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Androgenic hair, commonly known as body hair, is the terminal hair that develops on the human body during and after puberty. It is differentiated from the head hair and less visible vellus hair, which are much finer and lighter in color. The growth of androgenic hair is related to the level of androgens (male hormones) in the individual. Due to a normally higher level of androgen, men tend to have more androgenic hair than women.
From childhood onward, regardless of sex, vellus hair covers almost the entire area of the human body. Exceptions include the lips; the backs of the ears; the palms of hands; the soles of the feet; certain external genital areas; the navel; and scar tissue. The density of hair – i.e. the number of hair follicles per unit area of skin – varies from person to person. In many cases, areas on the human body that contain vellus hair will begin to produce darker and thicker body hair, such as the first growth of beard hair on a male adolescent's previously smooth chin.
Androgenic hair follows the same growth pattern as the hair that grows on the scalp, but with a shorter anagen phase and longer telogen phase. While the anagen phase for the hair on one's head lasts for years, the androgenic hair growing phase lasts a few months. The telogen phase for body hair lasts close to a year. This shortened growing period and extended dormant period explains why the hair on the head tends to be much longer than other hair found on the body. Differences in length seen in comparing the hair on the back of the hand and pubic hair, for example, can be explained by varied growth cycles in those two regions. The same goes for differences in body hair length seen in different people, especially when comparing men and women.
Development and growth
Hair follicles are to varying degrees sensitive to androgen, primarily testosterone and its derivatives, with different areas on the body having different sensitivity. As androgen levels increase, the rate of hair growth and the weight of the hairs increase. Genetic factors determine both individual levels of androgen and the hair follicle's sensitivity to androgen, as well as other characteristics such as hair colour, type of hair and hair retention.
Rising levels of androgen during puberty cause vellus hair to transform into terminal hair over many areas of the body. The sequence of appearance of terminal hair reflects the level of androgen sensitivity, with pubic hair being the first to appear due to the area's special sensitivity to androgen. The appearance of pubic hair in both sexes is usually seen as an indication of the start of a person's puberty. There is a sexual differentiation in the amount and distribution of androgenic hair, with men tending to have more terminal hair in more areas. This includes facial hair, chest hair, abdominal hair, leg hair, arm hair, and foot hair. Women retain more of the less visible vellus hair, although leg, arm, and foot hair can be noticeable on women. It is not unusual for women to have a few terminal hairs around their nipples as well. In the later decades of life, especially after the 5th decade, there begins a noticeable reduction in body hair especially in the legs. The reason for this is not known but it could be due to poorer circulation, lower free circulating hormone amounts or other reasons.
Like much of the hair on the human body, leg, arm, chest, and back hair begin as vellus hair. As people age, the hair in these regions will often begin to grow darker and more abundantly. This will typically happen during or after puberty. Men will generally have more abundant, coarser hair on the legs, arms, chest, and back, while women tend to have a less drastic change in the hair growth in these areas. However, some women will grow darker, longer hair in one or more of these regions.
Chest and abdomen
Vellus hair grows on the chest and abdomen of both sexes at all stages of development. After puberty and extending into adulthood, most males grow increasing amounts of terminal hair over the chest and abdomen areas. Adult women also typically can grow terminal hairs around the areola though in Western culture these hairs are typically removed.
Arm hair grows on a human's forearms, sometimes even on the elbow area. Terminal arm hair is concentrated on the wrist end of the forearm, extending over the hand. Terminal hair growth in adolescent and adult males is often much more intense than that in females, particularly for individuals with dark hair. In some cultures, it is common for women to remove arm hair, though this practice is less frequent than that of leg hair removal.
Terminal hair growth on arms is a secondary sexual characteristic in boys and appears in the last stages of puberty. Vellus arm hair is usually concentrated on the elbow end of the forearm and often ends on the lower part of the upper arm. This type of intense arm vellus hair growth sometimes occurs in young women and people of both sexes until puberty. Even though this causes the arms to appear hairy, it is not caused by testosterone. The hair is softer and different from men's arm hair, in texture.
Visible hair appearing on the top surfaces of the feet and toes generally begins with the onset of puberty. Terminal hair growth on the feet is typically more intense in adult and adolescent males than in females.
Leg hair generally appears at the onset of adulthood, with the legs of men most often hairier than those of women. For a variety of reasons, people may shave their leg hair, including cultural practice or individual needs. Around the world, females generally shave their leg hair more regularly than men, to conform with the social norms of many cultures, many of which perceive smooth skin as a sign of youth, beauty, and in some cultures, hygiene. However, athletes of both sexes – swimmers, runners, cyclists and bodybuilders in particular – may shave their androgenic hair to reduce friction, highlight muscular development or to make it easier to get into and out of skin-tight clothing. The main reason why professional cyclists shave their legs is to make treatment of road rash less difficult.
Pubic hair is a collection of coarse hair found in the pubic region. It will often also grow on the thighs and abdomen. Zoologist Desmond Morris disputes theories that it developed to signal sexual maturity or protect the skin from chafing during copulation, and prefers the explanation that pubic hair acts as a scent trap. Also, both sexes having thick pubic hairs act as a sort of cushion during intercourse.
The genital area of males and females are first inhabited by shorter, lighter vellus hairs that are next to invisible and only begin to develop into darker, thicker pubic hair at puberty. At this time, the pituitary gland secretes gonadotropin hormones which trigger the production of testosterone in the ovaries and testicles, respectively, promoting pubic hair growth. The average ages pubic hair begins to grow in males and females are 12 and 11, respectively. However, in some males and females, it has been known for pubic hair to start as early as 9 and 8, respectively.
Just as individual people differ in scalp hair color, they can also differ in pubic hair color. Differences in thickness, growth rate, and length are also evident.
Pubic hair removal occurs in certain cultures; for example, it has long been practiced widely, if not universally, in the Muslim world due to a religious injunction. In Western culture, beginning in the 1980s, there has been a trend among women to remove or trim pubic hair, either partially or completely; the trend later spread to men as well.
Facial hair, as the name suggests, grows primarily on or around one's face. Both men and women experience facial hair growth, even though it is commonly accepted as a masculine trait. Like pubic hair, non-vellus facial hair will begin to grow in around puberty. Moustaches in young men usually begin to grow in at around the age of 13, although some men may not grow a moustache until they reach late teens. For some cases facial hair development may take longer to mature than in the late teens.
Androgenic hair provides tactile sensory input by transferring hair movement and vibration via the shaft to sensory nerves within the skin. Follicular nerves detect displacement of hair shafts and other nerve endings in the surrounding skin detect vibration and distortions of the skin around the follicles. Androgenic hair extends the sense of touch beyond the surface of the skin into the air and space surrounding it, detecting air movements as well as hair displacement from contact by insects or objects.
Determining the evolutionary function of androgenic hair must take into account both human evolution and the thermal properties of hair itself.
The thermodynamic properties of hair are based on the properties of the keratin strands and amino acids that combine into a 'coiled' structure. This structure lends to many of the properties of hair, such as its ability to stretch and return to its original length. It should be noted that this coiled structure does not predispose curly or frizzy hair, both of which are defined by oval or triangular hair follicle cross-sections.
Evolution of less body hair
Hair is a very good thermal conductor and aids heat transfer both into and out of the body. When goose bumps are observed, small muscles (Arrector pili muscle) contract to raise the hairs both to provide insulation, by reducing cooling by air convection of the skin, as well as in response to central nervous stimulus, similar to the feeling of 'hairs standing up on the back of your neck'. This phenomenon also occurs when static charge is built up and stored in the hair. Keratin however can easily be damaged by excessive heat and dryness, suggesting that extreme sun exposure, perhaps due to a lack of clothing, would result in perpetual hair destruction, eventually resulting in the genes being bred out in favor of high skin pigmentation. It is also true that parasites can live on and in hair thus peoples who preserved their body hair would have required greater general hygiene to prevent diseases. 
Markus J. Rantala of the Department of Biological and Environmental Science, University of Jyväskylä, Finland, said humans evolved by "natural selection" to be hairless when the trade off of "having fewer parasites" became more important than having a "warming, furry coat".
P.E. Wheeler of the Department of Biology at Liverpool Polytechnic said quadrupedal Savannah mammals of similar volume to humans have body hair to keep warm while only larger quadrupedal Savannah mammals lack body hair, because their body volume itself is enough to keep them warm. Therefore, Wheeler said humans who should have body hair based on predictions of body volume alone for Savannah mammals evolved no body hair after evolving bipedalism which he said reduced the amount of body area exposed to the sun by 40%, reducing the solar warming effect on the human body.
Decreased body hair began in humanoids over 2 million years ago and aided persistence hunting (the ability to catch prey in very long distance chases) in the warm savannas where humanoids first evolved. This is well before homo sapiens evolved. The two main advantages are felt to be bipedal locomotion and better greater thermal load dissipation capacity due to better sweating and less hair.
Cambridge University zoologist Charles Goodhart believed men have long preferred the "hairless trait" in women, ever since the existence of the "hairless trait" occurred in our hairy forebears 70,000–120,000 years ago during the last episode of global warming. Goodhart argued that humans are relatively hairless today compared to our ancestors because women who were sexually selected for their "hairless trait" passed it on to both their male and female offspring.
Markus J. Rantala of the Department of Biological and Environmental Science, University of Jyväskylä, Finland, said the existence of androgen dependent hair on men could be explained by sexual attraction whereby hair on the genitals would trap pheromones and hair on the chin would make the chin appear more massive.
|Lower arm and leg||97%|
|Upper arm and leg||85%|
|mid-phalangeal (fingers and toes)||67%|
According to Ashley Montagu who taught anthropology at Princeton University, the Asian people, San people, and black people are less hairy than white people and Montagu said that the hairless feature is a neotenous trait.
Rodney P.R. Dawber of the Oxford Hair Foundation and Clinical Lecturer in Dermatology said East Asian males have little or no facial or body hair and Dawber also said that Mediterranean males are covered with an exuberant pelage.
C.H. Danforth and Mildred Trotter of the Department of Anatomy at Washington University did a study using army soldiers of European origin where they concluded that dark-haired white men are generally more hairy than fair-haired white men.
H. Harris who published in the British Journal of Dermatology said American Indians have the least body hair, Chinese and black people have little body hair, white people have more body hair than blacks and Ainu have the most body hair.
Milkica Nešić et al. of the Department of Physiology at the University of Niš, Serbia, cited prior studies to indicate that the frequency of hair on the middle finger joint (mid-phalangeal hair) in whites is significantly higher than in black populations.
Eike-Meinrad Winkler and Kerrin Christiansen of the Institut für Humanbiologie, Hamburg, Germany, did a study using Kavango people and !Kung people of body hair and hormone levels to investigate the reason black Africans did not have bodies as hairy as Europeans. Winkler and Christiansen concluded the difference in hairiness between black Africans and Europeans had to do with differences in androgen or estradiol production, in androgen metabolism, and in sex hormone action in the target cells.
Valerie Anne Randall of the Department of Biomedical Sciences, University of Bradford, said beard growth in Caucasian men increases until the mid-thirties due to a delay caused by growth cycles changing from vellus hair to terminal hair. Randall said white men and women are hairier than Japanese men and women even with the same total plasma androgen levels. Randall says that the reason for some people being hairy and some people not being hairy is unclear, but that it probably is related to differing sensitivity of hair follicles to 5α-reductase.
Stewart W. Hindley and Albert Damon of the Department of Anthropology at Stanford University have studied the frequency of hair on the middle finger joint (mid-phalangeal hair) of Solomon Islanders, as a part of a series of anthropometric studies of these populations. They summarize other studies on prevalence of this trait as reporting, in general, that Caucasoids are more likely to have hair on the middle finger joint than Negroids and Mongoloids, and collect the following frequencies from previously published literature: Andamanese 0%, Eskimo 1%, African American 16% or 28%, Ethiopians 25.6%, Mexicans of the Yucatan 20.9%, Penobscot and Shinnecock 22.7%, Gurkha 33.6%, Japanese 44.6%, various Hindus 40-50%, Egyptians 52.3%, Near Eastern peoples 62-71%, various Europeans 60-80%. Although they never made an Androgenic hair map. 
Anthropologist Joseph Deniker said that the very hairy peoples are the Ainus, Iranians, Australian aborigines (Arnhem Land being less hairy), Toda, Dravidians and Melanesians while Deniker said the American Indian, San, east Asian people that includes Chinese who are from China mainland, Mongol and Malay people the most are glabrous (not hairy). Deniker said hairy peoples have thicker beards, eyelashes and eyebrows, but Deniker said hairy people tend to have fewer hairs on their scalp.
- (Hair Growth: EMedicine Otolaryngology and Facial Plastic Surgery)
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- Wheeler, P.E. (1985). The Loss of Functional Body Hair in Man: the Influence of Thermal Environment, Body Form and Bipedality. Journal of Human Evolution. 14, 23-28
- Danny Penman (1995-11-10). "Zoologist claims sexual preference led the naked ape to lose its hair - News". London: The Independent. Retrieved 2011-12-28.
- Garn, S. M. (1951), TYPES AND DISTRIBUTION OF THE HAIR IN MAN. Annals of the New York Academy of Sciences, 53: 498–507. doi:10.1111/j.1749-6632.1951.tb31952.x
- Montagu, Ashley. Growing Young. Published by Greenwood Publishing Group, 1989 ISBN 0-89789-167-8
- Dawber R.P.R. (1997). Diseases of the head and scalp (3rd ed.). Virginia:Blackwell Science Ltd.
- Arnold Henry Savage Landor (1970). Alone with the hairy Ainu: or, 3.800 miles on a pack saddle in Yezo and a cruise to the Kurile islands
- Danforth, C. H. and Trotter, M. (1922), The distribution of body hair in white subjects. American Journal of Physical Anthropology, 5: 259–265. doi:10.1002/ajpa.1330050318
- HARRIS, H. (1947), THE RELATION OF HAIR-GROWTH ON THE BODY TO BALDNESS. British Journal of Dermatology, 59: 300–309. doi:10.1111/j.1365-2133.1947.tb10910.x
- Nešić, M. et al. (2010). Middle phalangeal hair distribution in Serbian high school students. Arch. Biol. Sci., Belgrade, 62 (3), 841-850, doi:10.2298/ABS1003841N
- Winkler, E.-M. and Christiansen, K. (1993), Sex hormone levels and body hair growth in !Kung San and Kavango men from Namibia. American Journal of Physical Anthropology, 92: 155–164. doi:10.1002/ajpa.1330920205
- Randall, V. A. (1994), Androgens and human hair growth. Clinical Endocrinology, 40: 439–457. doi:10.1111/j.1365-2265.1994.tb02483.x
- Hindley, S. W. and Damon, A. (1973), Some genetic traits in Solomon Island populations IV. Mid-phalangeal hair. American Journal of Physical Anthropology, 39: 191–194. doi:10.1002/ajpa.1330390208
- Joseph Deniker. (1901). The races of man: an outline of anthropology and ethnography