Secondary sex characteristic
A secondary sex characteristic is a physical characteristic of an organism that is related to or derived from its sex, but not directly part of its reproductive system.[1] In humans, these characteristics typically start to appear during puberty. In animals, they can start to appear at sexual maturity.[2][3] In humans, secondary sex characteristics include enlarged breasts and widened hips of females, facial hair and Adam's apples on males, and pubic hair on both.[1][4] In non-human animals, secondary sex characteristics include, for example, the manes of male lions,[3] the bright facial and rump coloration of male mandrills, and horns in many goats and antelopes.
Secondary sex characteristics are particularly evident in the sexually dimorphic phenotypic traits that distinguish the sexes of a species.[5] In evolution, secondary sex characteristics are the product of sexual selection for traits that show fitness, giving an organism an advantage over its rivals in courtship and in aggressive interactions.[6]
The characteristics are believed to be 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.
Secondary sex characteristics vs. primary sex characteristics[edit]
The reproductive organs in male or female mammals that are usually identifiable at birth are described as the primary sex characteristics or sex organs. In the male, these are the penis, testes, scrotum. In the female, these are the ovaries, fallopian tubes, uterus, cervix, vagina and vulva. The primary sex organs are different from the secondary sex organs because at maturity they produce gametes, which are haploid male or female germ cells which can unite with another of the opposite sex during sexual reproduction to form a zygote.
The secondary sex characteristics differ in that they will not be identifiable at birth, they will develop as the subject becomes sexually mature. In mammals, these characteristics include breasts in females and greater muscle mass in males. Secondary sexual characteristics have an evolutionary purpose: increase the chance of breeding.[7]
In non-human animals[edit]
Examples of secondary sex characteristics in non-human animals include manes of male lions[3] and long feathers of male peafowl, the tusks of male narwhals, enlarged proboscises in male elephant seals and proboscis monkeys, the bright facial and rump coloration of male mandrills, horns in many goats and antelopes,[10] and the swollen upper lip and elongated premaxillary and maxillary teeth of male spikethumb frogs.[11] Male fish develop "nuptial tubercles",[12] mainly on their snouts, in the breeding season. These are an honest signal of health, and may assist females in sexual selection for species that use lek mating, such as the roach Rutilus rutilus.[13]
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.[14]
Male jumping spiders have visual patches of UV reflectance, which are ornamentations used to attract females.[15]