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Modern human beings, especially women, cut their armpit hair. It seems to me the armpit hair is trivial/useless. Shortly speaking, what is the armpit hair for?

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While I won't answer fully at this time: Insulation is always one purpose. Bare pits = cold! Also, for men at least, we have glands that emit scents that correspond to our MHC allele makeup (immune system variation) - and women tend to prefer scents that differ from their family which results in more MHC diversity in the bloodline. A mat of hair can act as a scent-sponge, increasing the effect. It can also be a sign of cleanliness; bad smells are often bacteria in origin - having hair but no bad smell = clean. Hope this helps! –  MCM Nov 12 '12 at 18:59
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There are three theories on Wikipedia: en.wikipedia.org/wiki/Underarm_hair –  Alan Boyd Nov 12 '12 at 18:59
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Hair has been used for defense, think of lions, horses and roman centurion helmets. It's hard to cut or bite through. I'm not an expert, but it may be that beards, pubic and underarm hair cover exposed parts of the body that have vital arteries under the skin(neck, armpit and groin). –  Alex Stone Nov 13 '12 at 0:32
    
I don't think trivial is the word you wanted to use... possibly redundant, or useless? –  nico Nov 16 '12 at 7:21

4 Answers 4

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I have been trying to read up a bit on this. I started out looking on wikipedia and it seems there are three hypotheses which are not necessarily mutually exclusive.

1.Aid the wicking of sweat away from the skin

2.Reduce friction between the thorax and upper arm

3.Facilitate the release of sex pheromones

The first would seem possible because it would help to prevent over-heating of the lymph nodes under the armpits and that also fits with the fact there are a lot of sweat glands there.

The second, I can't find any citations which support this but I understand that people who shave their armpits tend to suffer from dry underarms. This could be a result of increased friction.

The third is apparently controversial although it seems perfectly reasonable and highly likely to me. There is a famous experiment by Claus Wedekind who got men to wear a t-shirt for a few days. He then got women to choose a most preferred male purely by the scent of the t-shirt and found interesting results relating to the MHC profiles. The armpit hair could help to trap the odour produced by the sweat and help with mate attraction, like some kind of scent sponge. I don't understand why this is so controversial, loads of animals influence mate choice with scent - perhaps people don't like to think we are genetically attracted to each other by sweat because it seems animalistic and less romantic.

In summary, there is no sure answer and it could well be all of the above, or something entirely different.

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It's not proper to think of evolution in teleological (purpose-driven) terms--armpit hair is probably a trait that was never eliminated because it is not harmful to the species, rather than a useful trait that was specifically "selected for".

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Good point about avoiding thinking in teleological terms. I think what is difficult to explain is why most body hair has been lost yet some retained. I would have thought that the loss of body hair would have affected the armpits just as it has the rest of the body (I know we have hair all over our bodies but it is only so dense in a few places). It suggests that there is a reason the armpit hair (and other areas) has been retained and that the genes affecting hair growth are up-regulated in these localities. This would suggest to me that there is some purpose of having armpit hair. Any ideas? –  GriffinEvo Nov 16 '12 at 10:11
    
ps, production/growth of the hair is an energetic and nutritional cost - though it is probably minor. –  GriffinEvo Nov 16 '12 at 10:13
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@rg255 Hair was probably retained in the most sensitive areas: the pubic area, the armpits, and outside various openings and membranes such as the eyes, ears, and mouth. –  QuietThud Nov 17 '12 at 7:59
    
@rg255 To speculate further, it is possible that men on average have more body hair because they on average have a lower body fat percentage, leaving them more exposed. This could also explain the high prevalence of limb hair in both sexes: limbs and their appendages (fingers, toes) are farther away from the heart, and therefore more vulnerable to the cold. –  QuietThud Nov 17 '12 at 8:23
    
I think that has a lot to do with testosterone (and other hormone) levels: give a woman testosterone pills and she will likely get rather hairy! –  GriffinEvo Nov 17 '12 at 8:32

Microbiome manipulation is a common theme in these answers/comments, and there are experimental methodologies for investigating such things. For example: "Topographical and Temporal Diversity of the Human Skin Microbiome" at http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2805064.

One selection pressure that hasn't been mentioned is the reduction of harmful bacteria that occurs when providing a growth advantage to more benign bacteria. Many hairy areas are those where bacteria in general can live and prosper. Various ways hair could impact the microbioime are are easily imagined, for example greatly increased surface area should favor aerobic vs anaerobic bacteria. More specific host-bacteria interaction, e.g. specific bacteria being selected for due to the presence of certain hair surface proteins or the microstructure of the hairs surface might be harder to investigate. How hair surface varies in different location might be informative.

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The explanation I heard from the documentary called "Curiosity: World's Dirtiest Man" is that armpit hairs allows for the growth of microbes which allow for the production of smell. This smell is useful for the attraction of mates in primates (although this function is probably lost in humans as our society became more hygienic).

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