Contents 1 Evolutionary roots 2 In humans 2.1 Females 2.2 Males 3 References

Evolutionary roots[edit] See also: Mate choice A red deer stag's antlers are secondary sexual characteristics. Charles Darwin hypothesized that sexual selection, or competition within a species for mates, can explain observed differences between sexes in many species.[1] 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.[2] 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. Male jumping spiders have visual patches of UV reflectance, which are ornamentations used to attract females.[3]

In humans[edit] This section may lend undue weight to certain ideas, incidents, or controversies. Please help to create a more balanced presentation. Discuss and resolve this issue before removing this message. (December 2016) This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (December 2016) (Learn how and when to remove this template message) Anatomical characteristics of the human male and female Main article: Sexual selection in human evolution Sexual differentiation begins during gestation, when the gonads are formed. The general structure and shape of the body and face, as well as sex hormone levels, are similar in preadolescent boys and girls. As puberty begins and sex hormone levels rise, differences appear, though some changes are similar in males and females. Male levels of testosterone directly induce the growth of the genitals, and indirectly (via dihydrotestosterone (DHT)) the prostate. Estradiol and other hormones cause breasts to develop in females. However, fetal or neonatal androgens may modulate later breast development by reducing the capacity of breast tissue to respond to later estrogen.[citation needed] Females[edit] In females, breasts are a manifestation of higher levels of estrogen; estrogen also widens the pelvis and increases the amount of body fat in hips, thighs, buttocks, and breasts. Estrogen also induces growth of the uterus, proliferation of the endometrium, and menses. Enlargement of breasts and erection of nipples.[4] Growth of body hair, most prominently underarm and pubic hair Greater development of thigh muscles behind the femur, rather than in front of it (as is typical in mature males) Widening of hips;[5] lower waist to hip ratio than adult males Elbows that hyperextend 5-8° more than male adults[6] Face is more rounded, with softer features Smaller waist Upper arms approximately 2 cm longer, on average, for a given height[7] Changed distribution in weight and fat; more subcutaneous fat and fat deposits, mainly around the buttocks, thighs, and hips Labia minora, the inner lips of the vulva, may grow more prominent and undergo changes in color with the increased stimulation related with higher levels of estrogen.[8] Males[edit] A beard is a human male secondary sexual characteristic In males, testosterone directly increases size and mass of muscles, vocal cords, and bones, deepening the voice, and changing the shape of the face and skeleton. Converted into DHT in the skin, it accelerates growth of androgen-responsive facial and body hair but may slow and eventually stop the growth of head hair. Taller stature is largely a result of later puberty and slower epiphyseal fusion. Growth of body hair, including underarm, abdominal, chest hair and pubic hair. Loss of scalp hair due to androgenic alopecia can also occur. Greater mass of thigh muscles in front of the femur, rather than behind it (as is typical in mature females) Growth of facial hair Enlargement of larynx (Adam's apple) and deepening of voice[9] Increased stature; adult males are taller than adult females, on average Heavier skull and bone structure Increased muscle mass and strength hands, feet and nose grow larger Larger bodies Face is square, with more angular features Small waist, but wider than females Broadening of shoulders and chest; shoulders wider than hips[4] Increased secretions of oil and sweat glands[9] Coarsening or rigidity of skin texture due to less subcutaneous fat Higher waist-to-hip ratio than prepubescent or adult females or prepubescent males, on average Lower bodyfat percentage than prepubescent or adult females or prepubescent males, on average

References[edit] ^ Darwin, C. (1871) The Descent of Man and Selection in Relation to Sex John Murray, London ^ Weatherhead PJ, Robertson RJ (Feb 1979). "Offspring quality and the polygyny threshold: "The sexy son hypothesis"". American Naturalist. 113 (2): 201–208. doi:10.1086/283379.  ^ Lim, Matthew L. M., and Daiqin Li. "Courtship and Male-Male Agonistic Behaviour of Comsophasis Umbratica Simon, an Ornate Jumping Spider (Araneae: Salticidae)." The Raffles Bulletin of Zoology (2004): 52(2): 435-448. National University of Singapore. 20 September 2015 ^ a b "Secondary Characteristics". Archived from the original on 2011-09-27.  ^ "Sexual Maturity". Technical Issues in Reproductive Health. Columbia University. May 2, 2008.  ^ Amis AA, Miller JH (Dec 1982). "The elbow". Clinics in rheumatic diseases. 8 (3): 571–93. PMID 7184689.  ^ Manwatching: A Field Guide to Human Behaviour, 1977, Desmond Morris ^ Lloyd, Jillian (May 2005). "Female genital appearance: 'normality' unfolds". British Journal of Obstetrics and Gynecology. 12 (5): 643–646. doi:10.1111/j.1471-0528.2004.00517.x.  ^ a b Sexual reproduction Archived 2009-02-08 at the Wayback Machine. Retrieved from "" Categories: Secondary sexual characteristicsSexual selectionHidden categories: Webarchive template wayback linksArticles needing POV-check from December 2016Articles needing additional references from December 2016All articles needing additional referencesAll articles with unsourced statementsArticles with unsourced statements from December 2016

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