{"id":32947,"date":"2023-10-17T14:41:33","date_gmt":"2023-10-17T12:41:33","guid":{"rendered":"http:\/\/1fd5c48d-bd19-400a-b2ed-77dd47fa2c5c"},"modified":"2023-10-17T15:35:39","modified_gmt":"2023-10-17T13:35:39","slug":"what-is-sexual-dimorphism-and-why-are-males-such-show-offs","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcwildlife\/rss_feed\/what-is-sexual-dimorphism-and-why-are-males-such-show-offs\/","title":{"rendered":"What is sexual dimorphism? And why are males such show-offs?"},"content":{"rendered":"

What is sexual dimorphism? Why does sexual dimorphism exist? JV Chamary explains all you need to know <\/p>

By <\/p>

Published: Tuesday, 17 October 2023 at 12:41 PM<\/p>

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Male and female animals of the same species usually come in different forms or \u2018morphs\u2019. The two sexes have distinctive features that include not only visible differences in features such as colour, size and shape, but also in unseen traits that dictate physiology and behaviour. So why does this sexual dimorphism exist?<\/p>

What\u2019s a sexual morph?<\/strong><\/h2>

Ignore the genitals! Biological sex is defined by an individual\u2019s capacity to create sperm, eggs or both \u2013 making them male, female or hermaphrodite. <\/p>

Organs that produce reproductive cells (like testes and ovaries), plus structures for fertilisation (like the penis and vagina), are primary <\/em>sexual characteristics. <\/p>

Dimorphism is defined by secondary<\/em> sexual characteristics \u2013 traits typical of males or females that aren\u2019t directly involved in reproduction. Such traits help animals identify the opposite sex without staring rudely at the nether regions.<\/p>

What are some examples of sexual dimorphism?<\/h2>

The clearest dimorphic traits involve colouration and ornamentation, the most famous example of which is the peafowl.A peacock has fancy tail feathers,<\/a> a peahen\u2019s plumage is subdued. Less obviously, songbirds produce different vocalisations: in zebra finches, females have a smaller brain circuit for learning and can\u2019t sing. Another trait is \u2018sexual size dimorphism\u2019. <\/p>

Females are often larger because it gives greater fecundity (more offspring). In one extreme example, male anglerfish live on females as parasites and serve as little more than sperm-producing organs.<\/p>

Are females always bigger?<\/strong><\/h2>

No. The most well-known exception is mammals: in species where size dimorphism occurs, males are generally larger. <\/p>

Females often do the hard work of nourishing offspring during pregnancy then rearing after birth, while males focus their time and energy on trying to maximise mating opportunities. That drives competition and the evolution<\/a> of bigger male bodies and\/or weapons such as horns and antlers.<\/p>

Fighting for a reproductive advantage can also continue through post-copulatory processes such as \u2018sperm competition\u2019. In some species, little swimmers not only race to fertilise an egg, semen can contain poisons that kill rival sperm cells.<\/p>

Why are males such show-offs?<\/strong><\/h2>

It\u2019s down to a discrepancy in the amount of resources each sex spends on reproduction. A mother contributes more than a father, whose input often ends at copulation, so males must impress females (the more invested sex) by spending time and energy on courtship dances or elaborate features.<\/p>

Fancy traits like the peacock\u2019s tail, which is very conspicuous to predators, ought to reduce an individual\u2019s chances of survival, so why do they persist? One theory is that males show off \u2018good genes\u2019 by being able to survive despite the handicap. Once the preference for a feature is established, a female is forced to choose a mate with that trait so her male offspring are attractive \u2013 the \u2018sexy son\u2019 hypothesis.<\/p>

Mate choice is the major driving force behind the evolution of dimorphic traits by sexual selection, a type of natural selection where environmental pressure is exerted by one sex\u2019s preferences for particular characteristics in the other.<\/p>

Can there be more than two morphs?<\/strong><\/h2>

Yes! Sexual dimorphism compares an average male and female (individuals vary). Many species have \u2018sex-specific morphs\u2019 too. Female-specific morphs seem rare \u2013 a famous example is queen and worker bees. <\/p>

Male-specific morphs are more common, seen in everything from scarab beetles to wild turkeys, and include territorial \u2018alpha\u2019 and \u2018sneaker\u2019 males. In some species, a male can develop from one morph into another, starting as a young sneaker before becoming older and territorial.<\/p>

Sex-specific traits reflect alternative mating strategies: alphas are typically large in order to compete and defend territory, sneakers are smaller and mate by stealth. The traits are discrete, not continuous \u2013 medium-sized morphs get weeded out as they aren\u2019t well-adapted to either strategy. <\/p>

There\u2019s also a trade-off between growing big to mate on demand, versus having relatively large testicles to aid sneakers who may only get one shot at mating. The options are either grow big or grow bigger testicles.<\/a><\/p>