Secondary sexual characteristics are characteristics that appear during puberty in humans and during sexual maturity in other animals . [1] [2] These features are particularly evident in sexually dimorphic phenotypic traits that differentiate the sexes of a species , [3] but are not directly part of the reproductive system , unlike sex organs (primary sex characteristics). [4] Sexual selection for traits displaying fitness to secondary sex characteristicsconsidered a product of , giving an organism an advantage over its rivals in courtship and aggressive interactions.
Secondary sex characteristics include, for example, the mane of male lions , [2] the bright face and rump of male mandrills , and horns in many goats and antelope . These characteristics are believed to be produced by a positive feedback loop known as Fisherian runaway that is produced by a secondary trait in one sex and desire for that trait in the other sex. Male birds and fish of many species have bright colors or other external ornamentation. Differences in the size of the sexes are also considered as secondary sexual traits.
In humans, visible secondary sex characteristics include pubic hair , enlarged breasts and wide hips on 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, could explain the differences observed between the sexes in many species. [7] [ better source needed ]
The English biologist Ronald Fisher, in his 1930 book The Genetic Theory of Natural Selection , developed several ideas related to secondary characteristics, including the concept of the Fisherian runaway, which postulates that a trait in females combined with that trait in males. Desire can create a positive feedback loop or runaway where the feature becomes immensely amplified. The 1975 constraint theory expands on this idea, stating that a peacock’s tail, for example, exhibits fitness by being a useless obstacle that is too hard to fake. One of Fischer’s ideas is the sexy son hypothesis, whereby women desire to have sons who have characteristics that they find sexually attractive in order to maximize the number of grandchildren. [8]An alternative hypothesis is that certain genes that enable males to develop impressive ornamentation 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 gene hypothesis.
in non-human animals
Secondary sexual characteristics in non-human animals include the manes of male lions [2] and the long plumage of male peacocks, the teeth of male narwhals, enlarged proboscises in male elephant seals and proboscis monkeys, the bright facial and caudal coloration of male mandrills, and many goats and horns in antelope. [9]
Biologists today distinguish between “male-to-male combat” and “mate choice”, usually the female choice of a male mate. The sexual traits are things like horns, horns and larger size due to fighting. Characteristics, often referred to as ornamentation, due to mate choice include brighter plumage , color, and other characteristics that have no immediate purpose for survival or warfare.
Male jumping spiders have visible patches of UV reflectance, an ornamentation used to attract females.
In humans
Sexual differentiation begins during pregnancy, when the gonads are formed. The general structure and shape of the body and face, as well as sex hormone levels, are similar in adolescent boys and girls. As puberty begins and sex hormone levels rise, differences appear, although some changes are similar in men and women. Male levels of testosterone directly induce the development of the genitals, and indirectly (via dihydrotestosterone (DHT)) the prostate. Estradiol and other hormones cause the development of breasts in women. However, fetal or neonatal androgens may modulate later breast development by reducing the ability of breast tissue to respond to estrogen.
Underarm hair and pubic hair are usually considered secondary sexual characteristics, [6] but may also be considered non-secondary sexual characteristics because they are characteristics of both sexes after puberty.
Sexual differentiation begins during pregnancy, when the gonads are formed. The general structure and shape of the body and face, as well as sex hormone levels, are similar in adolescent boys and girls. As puberty begins and sex hormone levels rise, differences appear, although some changes are similar in men and women. Male levels of testosterone directly induce the development of the genitals, and indirectly (via dihydrotestosterone (DHT)) the prostate. Estradiol and other hormones cause the development of breasts in women. However, fetal or neonatal androgens may modulate later breast development by reducing the ability of breast tissue to respond to estrogen.
Underarm hair and pubic hair are usually considered secondary sexual characteristics, [6] but may also be considered non-secondary sexual characteristics because they are characteristics of both sexes after puberty.
women
In women, breasts are a manifestation of high levels of estrogen; Estrogen also widens the pelvis and increases the amount of body fat in the hips, thighs, buttocks and breasts. [4] [6] Estrogen also stimulates uterine growth, proliferation of the endometrium, and menstruation. [4] Female secondary sex characteristics include:
- Enlargement of breast and erection of nipples. [4] [6]
- Growth of body hair, most prominently underarm and pubic hair. [2] [4] [6]
- widening of the hips; [4] [6] The waist-to-hip ratio is lower than that of adult males.
- Elbows that hyperextend 5-8° higher than in male adults. [15]
- For a given height, the upper arms are on average about 2 cm long. [16]
- The labia minora, the inner lip of the vulva, may become more prominent and undergo changes in color with increased arousal related to higher levels of estrogen. [17]
men
The increased secretion of testosterone from the testes during puberty causes the appearance of male secondary sex characteristics. [18] In men, testosterone directly increases the size and mass of muscles, vocal cords and bones, deepens the voice, and changes the shape of the face and skeleton. [4] Converted to DHT in the skin, it accelerates the growth of androgen-reactive facial and body hair but can slow and eventually inhibit the growth of scalp hair. Tall height is largely the result of later puberty and slower epiphyseal fusion. Male secondary sex characteristics include:
- Growth of body hair, including underarm, abdomen, chest hair and pubic hair. [2] [4]
- Facial hair growth. [4]
- Enlargement of the larynx (Adam’s apple) and deepening of the voice. [4] [19]
- increased stature; Adult males are, on average, taller than adult females. [4]
- Heavy skull and bone structure. [4]
- Increase in muscle mass and strength. [4]
- widening of the shoulders and chest; Shoulders wider than hips. [20]
- Increased secretion of oil and sweat glands.
