Why Do Females and Males not Have the Same Body Size?

Except for couples like Tom Cruise - Nicole Kidman, we know that the man is usually taller than the woman. But the body size, form and structure between the genders are not limited only to humans. And the evolutionary causes of these differences are not clearly understood.

A new scholarly book sums up the latest discoveries in evolutionary biology that can shed light on gender differences many organisms, from mammals to birds, reptiles, amphibians, insects, spiders and flowering plants possess.

"Sex, Size & Gender Roles: Evolutionary Studies of Sexual Size Dimorphism" (Oxford University Press, 2007) attempts to explain why males are usually bigger than females in most large mammals, while female being the towering sex in smaller species like insects and spiders, but also how the sex differences occur despite the substantially similar genetic material.

"In all species with separate sexes, males and females are striving to maximize their genetic contributions to future generations, but the ways of doing this vary enormously among species. We see this reflected in fantastic patterns of sexual dimorphism that we have only begun to appreciate." said co-author Daphne Fairbairn, a professor of biology.

Now we can deduce why female spiders are usually much larger than male spiders, while the opposite is valid for lions, deer and sheep.

"Male spiders, by being small, are adapted for searching for widely-dispersed females, while the females remain as sit-and-wait predators on their webs. On the other hand, in many large mammals such as deer and sheep, females and young tend to be found in areas that have high quality food or other resources," she said.

"Males are able to find groups of females in these predictable places and defend them against other males. Large males, often with big weapons such as horns or antlers, are most successful in these male-male contests. In most birds males are bigger than females because their large size helps males display themselves in a more striking manner to attract females."

Still, in birds of prey the female is larger, as the male is more agile in hunting and delivering food for the nesting female protecting the nest (that's why lion females, smaller and more agile, hunt better, while the big male defends the territory). In other birds, polyandry (females mating with several males, like in phalaropes, lily-trotters or buttonquails) is the cause of females being larger: they fight between themselves for the males.