Why does beards exist only on man?

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If beards were necessary for protecting the face, woman should also have developed a beard, but it never happened at least as strong as a man’s beard. Is it something visual? Or it has any function that was important back then on man but not on woman?

In: Biology

It’s all a matter of hormones

Women develop excessive body or facial hair due to higher-than-normal levels of androgens, including testosterone. All females produce androgens, but the levels typically remain low. Certain medical conditions can cause a woman to produce too many androgens.

Some women do grow beards. Not all men can. It depends more on hormones, ancestry, regional climate… It’s more likely early humans DID have beards and grew winter coats and shed them just like animals and later evolved NOT to have them.

It’s possible that men have beards due to [sexual selection](https://en.wikipedia.org/wiki/Sexual_selection). This means the purpose of a beard is to make men more attractive to women, a bit like a peacock’s tail. Of course, humans are far more complex than peafowl and there is far more variation in what we find attractive about the opposite sex. Attractiveness is also heavily influenced by societal trends. Therefore, beards are no longer as important as they may once have been for early humans.

Beards aren’t thought to be for protecting the face. The two main theories seem to be that (a) beards are a secondary sexual characteristic that indicates either sexual maturity or dominance to women, and (b) that beards are a side-effect of another selection process that themselves serve no particular purpose.

If a is true, and beards are an indicator of sexual maturity, then they’re unnecessary on women because women have plenty of other indicators already. It would take an extra step of evolution for beards to appear on women, which hasn’t happened. If a is true but beards are an indicator of dominance (ie, the better the beard, the better the man) then they’re unnecessary on women because on the evolutionary time scale, the men compete for the women’s attention; the women aren’t competing, so need no sign of dominance over other women. If b is true then beards are just a thing that exist with no particular purpose.

There is reasonable evidence for both theories. Beard growth is moderated by a molecule called dihydrotestosterone, which is produced in the body from testosterone. The same molecule is also heavily involved in other important aspects of male-ness, particularly maturation of the male genitalia. This means men capable of growing good beards likely also have good reproductive qualities like high sperm count, and will, on average, be more reproductively successful. So even if beards don’t serve their own purpose, they’re selected for accidentally by virtue of selecting for other traits caused by the same genes. Women have *way* less testosterone than men, so don’t normally grow beards, but disorders that cause significant increases in testosterone can produce beards on women.