MUCH ink and many electrons have been spilled on the question of human hairlessness: why, as Desmond Morris put it in the title of a book published in 1967, Homo sapiens is “The Naked Ape”. This lack of hair has been attributed to everything from a putative aquatic period in the species's past to the advantages of displaying a healthy skin to members of the opposite sex.

Less attention has been paid, though, to the fact that humans are not really hairless at all. Per square centimetre, human skin has as many hair follicles as that of other great apes. The difference is not in the number, but in the fineness of the hair that grows from those follicles. These fine human hairs do not seem to be performing any of the functions of their counterparts in more hirsute species (insulation and, through colouration, either signalling or camouflage). So what are they for?

That is a question addressed by Isabelle Dean and Michael Siva-Jothy of Sheffield University, in Britain, in a paper in Biology Letters. Their conclusion is that humans have fine body hair to serve as an alarm system.

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.