Humans have about the same number of hair follicles (seen here in cross-section) as would be expected of an ape of our size, but we grow only fine hair over most of our body. Why we evolved with very little thick hair is a matter of debate. One theory is that skin parasites were a trigger – coarsely hirsute mammals provide them with a hiding place, while our more delicate down actually makes their detection easier. Researchers tested this idea by placing bed bugs on the arms of volunteers and measuring the time they took to bite. The more fine hair on the volunteer’s arm, the longer the bugs searched before feeding on blood, making it more likely they would be detected and scratched. It seems that a good covering of fine hair is optimal, reducing risks associated with infected bites.
Written by Mick Warwicker
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