Sexual dimorphism

 

The header of this article is a phrase I have come across recently. To be honest, this brief note is an attempt to show off my newly-acquired vocabulary, but I guess you will enjoy if you read on, particularly if you are interested in the animal world or the evolution of life forms. 

“Dimorphic” means “representing two distinct forms”. Sexual dimorphism means the differences in sizes, strength, and behaviour patterns between the two sexes of the same species, and how different they look. 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 accessory, 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 has 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 humans in campuses and everywhere else. 

Interestingly, sexual dimorphism is much less pronounced among domesticated animals because they don’t have the same evolutionary pressures as wild animals! Their survival (or suffering) and procreation is ensured by their human masters. Among humans, in 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! Consequently, sexual dimorphism is less pronounced among humans. According to an estimate, 10% of US men weigh less than the average weight of an American woman. Men don’t need a bigger body or muscles to impress women these days. Instead, they try to enrol in Ivy Leage universities. IITs in our country.

However, certain forms of dimorphism, like bright colours among birds and antlers on deer should be a disadvantage for their survival. These make them more visible and easier targets for predators. But the paradox is easily 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 for young adults and children like me. The story is around the evolutionary success of migratory birds in particular and birds in general. The title of the book is অচিন পাখী  which literally mean An Exotic Bird.

The author of the book is Debashis Moitra, who besides writing in Bangla, teaches civil engineering. Humans are no less intriguing than birds!

More about the book later. Please watch this space!

20 March 2022 / edited on Friday, 22 March 2024

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