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Why are male animals more colorful?
Sexual selection, according to English naturalist Charles Darwin, is an evolutionary process that “depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.” That is, the reproductive edge that some animals have over others of their kind gives them a leg up in the mating game and hence increases their chances of leaving more offspring. The focus on reproductive advantage in Darwin’s definition distinguishes sexual selection from natural selection, which depends on the advantage certain individuals in a species have over others in adapting to their environment and surviving to reproduce.
A male animal’s reproductive advantage is expressed either by how well he competes for a mate with other members of his sex (intrasexual selection; also called male-male competition) or by how attractive he appears to individuals of the opposite sex (intersexual selection; also called female mate choice). Since, in most species, the number of females (and of their eggs) tends to be limited, males engage in competition for a mate or display striking physical traits to attract a mate. The bright colors of the feathers, fur, or fins of many male animals, particularly birds and mammals, serve to increase the animals’ “sex appeal.” This function is analogous to that of the antlers of the moose (Alces alces) and the horns of the rhinoceros beetle (subfamily Dynastinae).
The bright colors of a male signal to a female that he is sufficiently healthy and genetically robust. Although such colors can count against natural selection—making him more visible to predators and hence less likely to survive—a male who displays them and still thrives appears to be a suitable mate. Where body coloration results from what an animal eats, such as in the male house finch, bright colors also signal the quality of the male’s territory: a brighter male tends to have a higher-quality territory, which can provide more food for the female and the young than other territories. Interestingly, in the shore bird phalarope (Phalaropus species), the sex roles are reversed: it is the females that court males, and hence it is the females that display brighter plumage.
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A male’s bright color can also signify his vitality to other males competing for the same resources (such as a female or territory). In Jackson’s chameleons (Trioceros jacksonii xantholophus), for instance, the vivid yellow-green body color of the male is often used to signal dominance; in fact, a male that loses a contest turns a muted brown. The bright red shoulder patch of the male red-winged blackbird (Agelaius phoeniceus) and the red nose of the male mandrill (Mandrillus sphinx) are similar tools for intimidating competitors.
