Androgenic hair

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Androgenic hair, colloquially body hair, is the terminal hair on the human body developed during and after puberty. It is differentiated from the head hair and less visible vellus hair. Androgenic denotes its growth is related to the level of androgens (male hormones) in the individual. Due to a normally higher level of androgens, men tend to have more androgenic hair than women.

From childhood onward, regardless of sex, vellus hair covers the entire 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 (number of hair follicles per area of skin) varies from one person to another.

Contents 1 Development and growth 2 Evolution 3 Growth distribution 3.1 Arms 3.2 Feet 3.3 Legs 4 References

[edit] Development and growth

Rising levels of androgens during puberty cause vellus hair to transform into terminal hair over many areas of the body. Hair follicles respond to androgens, primarily testosterone and its derivatives. Different areas respond with different sensitivities. The rate of hair growth and the weight of the hairs increase. The sequence of appearance of androgenic hair reflects the gradations of androgen sensitivity. Since the pubic area is most sensitive to androgen, heavier hair usually grows there first. Genetic factors determine both individual levels of androgens and the hair follicle's sensitivity to androgens, and thus hair development.

Most noticeable in both sexes are the development of axillary hair and pubic hair. Thus it is sometimes viewed as a secondary sex characteristic. There is a sexual differentiation in the amount and distribution of androgenic hair, with men having 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, though leg, arm, and foot hair is noticeable.

[edit] Evolution

To truly understand the evolution of hair, both evolution theory must be considered as well as the thermodynamic properties of hair itself. At current the leading theory on human evolution is the 'Out of Africa' theory, suggesting that the African peoples as well as the Inuit peoples, were earlier emegences, both of which are almost hairless. Since human evolution is believed to have stemmed from an animal somewhat similar genetically, and thus in appearance, to the Chimpanzee^[1], it must also be noted that the hair patternation, less hair on the chest, armpits and groin, suggests an alternative mode of evolution for Homo Sapiens body hair, as opposed to a preserved archaic trait.

It is however noted that among European populations, Northern Europeans tend to have less and straighter hair. By genetic testing this is believed to be due to an ancient Inuit admixture that has both changed the hair follicles cross-section from an oval to a more rounded form, reducing the wavy/curly nature of the hair and lending to a decrease in general body hair formation and teminalisation by dihydro-testosterone's effects (the more potent '5 alpha reductase' of testosterone)^[2].

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.^[3]

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.^[4]

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.^[5]

[edit] Growth distribution

[edit] Arms

Arm hair is hair that grows on a human's forearms. Sometimes this covers even the elbow area. This is typically thought as a male trait as men have terminal hair on their arms, making them noticeable, particularly if they 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 vellus hair growth on arms sometimes occurs in young women and girls.

[edit] Feet

Foot hair is hair that grows on the feet of humans, generally appearing 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.

[edit] Legs

Leg hair is hair that grows on the legs of humans, generally appearing at the onset of adulthood. Male legs are most often hairier than female ones. For a variety of reasons, people may shave their leg hair. Women generally shave their leg hair more regularly than men because the social norm of many cultures expects them to show a sign of beauty. The amount of hair on an individuals legs can depend on their ethnic background. Men of Middle Eastern, South Asian, West European, North American, South American, or general Mediterranean descent generally having more, however this may also be due to members of these ethnic groups having darker hair. Men of East Asian and Northern European descent can have very little hair on their legs with some having virtually none.

The current Guinness World Record for world's longest leg hair belongs to Wes Pemberton of Scurry, Texas at 6.5 inches (16.51 cm).^[6]

[edit] References