original/file-20210414-21-pu5nwu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/395103/original/file-20210414-21-pu5nwu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/395103/original/file-20210414-21-pu5nwu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/395103/original/file-20210414-21-pu5nwu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/395103/original/file-20210414-21-pu5nwu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=501&fit=crop&dpr=1 754w, https://images.theconversation.com/files/395103/original/file-20210414-21-pu5nwu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=501&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/395103/original/file-20210414-21-pu5nwu.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">Human brain structure is the same in males and females.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/head-x-ray-brain-in-mri-royalty-free-image/168715138">Movus/iStock via Getty Images Plus</a></span>
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<p>Neuroscientists have long held out hope that bigger studies and better methods would finally uncover the “real” or species-wide sex differences in the brain. But the truth is, as studies have gotten bigger, the <a href="https://doi.org/10.1038/s41598-018-23976-1">sex effects have gotten smaller</a>. </p>
<p>This collapse is a telltale sign of a problem known as <a href="https://doi.org/10.3310/hta14080">publication bias</a>. Small, early studies which found a significant sex difference were <a href="https://doi.org/10.1038/s41598-018-23976-1">likelier to get published</a> than research finding no male-female brain difference.</p>
<h2>Software versus hardware</h2>
<p>We must be doing something right, because our challenge to the dogma of brain sex has received pushback from both ends of the academic spectrum. Some have labeled us as science <a href="https://quillette.com/2019/03/11/science-denial-wont-end-sexism/">“deniers”</a> and deride us for political correctness. On the other extreme, we are dismissed by <a href="https://doi.org/10.1038/d41586-019-01366-5">women’s health advocates</a>, who believe research has overlooked women’s brains – and that neuroscientists should intensify our search for sex differences to better treat female-dominant disorders, such as depression and <a href="https://www.alzdiscovery.org/cognitive-vitality/blog/how-does-alzheimers-affect-women-and-men-differently">Alzheimer’s disease</a>.</p>
<p>But there’s no denying the decades of actual data, which show that brain sex differences are tiny and swamped by the much greater variance in individuals’ brain measures across the population. And the same is true for most behavioral measures. </p>
<p>[<em>Deep knowledge, daily.</em> <a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=deepknowledge">Sign up for The Conversation’s newsletter</a>.]</p>
<p>About a decade ago, teachers were urged to <a href="https://doi.org/10.1007/s11199-011-0037-y">separate boys and girls</a> for math and English classes based on the sexes’ alleged learning differences. Fortunately, <a href="https://www.theguardian.com/commentisfree/2017/jul/14/gender-segregation-state-schools-disaster-damage-children">many refused</a>, arguing the <a href="https://core.ac.uk/download/pdf/4156138.pdf">range of ability</a> is always much greater among boys or among girls than between each gender as a group.</p>
<p>In other words, sex is a very imprecise indicator of what kind of brain a person will have. Another way to think about it is <a href="https://doi.org/10.1016/j.neubiorev.2020.11.018">every individual brain is a mosaic</a> of circuits that control the many dimensions of masculinity and femininity, such as emotional expressiveness, interpersonal style, verbal and analytic reasoning, sexuality and gender identity itself. </p>
<p>Or, to use a computer analogy, gendered behavior comes from running different software on the same basic hardware. </p>
<p>The absence of binary brain sex features also <a href="https://psycnet.apa.org/record/2018-32185-001">resonates</a> with the increasing numbers of people who <a href="https://doi.org/10.1016/j.jadohealth.2017.04.014">identify as nonbinary, queer, nonconforming or transgender</a>. Whatever influence biological sex exerts directly on human brain circuitry is clearly not sufficient to explain the multidimensional behaviors we lump under the complex phenomenon of gender. </p>
<p>Rather than “dimorphic,” the human brain is a sexually monomorphic organ – much more like the heart, kidneys and lungs. As you may have noticed, these can be transplanted between women and men with great success.</p><img src="https://counter.theconversation.com/content/158005/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lise Eliot receives funding from the Fred B. Snite Foundation. </span></em></p>Rather than distinctly male or female, the human brain is much more like the heart, kidneys and lungs – basically the same no matter the sex of the body it’s in.Lise Eliot, Professor of Neuroscience, Rosalind Franklin University of Medicine and ScienceLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1089112019-01-23T19:05:19Z2019-01-23T19:05:19ZHow genes and evolution shape gender – and transgender – identity<figure><img src="https://images.theconversation.com/files/250893/original/file-20181217-185264-166ktjm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">There are many genes involved in shaping not just our biological sex, but also our gender identity. </span> <span class="attribution"><a class="source" href="https://unsplash.com/photos/VLOVJl_mLCA">Limor Zellermayer/Unsplash</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>Mismatch between biological sex and gender identity, culminating in its severest form as <a href="https://www.webmd.com/sex/gender-dysphoria#1">gender dysphoria</a>, has been ascribed to <a href="https://edition.cnn.com/2018/06/20/health/transgender-people-no-longer-considered-mentally-ill-trnd/index.html">mental disease</a>, family dysfunction and <a href="https://onlineacademiccommunity.uvic.ca/ahdevor/2016/12/07/h-devor-1994-transsexualism-dissociation-and-child-abuse-an-initial-discussion-based-on-nonclinical-data-journal-of-psychology-and-human-sexuality-63-49-72/">childhood trauma</a>. </p>
<p>But accumulating evidence now implies <a href="http://sitn.hms.harvard.edu/flash/2016/gender-lines-science-transgender-identity/">biological factors</a> in establishing gender identity, and a role for <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4823815/">particular genes</a>. </p>
<p>Variants – subtly different versions – of genes linked with gender identity might simply be part of a spectrum of gender and sexuality maintained throughout human history.</p>
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Read more:
<a href="https://theconversation.com/explainer-what-treatment-do-young-children-receive-for-gender-dysphoria-and-is-it-irreversible-64759">Explainer: what treatment do young children receive for gender dysphoria and is it irreversible?</a>
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<h2>Transgender and gender dysphoria</h2>
<p>Some young boys show an early preference for dressing and behaving as girls; some young girls are convinced they should be boys.</p>
<p>This apparent mismatch of biological sex and gender identity can lead to severe <a href="https://theconversation.com/explainer-what-treatment-do-young-children-receive-for-gender-dysphoria-and-is-it-irreversible-64759">gender dysphoria</a>. Coupled with school bullying and family rejection, it can make lives a <a href="https://transequality.org/issues/youth-students">torment for young people</a>, and the rate of suicide is <a href="http://pediatrics.aappublications.org/content/142/4/e20174218">frighteningly high</a>.</p>
<p>As they move into adulthood, nearly <a href="https://www.psypost.org/2017/12/many-transgender-kids-grow-stay-trans-50499">half of these children</a> (or even more when the studies are <a href="https://theconversation.com/why-rapid-onset-gender-dysphoria-is-bad-science-92742">closely interrogated</a>), continue to feel strongly that they were born in the wrong body. Many seek treatment – hormones and surgery – to transition into the sex with which they identify. </p>
<p>Although male to female (MtF) and female to male (FtM) transitions are now much more available and accepted, the road to transition is still fraught with uncertainty and opprobrium.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/250895/original/file-20181217-185268-17x9ud2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/250895/original/file-20181217-185268-17x9ud2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=476&fit=crop&dpr=1 600w, https://images.theconversation.com/files/250895/original/file-20181217-185268-17x9ud2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=476&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/250895/original/file-20181217-185268-17x9ud2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=476&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/250895/original/file-20181217-185268-17x9ud2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=599&fit=crop&dpr=1 754w, https://images.theconversation.com/files/250895/original/file-20181217-185268-17x9ud2.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=599&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/250895/original/file-20181217-185268-17x9ud2.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=599&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">American television personality Caitlyn Jenner has spoken openly about being transgender.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/lisbon-portugal-november-2017-american-television-786032923?src=nwKJjOWtZhiFe6TIkmFxSw-1-89">from www.shutterstock.com</a></span>
</figcaption>
</figure>
<p>Transwomen (born male) and transmen (born female) have been a part of society in <a href="http://bilerico.lgbtqnation.com/2008/02/transgender_history_trans_expression_in.php">every culture at every time</a>. Their frequency and visibility is a function of societal mores, and in most societies they have suffered <a href="https://www.amnesty.org.au/transgender-people-face-casual-discrimination-up-to-60-times-a-day/">discrimination or worse</a>. </p>
<p>This discrimination stems from a persistent attitude that transgender identification is an aberration of normal sexual development, perhaps exacerbated by events such as trauma or illness. </p>
<p>However, over the last decades, growing recognition emerged that transgender feelings start very early and are very consistent – pointing to a biological basis.</p>
<p>This led to many searches for biological signatures of transsexualism, including reports of <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4681519">differences in sex hormones</a> and claims of <a href="https://www.the-scientist.com/features/are-the-brains-of-transgender-people-different-from-those-of-cisgender-people-30027">brain differences</a>.</p>
<h2>Sex genes and transgender</h2>
<p>In the 1980s I was swayed by the passionate advocacy of <a href="https://zagria.blogspot.com/2016/05/herbert-bower-1914-2004-psychiatrist.html#.XBiHVfxS_dQ">Herbert Bower</a>, a psychiatrist who worked with transsexuals in Melbourne. He was revered in the transgender community for his willingness to authorise sex change operations, which were highly controversial at the time. Aged in his 90s, he came to my laboratory in 1988 to explore the possibility that variation in the genes that determine sex could underlie transgender. </p>
<p>Dr Bower wondered if the gene that controls male development might work differently in transgender boys. This gene (called SRY, and which is <a href="https://theconversation.com/what-makes-you-a-man-or-a-woman-geneticist-jenny-graves-explains-102983">found on the Y chromosome</a>) triggers the formation of a testis in the embryo; the testis makes hormones and the hormones make the baby male.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/what-makes-you-a-man-or-a-woman-geneticist-jenny-graves-explains-102983">What makes you a man or a woman? Geneticist Jenny Graves explains</a>
</strong>
</em>
</p>
<hr>
<p>There are, indeed, variants of the SRY gene. Some don’t work at all, and babies who have a Y chromosome but a mutant SRY are <a href="https://www.nature.com/articles/348448a0">born female</a>. However, they are not disproportionately transgender. Nor are the many people born with variants of other genes in the <a href="http://theconversation.com/boy-girl-or-dilemmas-when-sex-development-goes-awry-49359">sex determining pathway</a>.</p>
<p>After many discussions, Dr Bower agreed that the sex determining gene was probably not directly involved – but the idea of genes affecting sexual identity took root. So are there separate genes that affect gender identity?</p>
<h2>Evidence for gene variants in transgender</h2>
<p>The search for gene variants that underlie any trait usually starts with twin studies. </p>
<p>There are <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0021982">reports</a> that <a href="https://www.tandfonline.com/doi/abs/10.1080/15532739.2013.750222">identical twins</a> are much more likely to be concordant (that is both transgender, or both <a href="https://theconversation.com/explainer-what-does-it-mean-to-be-cisgender-103159">cisgender</a>) than fraternal twins or siblings. This is probably an underestimation given that one twin may not wish to come out as trans, thus underestimating the concordance. This suggests a substantial genetic component.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-what-does-it-mean-to-be-cisgender-103159">Explainer: what does it mean to be 'cisgender'?</a>
</strong>
</em>
</p>
<hr>
<p>More recently, particular genes have been studied in detail in transwomen and transmen. One <a href="https://onlinelibrary.wiley.com/doi/abs/10.1111/jsm.12673">study</a> looked at associations between being trans and particular variants of some genes in the hormone pathway.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/250897/original/file-20181217-185252-1g5tku5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/250897/original/file-20181217-185252-1g5tku5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/250897/original/file-20181217-185252-1g5tku5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/250897/original/file-20181217-185252-1g5tku5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/250897/original/file-20181217-185252-1g5tku5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/250897/original/file-20181217-185252-1g5tku5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/250897/original/file-20181217-185252-1g5tku5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/250897/original/file-20181217-185252-1g5tku5.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">Studies of twins help us learn about the genes involved in determining identity.</span>
<span class="attribution"><a class="source" href="https://unsplash.com/photos/M-adWhDQd7Y">Keisha Montfleury/Unsplash</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>A recent and much larger <a href="https://academic.oup.com/jcem/advance-article-abstract/doi/10.1210/jc.2018-01105/5104458?redirectedFrom=fulltext">study</a> assembled samples from 380 transwomen who had, or planned, sex change operations. They looked in fine detail at 12 of the “usual suspects” – genes involved in hormone pathways. They found that transwomen had a high frequency of particular DNA variants of four genes that would alter sex hormone signalling while they had been developing in utero.</p>
<p>There may be many other genes that contribute to a feminine or a masculine sexual identity. They are not necessarily all concerned with sex hormone signalling – some may affect brain function and behaviour.</p>
<p>The next step in exploring this further would be to compare whole genome sequences of cis- and transsexual people. Whole genome epigenetic analyses, looking at the molecules that affect how genes function in the body, might also pick up differences in the action of genes. </p>
<p>It’s probable that many – maybe hundreds – of genes work together to produce a great range of sexual identities.</p>
<h2>How would “sexual identity genes” work in transgender?</h2>
<p>Sexual identity genes don’t have to be on sex chromosomes. So they will not necessarily be “in sync” with having a Y chromosome and an SRY gene. This is in line with observations that gender identity is separable from biological sex.</p>
<p>This means that among both sexes we would expect a spread of more feminine and more masculine identity. That is to say, in the general population of males you would expect to see a range of identities from strongly masculine to more feminine. And among females in the population you would see a range from strongly feminine to more masculine identities. This would be expected to produce transwomen at one end of the distribution, and transmen at the other.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/251402/original/file-20181218-27758-ngq5u3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/251402/original/file-20181218-27758-ngq5u3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/251402/original/file-20181218-27758-ngq5u3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/251402/original/file-20181218-27758-ngq5u3.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/251402/original/file-20181218-27758-ngq5u3.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/251402/original/file-20181218-27758-ngq5u3.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/251402/original/file-20181218-27758-ngq5u3.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">There is a natural range in masculine and feminine identity.</span>
<span class="attribution"><a class="source" href="https://unsplash.com/photos/pIsHOl77zzA">John Schnobrich/unsplash</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>This occurrence of a range of differing identities would be comparable with a trait such as height. Although men are about 14 cm taller than women on average, it’s perfectly normal to see short men and tall women. It’s just part of the normal distribution of a certain human characteristic expressed differently in men and women. </p>
<p>This argument is similar to that which I <a href="https://theconversation.com/born-this-way-an-evolutionary-view-of-gay-genes-26051">previously described</a> for so-called “gay genes”. I suggested same–sex attraction can readily be explained by many “male-loving” and “female-loving” variants of mate choice genes that are inherited independently of sex.</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>
</strong>
</em>
</p>
<hr>
<h2>Why is transgender so frequent then?</h2>
<p>Transgender is <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4823815/">not rare</a> (MtF of 1/200, FtM of 1/400). If gender identity is strongly influenced by genes, this leads to questions about why it is maintained in the population if transmen and transwomen have <a href="https://www.jahonline.org/article/S1054-139X(17)30854-6/fulltext">fewer children</a>. </p>
<p>I suggest genes that influence sexual identity are positively selected in the other sex. Feminine women and masculine men may partner earlier and have more kids, to whom they pass on their gender identity gene variants. Looking at whether the female relatives of transwomen, and the male relatives of transmen, have more children than average, would test this hypothesis. </p>
<p>I made much the same argument to explain why homosexuality is so common, although gay men have <a href="http://theconversation.com/born-this-way-an-evolutionary-view-of-gay-genes-26051">fewer children than average</a>. I suggested gay men share their “male loving” gene variants with their female relatives, who mate earlier and pass this gene variant on to more children. And it turns out that the female relatives of gay men <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1691850/pdf/15539346.pdf">do have more children</a>.</p>
<p>These variants of sexual identity and behaviour may therefore be considered examples of what we call “sexual antagonism”, in which a gene variant has different selective values in men and women. It makes for the amazing variety of human sexual behaviours that we are beginning to recognise.</p><img src="https://counter.theconversation.com/content/108911/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jenny Graves receives funding from the Australian Research Council. </span></em></p>It’s not about trauma or how you were raised: evidence now points to a biological basis for transgender, and to the action of particular genes in that determination.Jenny Graves, Distinguished Professor of Genetics, La Trobe UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/738272017-03-21T01:02:56Z2017-03-21T01:02:56ZWhat dung beetles are teaching us about the genetics of sex differences<p>Picture a lion: The male has a luxuriant mane, the female doesn’t. This is a classic example of what biologists call sexual dimorphism – the two sexes of the same species exhibit differences in form or behavior. Male and female lions pretty much share the same genetic information, but look quite different.</p>
<p>We’re used to thinking of genes as responsible for the traits an organism develops. But different forms of a trait – mane or no mane – can arise from practically identical genetic information. Further, traits are not all equally sexually dimorphic. While the tails of peacocks and peahens are extremely different, their feet, for example, are pretty much the same.</p>
<p>Understanding how this variation of form – what geneticists call phenotypic variation – arises is crucial to answering several scientific questions, including how novel traits appear during evolution and how complex diseases emerge during a lifetime.</p>
<p>So researchers have taken a closer look at the genome, looking for the genes responsible for differences between sexes and between traits within one sex. The key to these sexually dimorphic traits appears to be a kind of protein called a <a href="https://www.khanacademy.org/science/biology/gene-regulation/gene-regulation-in-eukaryotes/a/eukaryotic-transcription-factors">transcription factor</a>, whose job it is to turn genes “on” and “off.”</p>
<p><a href="http://dx.doi.org/10.1038/ncomms14593">In our own work with dung beetles</a>, my colleagues and I are untangling how these transcription factors actually lead to the different traits we see in males and females. A lot of it has to do with something called “alternative gene splicing” – a phenomenon that allows a single gene to encode for different proteins, depending on how the building blocks are joined together.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161596/original/image-20170320-9132-112g2nz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161596/original/image-20170320-9132-112g2nz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161596/original/image-20170320-9132-112g2nz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161596/original/image-20170320-9132-112g2nz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161596/original/image-20170320-9132-112g2nz.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161596/original/image-20170320-9132-112g2nz.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161596/original/image-20170320-9132-112g2nz.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161596/original/image-20170320-9132-112g2nz.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">The gene <em>doublesex</em> produces visually obvious sexual dimorphism in the butterfly <em>Papilio polytes</em>, the common Mormon. Female (top), male (bottom).</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Papilio_polytes_mating_in_Kadavoor.jpg">Jeevan Jose, Kerala, India</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>On/off, male/female</h2>
<p>Over the years, different groups of scientists independently worked with various animals to identify genes that shape sexual identity; they realized that many of these genes share a specific region. This gene region was found in both the worm gene <em>mab-3</em> and the insect gene <em>doublesex</em>, so they named similar genes containing this region DMRT genes, for “<a href="http://dx.doi.org/10.1038/35618">doublesex mab-related transcription factors</a>.” </p>
<p>These genes code for DMRT proteins that turn on or off the reading, or expression, of other genes. To do this, they seek out genes in DNA, bind to those genes, and make it <a href="https://www.khanacademy.org/science/biology/gene-expression-central-dogma/transcription-of-dna-into-rna/a/stages-of-transcription">either easier or harder to access the genetic information</a>. By controlling what parts of the genome are expressed, DMRT proteins lead to products characteristic of maleness or femaleness. They match the expression of genes to the right sex and trait. </p>
<p>DMRTs almost always confer maleness. For instance, without DMRT, <a href="http://dx.doi.org/10.1038/nature10239">testicular tissue in male mice deteriorates</a>. When DMRT is experimentally produced in female mice, they <a href="http://dx.doi.org/10.1242/dev.122184">develop testicular tissue</a>. This job of promoting testis development is common to most animals, from fish and birds to worms and <a href="http://dx.doi.org/10.1016/j.tig.2012.02.002">clams</a>. </p>
<p>DMRTs even confer maleness in animals where individuals develop both testes and ovaries. In fish that exhibit sequential hermaphroditism – where gonads change from female to male, or vice versa, within the same individual – the waxing and waning of DMRT expression results in the <a href="http://dx.doi.org/10.1111/j.1742-4658.2011.08030.x">appearance and regression of testicular tissue</a>, respectively. Likewise, in turtles that become male or female based on temperatures experienced in the egg, DMRT is produced in the genital tissue of <a href="http://dx.doi.org/10.1002/(SICI)1526-968X(200003)26:3%3C174::AID-GENE2%3E3.0.CO;2-J">embryos exposed to male-promoting temperatures</a>.</p>
<p>The situation is a little different in insects. First, the role of DMRT (<em>doublesex</em>) in generating sexual dimorphism has extended beyond gonads to other parts of the body, including <a href="http://dx.doi.org/10.1371/journal.pgen.1004098">mouthparts</a>, <a href="http://dx.doi.org/10.1038/nature13112">wingspots</a> and mating bristles aptly named “<a href="http://dx.doi.org/10.1371/journal.pbio.1001131">sex combs</a>.” </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/161588/original/image-20170320-9121-z0fsyj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161588/original/image-20170320-9121-z0fsyj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/161588/original/image-20170320-9121-z0fsyj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161588/original/image-20170320-9121-z0fsyj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161588/original/image-20170320-9121-z0fsyj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161588/original/image-20170320-9121-z0fsyj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161588/original/image-20170320-9121-z0fsyj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161588/original/image-20170320-9121-z0fsyj.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">Depending on how the pieces are put together, one gene can result in a number of different proteins.</span>
<span class="attribution"><span class="source">Cris Ledón-Rettig</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Secondly, male and female insects generate their own versions of the <em>doublesex</em> protein through what’s called “<a href="https://www.khanacademy.org/science/biology/gene-expression-central-dogma/transcription-of-dna-into-rna/a/eukaryotic-pre-mrna-processing">alternative gene splicing</a>.” This is a way for a single gene to code for multiple proteins. Before genes are turned into proteins, they must be turned “on”; that is, transcribed into instructions for how to build the protein.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/161602/original/image-20170320-9127-1d7sk76.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/161602/original/image-20170320-9127-1d7sk76.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/161602/original/image-20170320-9127-1d7sk76.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=831&fit=crop&dpr=1 600w, https://images.theconversation.com/files/161602/original/image-20170320-9127-1d7sk76.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=831&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/161602/original/image-20170320-9127-1d7sk76.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=831&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/161602/original/image-20170320-9127-1d7sk76.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1045&fit=crop&dpr=1 754w, https://images.theconversation.com/files/161602/original/image-20170320-9127-1d7sk76.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1045&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/161602/original/image-20170320-9127-1d7sk76.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1045&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Thanks to the <em>doublesex</em> gene, in the stag beetle <em>Cyclommatus metallifer</em>, mandibles of males (right) are much larger than those of females (left).</span>
<span class="attribution"><a class="source" href="http://dx.doi.org/10.1371/journal.pgen.1004098">http://dx.doi.org/10.1371/journal.pgen.1004098</a></span>
</figcaption>
</figure>
<p>But the instructions contain both useful and extraneous regions of information, so the useful parts must be stitched together to create the final protein instructions. By combining the useful regions in different ways, a single gene can produce multiple proteins. In male and female insects, it’s this alternative gene splicing that results in the <em>doublesex</em> proteins <a href="http://dx.doi.org/10.1016/0092-8674(89)90633-8">behaving differently in each sex</a>.</p>
<p>So in a female, instructions from the <em>doublesex</em> gene might include sections 1, 2 and 3, while in a male the same instruction might include only 2 and 3. The different resulting proteins would each have their own effect on what parts of the genetic code are turned on or off – leading to a male with huge mouthparts and a female without, for instance.</p>
<h2><em>Doublesex</em> in dung beetles</h2>
<p>How do male and female forms of <em>doublesex</em> regulate genes to produce male and female traits? <a href="http://ecoevodevo.com/">Our research group</a> answered this question using dung beetles, which are exceptionally numerous in species (over 2,000), widespread (inhabiting every continent except Antarctica), versatile (consuming about every type of dung) and show amazing diversity in a sexually dimorphic trait: <a href="https://en.wikipedia.org/wiki/Dung_beetle">horns</a>.</p>
<p>We focused on the bull-headed dung beetle <em>Onthophagus taurus</em>, a species in which males produce large, bull-like head horns but females remain hornless. We found that <em>doublesex</em> proteins can <a href="http://dx.doi.org/10.1038/ncomms14593">regulate genes in two ways</a>.</p>
<p>In most traits, it regulates different genes in each sex. Here, <em>doublesex</em> is not acting as a “switch” between two possible sexual outcomes, but instead bestowing maleness and femaleness to each sex independently. Put another way, these traits don’t face a binary decision between becoming male or female, they are simply asexual and poised for further instruction.</p>
<p>The story is different for the dung beetles’ head horns. In this case, <em>doublesex</em> acts more like a switch, regulating the same genes in both sexes but in opposite directions. The female protein suppressed genes in females that would otherwise be promoted by the male protein in males. Why would there be an evolutionary incentive to do this?</p>
<p>Our data hinted that the female <em>doublesex</em> protein does this to avoid what is known as “sexual antagonism.” In nature, fitness is sculpted by both natural and sexual selection. Natural selection favors traits increasing survival, whereas sexual selection favors traits increasing access to mates. </p>
<p>Sometimes these forces are in agreement, but not always. The large head horns of male <em>O. taurus</em> increase their access to mates, but the same horns would be a hassle for females who have to tunnel underground to raise their offspring. This creates a tension between the sexes, or sexual antagonism, that limits the overall fitness of the species. However, if the female <em>doublesex</em> protein turns “off” genes that in males are responsible for horn growth, the whole species does better. </p>
<h2>Beyond beetle sex</h2>
<p>Our ongoing research is addressing how <em>doublesex</em> has evolved to generate the vast diversity in sexual dimorphism in dung beetles. Across species, horns are found in different body regions, grow differently in response to different quality diets, and can even occur in females rather than males.</p>
<p>In <em>Onthophagus sagittarius</em>, for instance, it’s the female that grows substantial horns while males remain hornless. This species is only five million years diverged from <em>O. taurus</em>, a mere drop of time in the evolutionary bucket for insects. For perspective, beetles diverged from flies about 225 million years ago. This suggests that <em>doublesex</em> can evolve quickly to acquire, switch, or modify the regulation of genes underlying horn development. </p>
<p>How will understanding the role of <em>doublesex</em> in sexually dimorphic insect traits help us understand phenotypic variation in other animals, even humans?</p>
<p>Despite the fact that DMRTs are spliced as only one form in mammals and act primarily in males, the <a href="http://dx.doi.org/10.1038/nature07509">majority of other human genes are alternatively spliced</a>; just like insects’ <em>doublesex</em> gene, most human genes have various regions that can be spliced together in different orders with varying results. Alternatively spliced genes can have distinct or opposing effects based on which sex or trait they’re expressed in. Understanding how proteins that are produced by alternatively spliced genes behave in different tissues, sexes and environments will reveal how one genome can produce a multitude of forms depending on context.</p>
<p>In the end, the humble dung beetle’s horns can give us a peek into the mechanisms underlying the vast complexity of animal forms, humans included.</p><img src="https://counter.theconversation.com/content/73827/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Cris Ledón-Rettig is affiliated with Indiana University, Bloomington. </span></em></p>How can the same basic genome produce such different forms in the two sexes of a single species? It turns out one gene can encode for various things, depending on the order its instructions are read.Cris Ledón-Rettig, Postdoctoral Fellow of Biology, Indiana UniversityLicensed as Creative Commons – attribution, no derivatives.