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!

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