Why We Aren't Completely Naked Apes

Body Hair is one of those enduring mysteries of evolution.  Some scientists write papers about why we have hair, and others write about why we don’t have more of it. Since other apes are furrier than we are (well, most of us), it’s a natural assumption that we lost much of our body hair somewhere along the line. This latest hypothesis for the retention of body hair appeared in the journal Biology Letters, and was summarized here in Science:

"When it creeps into your bed at night and crawls across your skin, the bedbug (inset) has to navigate a forest of body hair before plunging its proboscis into your flesh for a meal. One wrong step, and it could get smushed. Tickled by the question of how people detect such microscopic pests, researchers recruited 19 volunteers with various amounts of body hair and shaved one of each of their arms. They then asked the subjects to look away while they dropped bedbugs onto their arms. The volunteers hit a button as soon as they felt something crawling on them. Participants, especially men, with more hair follicles per square inch and whose body hairs were longer, tended to be several seconds quicker than less hirsute individuals to notice the bugs on their unshaven arms, the researchers report online today in Biology Letters. And everyone took a long time to notice the bedbugs on the shaved arm. That might explain why humans still have hair on their bodies, the researchers conclude, since we no longer need it for keeping warm."

I hope they paid these volunteers well. This paper demonstrates a possible advantage of having body hair, but that doesn’t mean the need for bug protection was the reason people retained some hair. There are many unanswered questions here. If body hair evolved to warn us about insects, why are some of us much more naked than others? Why do men vary from smooth to furry?  

The study doesn’t explain why men are hairier than women, at least on average. When there’s a significant sex difference in a trait, some scientists suspect a phenomenon known as sexual selection. This is, roughly, the selective propagation of traits that incite the desire of the opposite sex. Could male body hair variation have been shaped by the varying regional tastes of women?

The bug detection idea also doesn’t explain why we have hair where we have it. I could see why you’d want to be extra safe in keeping bedbugs and the like away from your pubic zone, but is there some reason all of us need extra warning in our armpits?

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