Male and female animals of the same species usually come in different forms or ‘morphs’. 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?
What’s a sexual morph?
Ignore the genitals! Biological sex is defined by an individual’s capacity to create sperm, eggs or both – making them male, female or hermaphrodite.
Organs that produce reproductive cells (like testes and ovaries), plus structures for fertilisation (like the penis and vagina), are primary sexual characteristics.
Dimorphism is defined by secondary sexual characteristics – traits typical of males or females that aren’t directly involved in reproduction. Such traits help animals identify the opposite sex without staring rudely at the nether regions.
What are some examples of sexual dimorphism?
The clearest dimorphic traits involve colouration and ornamentation, the most famous example of which is the peafowl.
A peacock has fancy tail feathers, a peahen’s plumage is subdued. Less obviously, songbirds produce different vocalisations: in zebra finches, females have a smaller brain circuit for learning and can’t sing. Another trait is ‘sexual size dimorphism’.
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.
Are females always bigger?
No. The most well-known exception is mammals: in species where size dimorphism occurs, males are generally larger.
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 of bigger male bodies and/or weapons such as horns and antlers.
Fighting for a reproductive advantage can also continue through post-copulatory processes such as ‘sperm competition’. In some species, little swimmers not only race to fertilise an egg, semen can contain poisons that kill rival sperm cells.
Why are males such show-offs?
It’s 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.
Fancy traits like the peacock’s tail, which is very conspicuous to predators, ought to reduce an individual’s chances of survival, so why do they persist? One theory is that males show off ‘good genes’ 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 – the ‘sexy son’ hypothesis.
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’s preferences for particular characteristics in the other.
Can there be more than two morphs?
Yes! Sexual dimorphism compares an average male and female (individuals vary). Many species have ‘sex-specific morphs’ too. Female-specific morphs seem rare – a famous example is queen and worker bees.
Male-specific morphs are more common, seen in everything from scarab beetles to wild turkeys, and include territorial ‘alpha’ and ‘sneaker’ males. In some species, a male can develop from one morph into another, starting as a young sneaker before becoming older and territorial.
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 – medium-sized morphs get weeded out as they aren’t well-adapted to either strategy.
There’s 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.