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Androgenic hair, colloquially 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. 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 gender, 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 – the number of hair follicles per area of skin – varies from person to person.

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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 especial 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 and 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.

Growth distribution

Heavy terminal arm hair on a middle-aged man
Leg hair of an adolescent male

Arms

Arm hair grows on a human's forearms, sometimes even on the elbow area. This is typically thought as a male trait, as men have terminal hair on their arms, making it more noticeable, particularly for those who have black hair. Terminal arm hair is concentrated on the wrist end of the forearm, extending over the hand. 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 appear in the last stages of puberty. If boys have arm hair before this stage, it is long, heavy vellus hair, and this is rare. 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 arm vellus hair growth sometimes occurs in young women and girls.

Feet

Foot hair generally appears at the onset of adulthood. Male feet are generally hairier, with visible hair appearing on the top surfaces of the feet and toes from the onset of puberty.

Legs

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. Women generally shave their leg hair more regularly than men, to conform with the social norms of many cultures, which perceive smooth skin as a sign of youth and beauty. 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.

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 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.[1]

Hair is a very good thermal conductor and aids both heat transfer into and out of the body. This is often seen as a problem with straighteners and blow drying as the hair quickly transfers the heat into the inner hair shaft and heats the water in the hair to boiling point resulting in dry brittle hair if silicone insulating oils are not used. When goose pimples are observed, small muscles 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 phenomena 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 in order to prevent diseases caused by such as well as a need for grooming, two predominant factors in the civilization of homo sapiens.[2]

It had been believed that body hair was lost and replaced by increased fat storage. However, this is often found to be quite untrue, where people with excessive weight tend to also have increased body hair production. It is more correct to understand fat storage as a means of storing energy evolved due to infrequent sources of food being available commonly seen in people of African and Indian descent as well as some European peoples and hair production as a result of increased weight may be due to sensitive skin attempting to prevent abrasion where skin to skin contact may occur, e.g. under the chin.[3]

See also

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