Body hair helps with thermoregulation.
The hair on the head protects your scalp from direct sunlight, but also retains heat if you are in a cold climate.
The eyelashes are like screen doors for the eyes, keeping insects, dust and small particles of debris from entering when opened.
The eyebrows keep sweat from entering your eyes.
Armpit hair, technically called “axillary” hairs, collect and disseminate pheromones while acting like WD-40 from body hair, reducing friction between the skin under the arm and the skin on the side of the chest when we let’s walk and swing our arms.
Pubic hair also helps reduce friction and provides a layer of protection against bacteria and other pathogens.
But facial hair? You will notice that he does not appear on this handy list of adaptive hairy traits.
When we started studying this stuff, evolutionary biologists thought it might serve thermoregulatory or prophylactic purposes similar to body hair and pubic hair. Beards and mustaches are around the mouth, after all, and the mouth absorbs food and other particles that can transmit disease. Beards and mustaches are also on the face, which is connected to the head, which loses a lot of heat from its top if it is not covered with hair. It all makes sense when you look at it that way.
Except that there is a problem with this theory: it leaves out 50% of the population, that is to say females. Natural selection is ruthless, and it has sent MANY species down the path to the dodo – for example, the dodo – but it rarely, if ever, selects a trait in a species like this and leaves half the population behind. outstanding, especially the half that makes all the babies (i.e. the larger half). If facial hair was supposed to perform important functions, it would be present in both sexes. Instead, thick, mature facial hair is present almost exclusively on the male half of the species, and its only job is to sit there on its wearer’s face as a signal to anyone who crosses its path. path.
What signal does facial hair send? Well, this is where it gets a little tricky, as far as the ornamental traits go. Geoffrey Miller, professor at the University of New Mexico, one of the foremost evolutionary psychologists in the field, puts it this way: “The two main explanations for male facial hair are intersex attraction (attracting women ) and intrasexual competition (intimidating rival men). ” Basically, facial hair means one thing to potential mates (namely manhood and sexual maturity, hubba-hubba type stuff) and another thing to potential rivals (formidable and wisdom or piety). Taken together, these signals give their own mark of high status to men with the most majestic mustaches or the tallest, heaviest beards.
The signal sent by facial hair also tends to be stronger and more reliable between men, who are more often rivals, than between men and women, who are more often partners. In fact, evolutionary biologists will tell you (if you ask them) that while some women really like facial hair, some don’t, and others don’t care, more often than not attraction has as much to do with it. with the density of the beard. like anything else. That is, if you are in a place where there are a lot of beards – say, a lumberjack convention – then a clean shaven face is more attractive, but if you are surrounded by bare faces then a beard is preferable.
In evolutionary genetics this is called “negative frequency dependence” (NFD), which is scientific discourse for the idea that when a trait is rare in a population, it tends to have an advantage. . In guppies, for example, males with a unique combination of colored spots mate more often and are attacked less. It’s a huge competitive advantage. It’s like going to Vegas expecting to lose $ 1000 but hoping to break even, only to end up earning $ 1000 instead. That’s a $ 2000 swing! The same goes for a trait with NFD selection. The trait goes from fighting for his life to being the life of the party. The downside is that the competitive advantage can lead to overpopulation of other people with the same trait very quickly, because of everything the very interesting looking guppy does, which means it loses its rarity and becomes common. Don’t worry, nature has a solution for this: as more and more guppies carry this same trait, it leads to decreased partner interest and increased predator attention. What was once the new guppy thing is becoming old news, in other words.