Secondary sex characteristics are features that distinguish the two sexes of a species, but that are not directly part of the reproductive system. They are believed to be the product of sexual selection for traits which give an individual an advantage over its rivals in courtship and aggressive interactions. They are distinguished from the primary sex characteristics: the sex organs, which are directly necessary for reproduction to occur.
Well known secondary sex characteristics include manes of male lions and long feathers of male peacocks. Other dramatic examples include the tusks of male narwhals, enlarged proboscises in male elephant seals and proboscis monkeys, the bright facial and rump coloration of male mandrills, and horns in many goats and antelopes. Male birds and fish of many species have brighter coloration or other external ornaments. Differences in size between sexes are also considered secondary sexual characteristics.
In humans, visible secondary sex characteristics include enlarged breasts of females and facial hair on males.
Charles Darwin hypothesized that sexual selection, or competition within a species for mates, can explain observed differences between sexes in many species. Biologists today distinguish between "male to male combat" and "mate choice", usually female choice of male mates. Sexual characteristics due to combat are such things as antlers, horns and greater size. Characteristics due to mate choice, often referred to as ornaments, include brighter plumage, coloration and other features that have no immediate purpose for survival or combat.
Ornamentation might arise because of some arbitrary female preference that is initially amplified by random genetic drift, eventually being reinforced by active selection for males with the appropriate ornament. This is known as the sexy son hypothesis. An alternative hypothesis is that some of the genes that enable males to develop impressive ornaments or fighting ability may be correlated with fitness markers such as disease resistance or a more efficient metabolism. This idea is known as the good genes hypothesis.
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