Body hair

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Body hair
Hair on the arm
Typical distribution of body hair in females and males
Anatomical terminology

Body hair or androgenic hair is

head hair and also from less visible vellus hair, which is much finer and lighter in color. Growth of androgenic hair is related to the level of androgens (male hormones
) and the density of androgen receptors in the dermal papillae. Both must reach a threshold for the proliferation of hair follicle cells.

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, palms of hands, soles of the feet, certain external genital areas, the navel, and scar tissue. 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 during puberty, such as the first growth of beard hair on a male and female adolescent's previously smooth chin; although it may appear thinner on the female.

Androgenic hair follows the same

women
.

Distribution

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 begins to grow darker and more abundantly. This growth occurs during or after puberty. Men will often have more abundant, coarser hair on the arms and back, while women tend to have a less drastic change in the hair growth in these areas but do experience a significant change in thickness of hairs. However, some women will grow darker, longer hair in one or more of these regions.

Chest and abdomen

Men's chest hair
Main article: Abdominal hair

Vellus hair grows on the chest and abdomen of both sexes at all stages of development. During the final stages of puberty and extending into adulthood, men grow increasing amounts of terminal hair over the chest and abdomen areas. Adult women can also grow terminal hairs around the areola, though in many cultures these hairs are removed.[citation needed]

Arms

Arm hair

Arm hair grows on a human's

forearms, sometimes even on the elbow area, and rarely on a human's bicep, triceps, and/or shoulders. Terminal arm hair is concentrated on the wrist end of the forearm, extending over the hand
. Terminal hair growth in adolescent 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

upper arm
. This type of intense arm vellus hair growth sometimes occurs in girls and children of both sexes until puberty. Even though this causes the arms to appear hairy, it is not caused solely by testosterone. The hair is softer and different from terminal arm hair, in texture.

The longest arm hair ever recorded was done so in California by David Reed in 2017. In 2024, Macie Davis-Southerland measured one hair at 7.24 inches long.[2]

Feet

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.

Legs

Main article: Leg hair
Leg hair of an adolescent male

Leg hair sometimes appears at the onset of adulthood, with the legs of men more often hairier than those of women. For a variety of reasons, people may

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.

Pubic

Main article: Pubic hair

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 partial cushion during intercourse.[3]

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 testicles and ovaries, promoting pubic hair growth. The average ages pubic hair begins to grow in males and females are 12 and 11, respectively. However, in some females, pubic hair has been known to start growing as early as age 8.

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.

Axillary

Main article: Underarm hair

Underarm hair normally starts to appear at the beginning of puberty, with growth usually completed by the end of the teenage years.

Today in much of the world, it is common for women to regularly shave their underarm hair. The prevalence of this practice varies widely, though. The practice became popular for cosmetic reasons around 1915 in the United States and United Kingdom, when one or more magazines showed a woman in a dress with shaved underarms. Regular shaving became feasible with the introduction of the safety razor at the beginning of the 20th century. While underarm shaving was quickly adopted in some English speaking countries, especially in the US and Canada, it did not become widespread in Europe until well after World War II.[4][5] Since then the practice has spread worldwide; some men also choose to shave their armpits.

Facial

Main article: Facial hair
Facial hair on an adult male

Facial hair grows primarily on or around one's face. Both men and women experience facial hair growth. 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 puberty, although some men may not grow a moustache until they reach late teens or at all. In some cases facial hair development may take longer to mature than the late teens, and some men experience no facial hair development even at an older age.

It is common for many women to develop a few facial hairs under or around the chin, along the sides of the face (in the area of sideburns), or on the upper lip. These may appear at any age after puberty but are often seen in women after menopause due to decreased levels of estrogen. A darkening of the vellus hair of the upper lip in women is not considered true facial hair, though it is often referred to as a "moustache"; the appearance of these dark vellus hairs may be lessened by bleaching. A relatively small number of women are able to grow enough facial hair to have a distinct beard. In some cases, female beard growth is the result of a hormonal imbalance (usually androgen excess), or a rare genetic disorder known as hypertrichosis.[6] Sometimes it is caused by use of anabolic steroids. Cultural pressure leads most women to remove facial hair, as it may be viewed as a social stigma.

Development

Main article: Human hair growth

Hair follicles are to varying degrees sensitive to androgens, primarily testosterone and its derivatives, particularly dihydrotestosterone, 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 androgens 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. (See Table 1 for development of male body hair during puberty.) 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 fifth 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.

Source:[7]

Function

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.[8][9]

Evolution

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

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. This coiled structure does not predispose curly or frizzy hair, both of which are defined by oval or triangular hair follicle cross-sections.[10]

Evolution of less body hair

Hair is a very good thermal conductor and aids heat transfer both into and out of the body. When

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.[11]

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".[12]

P. E. Wheeler of the Department of Biology at

Liverpool Polytechnic said quadrupedal savanna mammals of similar volume to humans have body hair to keep warm while only larger quadrupedal savanna mammals lack body hair, because their body volume itself is enough to keep them warm.[13] Therefore Wheeler said that humans, who should have body hair based on predictions of body volume alone for savanna 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.[13]

Loss of fur occurred at least two million years ago, but possibly as early as 3.3 million years ago judging from the divergence of head and pubic lice, and aided persistence hunting (the ability to catch prey in very long distance chases) in the warm savannas where hominins first evolved. The two main advantages are felt to be bipedal locomotion and greater thermal load dissipation capacity due to better sweating and less hair.[14]

Sexual selection

Main article: Sexual selection

Markus J. Rantala of the Department of Biological and Environmental Science,

pheromones and hair on the chin would make the chin appear more massive.[12]

Across populations

In 1876,

Indo-Europeans, and Southern Europeans (especially the Portuguese and Spanish) have extensive body hair.[15]

Anthropologist

Malays.[16] Deniker said that hirsute peoples tend to have thicker beards, eyelashes, and eyebrows but fewer hairs on their scalp.[16]

C.H. Danforth and Mildred Trotter of the Department of Anatomy at Washington University in St. Louis conducted a study using army soldiers of European origin in 1922 where they concluded that dark-haired white men are generally more hairy than fair-haired white men.[17]

H. Harris, publishing in the British Journal of Dermatology in 1947, wrote American Indians have the least body hair, Chinese and black people have little body hair, white people have more body hair than black people and Ainu have the most body hair.[18]

Anthropologist Arnold Henry Savage Landor described the Ainu as having hairy bodies.[19]

Stewart W. Hindley and Albert Damon of the Department of Anthropology at

Near Eastern peoples 62–71%, various Europeans 60–80%. However, they never completed an Androgenic hair map.[20]

According to anthropologist and professor

West Asian peoples. Montagu said that the hairless feature is a neotenous trait.[21]

Eike-Meinrad Winkler and Kerrin Christiansen of the Institut für Humanbiologie studied, in 1993,

Europeans.[22] 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.[22]

Valerie Anne Randall of the Department of Biomedical Sciences,

5α-reductase.[23]

Rodney P. R. Dawber of the Oxford Hair Foundation said in 1997 that East Asian males have little or no facial or body hair and Dawber also said that Mediterranean males are covered with an exuberant pelage.[24]

Milkica Nešić and her colleagues from the Department of Physiology at the

whites is significantly higher than in Black populations.[25]

Androgenic hair as biometric

It has been shown that individuals can be uniquely identified by their androgenic hair patterns. For example, even when one's particular distinguishing features such as face and tattoos are obscured, persons can still be identified by their hair on other parts of their body.[26][27]

See also

References

  1. PMID 29763123
    , retrieved 8 March 2024
  2. ^ "California woman proudly grows out the longest arm hair ever". Guinness World Records.
  3. ^ Morris, Desmond (1985). Bodywatching: a field guide to the human species. London: Jonathan Cape. p. 209.
  4. doi:10.1111/j.1542-734X.1982.0501_93.x
    .
  5. ^ Adams, Cecil (6 February 1991). "Who decided women should shave their legs and underarms?". The Straight Dope. Retrieved 22 January 2015.
  6. ^ Congenital Hypertrichosis Lanuginosa at eMedicine
  7. S2CID 42483235
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  8. PMID 4811387
    .
  9. ^ "Neuroscience for Kids – Receptors". Faculty.washington.edu. Retrieved 28 December 2011.
  10. ^ "Hair Shape". Hair-science.com. 1 February 2005. Archived from the original on 22 December 2008. Retrieved 18 September 2009.
  11. ^ "Properties Of Hair". Hair-science.com. 1 February 2005. Archived from the original on 4 April 2018. Retrieved 18 September 2009.
  12. ^ a b Rantala, M.J. (1999). Human nakedness: adaptation against ectoparasites? International Journal for Parasitology 29 1987±1989
  13. ^
    doi:10.1016/s0047-2484(85)80091-9
    .
  14. doi:10.1016/S0960-9822(03)00507-4
    . Erratum: Curr Biol 2004, 14:2309.
  15. ^ Peschel, O. (1876). The Races of Man and Their Geographical Distribution. London: Henry S. King & Co. pp. 96, 97 & 403. Retrieved 21 January 2017, from link.
  16. ^ a b Deniker, Joseph (1901). "Chapter I. Somatic Characteristics". The Races of Man: An Outline of Anthropology and Ethnography. London: The Walter Scott Pub. Co. Archived from the original on 20 January 2018. Retrieved 20 January 2018.{{cite book}}: CS1 maint: bot: original URL status unknown (link)
  17. doi:10.1002/ajpa.1330050318
    .
  18. S2CID 9479118
    .
  19. ISBN 978-1-108-04941-2.[page needed
    ]
  20. PMID 4750670
    .
  21. ISBN 0-89789-167-8
  22. ^
    PMID 8273828
    .
  23. ^
    S2CID 12391727
    .
  24. ^ Dawber R.P.R. (1997). Diseases of the Head and Scalp (3rd ed.). Virginia: Blackwell Science Ltd.
  25. doi:10.2298/ABS1003841N
    .
  26. S2CID 14132375
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  27. S2CID 806491
    .

External links

Look up body hair or androgenic hair in Wiktionary, the free dictionary.
  • Media related to Body hair at Wikimedia Commons
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