Why do men grow facial hair?

Question

Ancestral facial hair.

Our closest ancestors, the chimpanzee and bonobo, both grow facial hair around the mouth area. What is noteworthy is that this is the case in both males and females of the species.

Female chimpanzee with facial hair:

Dichotomy of facial hair in humans.

Our own species has a notable dichotomy in facial hair between males and females. I'm interested in what broadly speaking are the theories on facial hair. Could it be as simple as sexual selection over tens, or hundreds of millennia?

A previous answer eludes to a sexual selection direction, but my question is primarily why do human females not grow nearly as much facial hair as their male counterparts? Is this just a typical sexual dichotomy? I suppose here I am interested at a molecular level - is beard growth absent, or suppressed in women?

Speaking physiologically androgen is a hormone that drives hair growth and this hormone is more prevalent in males. Although females do have vellus hair coverage on their face, the thicker androgenic hair isn't typically present. A cursory search didn't reveal to me if this is due to less follicles in the first place so androgen would have little effect, or less hormone release.

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To answer the question "Why do men grow facial hair" I think three smaller questions are raised:

• What is the point of facial hair? (If there is any beyond sexual selection...)
• What selected thick androgenic facial hair out of human females?
• At a molecular level, have females lost the "facial hair gene", or gained a "facial hair suppressor gene"?

Answer 1

Hormones explain the sexual dimorphism.

Expose females to androgens and they develop masculine facial hair. See hirsutism. Also research:

... women’s intermediate facial follicles respond to men’s higher androgen levels by synthesizing more hair over several days ....

Answer 2

I don't think this is a complete answer, but I know that historically hair, like horse tail has been used as a part of armor, because it is flexible and difficult to cut through (see dragoon helmet picture).

Protection could be one of the primary purposes of facial hair in human males, who are more likely to experience aggression or be hunters.

Answer 3

A factor to add is also whiskers... most mammals have whiskers leftover from nocturnal sensing and fast-bite-reactions. For example felines that don't see well from upclose can use their whiskers in the final phase of an attack.

The age of mammal facial hair being up to 80 million years ( rodent carnivora divergeance ), the genes are more deeply encoded as very useful elements of mammal anatomy, and they tend to stay, plus the other factors described above.

Answer 4

I think @feetwet has given a good argument why the facial hair in females are suppressed rather than absent.

Facial hair signal sexual maturity and play role both in inter-sexual selection (attracting females, like peacock's tail) and intra-sexual selection - demonstrating dominance over other members of the same sex.

Another "hairy" trait also exclusive to males is baldness. For example:

Both male facial hair and male pattern baldness are genetically based, suggesting that they contributed to fitness. The multiple fitness model provides an evolutionary interpretation of the social perception of male pattern baldness and beardedness in terms of the multidimensional meaning of physical maturational stages. Male facial beardedness is associated with the sexual maturation stage and is hypothesized to signal aggressive dominance. Male pattern baldness, by contrast, is associated with the next stage of physical maturation, termed senescence. Pattern baldness may signal social maturity, a non-threatening form of dominance associated with wisdom and nurturance.