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Secondary sex characteristics are features that appear at sexual maturity in animals and during puberty in humans, especially the sexually dimorphic phenotypic traits that distinguish the two sexes of a species (male and female), but that, unlike the sex organs, are not directly part of the reproductive system. They are believed to be the product of sexual selection for traits which display fitness, giving 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 sexual reproduction to occur. Well-known secondary sex characteristics include manes of male lions and long feathers of male peacock, 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, and these are often produced by a positive feedback loop known as the Fisherian runaway produced by the secondary characteristic in one sex and the desire for that characteristic in the other sex. 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 and adam's apple on males. ==Evolutionary roots== Charles Darwin hypothesized that sexual selection, or competition within a species for mates, can explain observed differences between sexes in many species.〔Darwin, C. (1871) ''The Descent of Man and Selection in Relation to Sex'' John Murray, London〕 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. Ronald Fisher, the English biologist developed a number of ideas concerning secondary characteristics in his 1930 book, including the Fisherian runaway, which postulates that the desire for a characteristic in females combined with that characteristic in males can create a positive feedback loop or runaway where the feature becomes hugely amplified. The 1975 handicap principle extends this idea, pointing out that a peacock's tail, for instance, displays fitness by being a useless impediment that it is very hard to fake. Another Fisher idea is the sexy son hypothesis, whereby females will desire to have sons that possess the characteristic that they find sexy in order to maximize the number of grandchildren they produce. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Secondary sex characteristic」の詳細全文を読む スポンサード リンク
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