Beyond the obvious difference between
boys and
girls (which, hopefully has already been explained to you), sexual dimorphism is a marked
physical difference between the sexes of a given
species.
The most common
disparity between the sexes is body size. In many
apes, for example, the
male is larger than the female: in
gorillas, the
male can be up to one and a half times the
female's size, and in
orangutans, the
male can be twice the
female's size. Outside of
primates, the
female is very often larger: this is common in some
fishes, and in
spiders, the
male spider is often extremely small in comparison with
females of the same
species.
Sexual dimorphism tends to occur in
species that engage in
polygamous or
polyandrous behavior, both of which tend to arise from an unstable
environment. In
species that generally are
monogamous (which tends to occur in more stable
environments), sexual dimorphism is less pronounced.
Most species of penguins do not present sexual dimorphism (but among others, rockhopper penguins and crested penguins do), and also most are generally thought to be monogamous. But that's all you'll get out of me on the mating habits of penguins for today!