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Given that most Americans seem to do fine using anti-perspirant, it got me wondering: is there an evolutionary purpose to underarm perspiration? I can't imagine it acts to attract the opposite sex.

Or is it just likely an unfortunate side effect of whole-body evaporative cooling via sweat?

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There may be an evolutionary purpose to underarm perspiration, as alluded to in the "hidden hint" link of @DevashishDas, and also hinted at by the answer by @Chris. The apocrine sweat glands of our armpits produce steroids. One group of steroids secreted by the axillary apocrine glands are sex pheromones. Although we may not consciously notice the chemical odors of sex pheromones, they do likely affect our behaviors.

For example, Wedekind et al. showed that females showed distinct preferences for certain male odors. Women tended to prefer the odors of males that were genetically different at the major histocompatibility complex (MHC) loci, a group of genes associated with our immune response system. Greater genetic variation of MHC genes generally translates to greater ability to combat disease. Females that select males that are genetically different at the MHC would produce offspring with greater genetic variability at the complex, and therefore more likely to have greater resistance to disease or ability to fight disease.

These results suggest that the pheromones secreted in underarm perspiration is a sexually selected trait in humans. If so, underarm perspiration would have an evolutionary (fitness) advantage.

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First: Not everything what happens in biology is necessarily directly selected for. Sweating is of course selected for, so our body can maintain its normal body temperature in hot climates.

There is a difference between sweat glands under our armpits and for example on the forearm. Under our armpits we have the so called "apocrine sweat glands" while most of the rest of our body is covered with "eccrine sweat glands". They are not only different in size, but produce different types of sweat. Eccrine sweat glands produce sweat, that is made mostly of water and salt, while the apocrine sweat glands sweat contains additionally lipids, proteins and also steroids. Decomposition of the sweat by bacteria which live on our skin produces the typical sweaty odor. The sweat glands also differ in their activation (see here for more details), so can the apocrine glands be triggered by stress.

According to this source ("A short history of sweat gland biology") is roughly comparable, although the total numbers vary quite a lot between the single studies. Assuming that the amount of sweat is comparable, than another factor comes into play: The different evaporation from our arms and from under our armpits. The last form a kind of cavity where the sweat can not easily evaporate and will then stay. It might also play a role, that they can be activated by stress, which will happen in situations which are uncomfortable for us and make the situation worse. A paper which also looks very interesting in this context (to which I unfortunately have no access) is: "The evolution of sweat glands."

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Armpit sweat does not have a strong intrinsic odor. What you smell is the waste of the microorganisms which colonize your armpits. It is a similar mechanism responsible for foot odor and bad breath. All external bodily surfaces have particular microbes which prefer different areas. This also includes the intestinal, oral and nasal regions.

If you charted these microbes on our body, their colonies would read like a map of microbial nations. Different lipids, sugars, proteins on individuals promote different types of microbes and different metabolic pathways giving individuals different scents.

Our ability to perceive a wide range of microbial odors might have an evolutionary pressure given the importance of detecting microbes in spoiled food, etc. I hypothesize our ability to detect body odor is inherited from detecting microorganisms in general.

Surely as our olfactory sense has diminished over evolution, these microbial odors are being taken out of some ancient context.

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