Hair growth and dhea

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Sometimes, the term body hair is used, to distinguish hair on the body from hair on the head. The difference between body hair and scalp hair (and, in males, chin and moustache hair) is that head hair for practical purposes grows continuously, whereas body hair alternates regular periods of growth and dormancy. During the growth portion of the cycle, body hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After a few weeks, body hair growth stops. The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows to great length, whereas body hair does not. Anthropologists speculate that the functional significance of long head hair is almost certainly adornment, a by-product of natural selection.

Among humans, nature selected for little body hair as part of a set of adaptations including bipedal locomotion and an upright posture. Bipedal locomotion is extremely inefficient, and many animals can outrun human beings for short periods of time; such animals, however, are inefficient radiators of heat, and cannot run for long periods of time. Thus, human hunters must be able to chase animals for long periods of time, and must therefore have an efficient mechanism for radiating body heat. Upright posture, which exposes less surface area of the body to direct solar radiation, and subcutaneous sweat glands, which operate best with an absence of hair.

Hair serves a number of different functions. It provides insulation from cold weather and, in some species, from particularly hot weather. Because hair is often pigmented, it provides coloration. This might serve to camouflage an individual; in some mammals, the pigmentation changes with the seasons, becoming white during the snowy winter, for example.