the not so naked ape

human body hair, once thought to be an evolutionary relic, has a real job.

Ms Dean and Dr Siva-Jothy were testing the idea that fine body hairs (known, technically, as vellus and terminal hairs) are there to alert their owner to creepy crawlies such as bed bugs, which might be intent on biting them, and that the hair may also get in the way of such arthropods’ activities, giving the owner more time to react before he is bitten.

The standard “lab rat” for this sort of experiment is the university student, and Ms Dean and Dr Siva-Jothy managed to recruit 29 eager volunteers for their study—19 men and ten women. Each had a patch of skin on one arm shaved, marked with a pen and surrounded by petroleum jelly (to fence the bed bugs in), and a commensurate patch on the other marked and surrounded, but not shaved.

It was then time to get the bed bugs out. The bugs in question had been fed a week previously, and then starved, so they were eager to eat. Volunteers were asked to look away while a researcher put a bug on one of the skin patches. The volunteer was then supposed to record, using a press-button counter, the number of times he perceived the insect moving on his skin.

The difference was significant. When the bug was on a hairy patch it was detected, on average, every four seconds. When it was on a shaved patch, more than ten seconds elapsed between detections. Moreover, the bugs seemed to find it harder to locate a good spot to bite when they were surrounded by hair. Though no volunteer was actually bitten, because the vigilance of the watching researcher meant the insects were removed when they extended their probosces prior to biting, bugs on hairy skin took about a fifth longer than those on shaved skin to attempt to bite their hosts.

In both cases men (who are hairier than women, as measured by the density of follicles and the length of the vellus hair) were better off than women when the bugs were released on unshaven patches of skin, though there was no significant difference between the sexes when they were shaved. The upshot is that, whatever the reason why human body hair has shrunk, one reason it has not disappeared completely is because it warns and protects those who sport it from the attentions of hostile insects.

read further in the economist