Sexual dimorphism

The header above is a phrase I have come across recently. This little note is surely an attempt to show off my newly-acquired knowledge, but I guess you will enjoy if you read on, particularly if you are interested in evolution of life forms.

The adjective “dimorphic” means “representing two distinct forms”. Sexual dimorphism means the differences in size, strength, and behaviour patterns between the two sexes of the same species. For example, among wild mammals, males are often bigger and more aggressive than females. Females, on the other hand, take care of their infants. Among birds, males not only have attractive plumes and colours, usually, they are the ones that sing! The male deer has antlers, a cumbersome attachment, but it adds to his visual appeal, besides being useful during fights with other males.

Males and females are different because the struggle for reproductive success takes each of them along different evolutionary trajectories. Male birds / mammals have to win sexual contests with other males. To put it simply, they must impress females, a situation you come across in college campuses and elsewhere.

Interestingly, sexual dimorphism is much less pronounced among domesticated animals because they don’t have the same evolutionary pressures as wild animals! Their survival (and usually, suffering) is ensured by their human masters. Among humans, in the so-called civilized societies, physical strength has taken a back seat in personal contacts. Even if a man has a contest with another to win the favours of a woman, they don’t try to beat each other to pulp. Usually!

Sexual dimorphism is therefore less pronounced among humans. According to an estimate, in the US, 10% of men have less weight than the average weight of a woman. Men don’t need a bigger body or stronger muscles to impress women these days. Instead, they try to enrol in IITs.

However, certain forms of dimorphism, like bright colours for birds and antlers for deer should be a disadvantage for their survival. These make them more visible and easier targets for predators. But the paradox is resolved if you consider that the reproductive success of an animal is often more important than his/her life span. For example, a wild male pheasant lives half as long as a female. (Nature is perhaps not as partial to males as it might seem at the first glance!) A male pheasant’s reproductive success depends not on how long he lives, but on how many females he can mate with. Bright plumage improves his chances. On the other hand, among insects, females are often bigger than males as they need volume to produce a large number of eggs.

I got interested in the topic while reading some books. Of these, one I read at a stretch over the last two days: a beautiful Bangla novel around the evolutionary success of migratory birds in particular and birds in general. The book has been written by a relatively unknown author, Debashis Moitra, who besides writing, teaches civil engineering. Humans are no less intriguing than birds, it seems.

More about the book later. Please watch this space!

20 March 2022

Picture of male and female orioles from Marin Independent Journal, e-edition