Treblaine said:
Most mammals don't have such extreme gender dimorphism as humans do, except for lions where the males is much larger and stronger and has that distinct mane, analogous to how men naturally will grow a beard. Out closest ancestors, the Chimpanzees, do not have such gender dimorphism, it seems this has been a relatively recent adaptation in our evolution from non-human apes.
I think sexual dimorphism is more common in mammals than you make it out to be. In almost all species of Carnivora (the group of mammals that includes dogs and cats) the males are usually larger than the females. This is particularly pronounced in big cats (e.g. male tigers are usually 90-100kg, whereas females are only 65-80kg). In hyenas (and, strangely, blue whales!) this trend is reversed, with females usually larger than males.
Chimpanzees exhibit size difference too, with adult males on average about 10kg heavier than females. Although this difference seems insignificant next to gorillas, where males are often
twice as heavy as females. Female orangutans are similarly only half the size of males, and also lack the fleshy pads that males have on either side of their face. Looking at another type of primate, most lemurs exhibit sexual dimorphism of some sort (often in terms of colour rather than size). Baboon males typically have far larger canine teeth than females (although this varies from species to species). But mandrills are probably the most sexually dimorphic of all primates, with males both double the size
and vastly more colourful than females.
In fact, I've always thought that humans are quite boringly
un-dimorphic when it comes to sex. It really is just a difference in average height and muscle mass (with slight differences in fat distribution, and some additional body hair).
On a related note, do people think gender equality in our society would take (or have taken, depending on your view) longer to come about if humans showed greater sexual dimorphism?