Sexual dimorphism is used to refer to the differences in appearance between males and females of the same species, such as in color, shape, size, and structure. The differences are caused by the inheritance of one or the other sexual pattern in genetic material. The condition occurs in many animals, insects, birds and some plants. Differences may include secondary sex characteristics, size, color, markings, and may also include behavioral differences. These differences may be subtle or exaggerated, and may increase over time as a result of sexual selection.

We see for example that the mountain spiny lizard (Sceloporus jarrovi) is sexually dimorphic in feeding habits: the equal-sized males and females seek out different sizes of prey. Also, we note that pronounced size differences may occur between the sexes. For example, male baboons are more than twice as large as females. Male northern, or Steller, sea lions (Eumetopias jubatus) weigh about 2,200 pounds, roughly three times as much as females.

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