Mate choice—the decision about whom to mate with—is a type of natural selection, since animals must reproduce to pass down their genes. Mate choice is also called intersexual selection because the behavior occurs between the sexes.

In species with mate choice, one sex (usually, but not always, the female) is “choosy,” selecting a mate from individuals of the opposite sex based on appearance or behavioral characteristics. Often, females will choose “showier” males who have brighter coloration, more elaborate ornamentation (such as the peacock’s tail), or more complex songs or courtship displays.

Sexual Dimorphisms

Over time, this has led to the evolution of many sexual dimorphisms—phenotypic differences between males and females. Because males tend to have more reproductive success if they are showier, they typically have more extravagant characteristics than females.

There are several theories about why females tend to choose showier males. Brighter colors and elaborate courtship behaviors may indicate good health, genetic fitness, and the ability to defend a territory and provide resources for the female and her offspring. Females may also have inherent preferences for certain characteristics—such as colors—that give the males displaying those characteristics a reproductive advantage.

Once the animals reproduce, genes for both the preferred traits and the preference for them are passed down from the parents to their offspring—increasing these phenotypes in the population. Although showy characteristics can increase the risk of predation, this cost is often outweighed by the increased reproductive success of these individuals and their offspring.