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Do cheating females have better kids? Shake ya tail feather for yes

Why do females cheat on their partners? A new study of songbirds in the US – published today – suggests cheating females are more successful in the long-run and get more grandchildren. Why? Because the…

Male crimson chat feeding his offspring … but are they all his? Simon Griffith

Why do females cheat on their partners? A new study of songbirds in the US – published today – suggests cheating females are more successful in the long-run and get more grandchildren.

Why? Because the offspring sired as a result of promiscuous matings are more successful at reproducing than those sired by her long-term partner.

These intriguing results help us understand the evolutionary dynamics underlying social monogamous pairbonds – something that has control of our own hearts.

In socially-monogamous animals, females form partnerships with males to get access to male-held resources, such as territories, or direct help in raising offspring.

Despite the benefits females appear to get from forming these social bonds, some females are sexually unfaithful.

This is as risky a strategy for a bird as it would be in human relationships. Promiscuous females risk being deserted by their partner, or at least suffering a reduction in the contribution he will make to the rearing of their offspring.

Natural selection has made males very sensitive to the risk of cuckoldry and the expense of raising someone else’s offspring.

High infidelity

Given the potential cost of infidelity to females, there has been much research in evolutionary biology over the past three decades into the motivations behind extrapair copulations by females.

The advent of DNA profiling in the late 1980s provided new impetus to this field as wildlife biologists could use blood samples collected from families of birds to identify offspring that resulted from a cuckolding male.

In the best studies, researchers were even able to sample all of the males in a population and identify the extrapair male as well.

More than 200 species of bird around the world have been investigated in this way, and on average about 15% of all offspring are the result of female infidelity.

(It may be of interest that one of the highest levels of infidelity recorded is in a population of fairy wrens in the Botanic Gardens in Canberra, in which over three quarters of all offspring are sired by extrapair males).

Fairy wrens … very naughty birds. Timmy Toucan.

Usually in birds, the copulations that lead to this extrapair paternity are very brief and cryptic – females don’t appear to gain much from the cuckolding male other than sperm.

Many studies have therefore looked for potential genetic benefits in the resulting offspring, comparing offspring that have been sired by extrapair males with those sired by social partners.

To date, most studies have had to tackle the question during the short time-frame of a typical research grant, which means they have only looked for differences in the short-term.

Many have searched, to no avail, for differences between offspring at early stages of development while the offspring are still in the nest.

Charting a new flightpath

In their PRSB study of the dark-eyed junco (an American sparrow), conducted over 18 years, Nicole Gerlach and her co-authors genotyped more than 2,000 offspring and tracked those that returned to the study site, each year measuring how many offspring they produced.

Sons and daughters sired by extrapair males produced almost twice as many offspring in their lifetimes as those individuals fathered by the social male at a nest.

Sons produced by extrapair males were more likely to themselves father additional offspring themselves through promiscuous matings.

Daughters produced by unfaithful mothers were themselves better at producing fit and healthy offspring.

So, in this species of bird, infidelity does pay – even though the benefits are only seen many years later.

The study illustrates very nicely how behaviour can be shaped by selection acting in quite subtle ways and over long periods of time.

All of which may be worth getting in a flap about.

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6 Comments sorted by

  1. Bradley Twynham

    logged in via Facebook

    As an amateur ornithologist I find this article very interesting. However, the notion that this research could possibly provide any relevance to why female humans cheat is absurd. Birds do not mate for pleasure unlike human beings so the behavioural motivation is completely different. Linking bird behaviour to human behaviour seems a big stretch. I don't know if this is journalistic license or not I am sure the research is valid and important to understanding bird behaviour.

    1. Simon Griffith

      Associate Professor of Avian Behavioural Ecology at Macquarie University

      In reply to Bradley Twynham

      The research is not motivated by an attempt to understand human behaviour. As you write, the research is important in developing an understanding of avian mating systems and breeding behaviour. The only link I drew specifically with human behaviour was that there is a risk of cheating on a partner and I stand by that. Regardless of whether mating is primarily for 'pleasure' or for reproduction (or both), individuals whose partners are unfaithful are likely to respond to that infidelity.
      Socially monogamous birds are actually a better proxy for human mating behaviour than most mammals (that do not form extended social bonds with sexual partners). As such we can learn a lot about the dynamics of such relationships from birds, even though I agree with you we should treat such comparisons with a degree of caution.

    2. Dale Bloom


      In reply to Simon Griffith

      The more often mating occurs with different partners, the greater the risk of sexually transmitted diseases, which is a major disease factor in many species.

      It is debateable that offspring will be healthier over a longer time frame.

      I am also in agreement with Bradely that species of birds do not equate with many other species that often require longer periods of time to raise children or offspring.

      Within the human species, it normally requires 15 to 20 years to raise a child until they become independent enough, and this requires long-term bonding between the parents, and often requires support from an extended family.

      The children do not leave the nest within one year.

    3. Simon Griffith

      Associate Professor of Avian Behavioural Ecology at Macquarie University

      In reply to Dale Bloom

      The actual time frame that it takes offspring to mature is not really that relevant in this context. Evolution works by acting on the individuals within every cohort and in this context on the number of individuals that leave descendants. It does not really matter whether the offspring take 60 days to reach sexual maturity (as in the zebra finch), or 15-20 years (as in humans). What matters is all of the sources of variation that contribute to variation in the quality of those individuals. In long-lived…

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    4. Dale Bloom


      In reply to Simon Griffith

      Hi Simon
      There are positive and negatives in all things, genetic or behavioural variations in a species included.

      Increased mating may be a learned behavioural trait that may produce more offspring in the short term, but in the longer term, more offspring may overpopulate an area.

      I would think gestation periods do have a major affect on eventual species traits, together with the environment the species lives in.

      The human species for example does not have a mating season, and parents do not produce…

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  2. Nicholas Parsons

    logged in via Facebook

    I have no scientific background whatsoever, but I’m going to put my lay person’s two cents in all the same. So please view everything I say with appropriate scepticism.

    To me, it seems that even though this study of birds may not show THE reason for female infidelity in humans, it surely gives a good clue into what may be A reason for female infidelity in humans.

    It seems plausible (not that plausibility is any kind of proof) that adulterous couplings would produce stronger offspring. In choosing…

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