Underarm hair
Underarm hair, also known as axillary hair, is the hair in the underarm area ("axilla"). Development and function. Underarm hair, as human body hair, usually starts to appear at the beginning of puberty, with growth usually completed by the end of the teenage years. Axillary hair goes through four stages of development, driven by weak androgens produced by the adrenal in males and females during adrenarche, and testosterone from the testicle in males during puberty. Like Tanner Staging for pubic hair, axillary hair can be staged according to the Wolfsdorf Staging system, named after pediatric endocrinologist Joseph Wolfsdorf , as follows: Staging a patient's axillary hair will allow the physician to track the child's development longitudinally. The importance of human underarm hair is unclear. It may naturally wick sweat or other moisture away from the skin, aiding ventilation. Colonization by odor-producing bacteria is thereby transferred away from the skin (see skin flora). Overall, the main key feature armpit hair provides is to ease from skin-skin friction, and thus irritation, as the armpit when in its natural position and used in its natural function is constantly exposed to itself and naturally rubs against itself on a  daily basis, even more so where a person is doing any sort of physical labor, be it leisure or intense. As stated previously, air ventilation and oil dispersion go hand-in-hand with this natural phenomenon. Cultural attitudes. Today in much of the Western world, it is common for women to regularly shave or wax their underarm hair. Some men trim or shave the underarm, and the practice has increased among younger men. The prevalence of this practice varies widely, though. Religious reasons are sometimes cited; for example, in Islamic culture both Muslim men and women remove underarm hair to  meet religious guidelines of cleanliness. Removal of underarm hair was part of a collection of hygienic or cosmetic practices recommended by the Islamic prophet Muhammad (570–632) as consistent with "fitra" for both Muslim women and men, and has since usually been regarded as a requirement by most Muslims. In much of the Western world, some men also choose to remove their underarm hair for aesthetic reasons. Many competitive swimmers remove nearly all of their body hair, including their underarm hair, believing it makes their bodies more streamlined during races. Many male bodybuilders and professional wrestlers also remove their body hair  for cosmetic purposes. Seneca the Younger suggests it was common practice in ancient Rome: "One is, I believe, as faulty as the other: the one class are unreasonably elaborate, the other are unreasonably negligent; the former depilate the leg, the latter not even the underarm." (). In the West, the practice of removing armpit hair began for cosmetic reasons around 1915 in the United States and the United Kingdom, when one or more magazines (such as "Harper’s Bazaar") 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. In art. In works of art, the underarm hair is usually removed; showing it is a mark of modernism. This contrasts to the depiction of pubic hair in art, which is rarely portrayed in works created in the Middle Ages, increasingly common in Renaissance art, and quite frequent in modern times.