Androgenic hair
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2008-07-14T23:50:10Z
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/* Development and growth */ Sensitive pubic area is ambiguous
{{Unreferenced|date=June 2008}}
{{Expand|date=January 2007}}
[[Image:Androgenic hair.JPG|thumb|Distribution of androgenic hair on human female and male bodies]]
{{Human hair}}
'''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 [[androgen]]s (male [[hormone]]s) in the individual. Due to a normally higher level of androgens, [[man|men]] tend to have more androgenic hair than [[woman|women]].
From [[childhood]] onward, regardless of [[sex]], [[vellus hair]] covers the entire human body. Exceptions include the [[lips]], the backs of the [[ear]]s, the palms of [[hand]]s, the soles of the [[foot|feet]], certain external genital areas, the [[navel]] and [[scar]] tissue. The density of [[hair]] (number of [[hair follicle]]s per [[area]] of skin) varies from one person to another.
== Development and growth ==
Rising levels of [[androgens]] during puberty cause [[vellus hair]] to transform into [[terminal hair]] over many areas of the body. [[Hair follicle]]s 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.
[[Image:Malepuberty.jpg|thumb|280px|Development of androgenic hair on a human male]]
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 males having more terminal hair in more areas. This includes [[facial hair]], [[chest hair]], [[abdominal hair]], [[leg hair|leg]] and [[arm hair]], and [[foot hair]]. Females retain more of the less visible [[vellus hair]], though [[leg hair|leg]], [[arm hair|arm]], and [[foot hair]] is noticeable.
==Evolution==
<!-- Commented out because image was deleted: [[Image:Laetoliafar.jpg.jpg|thumb|200px|Re-creation of ''[[Australopithecus afarensis]]'' from [[Laetoli]], ca. 3 million years ago ([[American Museum of Natural History]]){{3d alt|Laetoli 3d.jpg}}]] -->
Evolutionary and genetic evidence suggests human androgenic hair is the remnant of body hair of the type found on today's [[great apes]]. Full-on body hair phased out in the course of [[human evolution]] roughly 2-3 million years ago{{Fact|date=July 2008}}. Loss of body hair is related to several unique aspects of human evolution. Homo Erectus, and later pre-modern humans, began relying on sweating for [[thermoregulation]] instead of insulating hair. Because of the intensity of sunlight, and harmful ultraviolet radiation, loss of hair put positive selective pressures on development of protective dark [[skin colour]]. Based on the numbers and spread of variations in the [[melanocortin 1 receptor|MC1R]] nucleotide sequences between humans and chimpanzees, modern hair patterns were seen by 1.2 million years ago. However, hair densities and patterns on humans differ for populations in differing climates.
[[Category:Body hair]]
[[Category:Human development]]
[[ca:Pèl corporal]]
[[de:Körperbehaarung]]
[[es:Vello androgénico]]
[[nl:Lichaamsbeharing]]
[[pl:Owłosienie ciała]]