The question of why human beings have virtually no body hair – as discussed yesterday on The Conversation – has puzzled evolutionary theorists since Darwin’s Descent of Man (1871).

It’s puzzling because we are alone among the primates in being virtually hairless. Related to this problem, but not so widely discussed, is the question of why we nevertheless have a coating of fine, short, and nearly invisible body hair. If we lost most of our body hair in the course of human evolution, why did we retain the nearly invisible body hairs we have today?

A common view is that, although these hairs play some minor roles in sweat-gland functioning and tactile perception, they are largely non-functional, perhaps a remnant of early times when we had a thick coat of fur. But now, a couple of researchers in the UK have proposed a new answer.

In their research paper – published today in Biology Letters – Isabelle Dean and Michael T. Siva-Jothy from the University of Sheffield argue that human fine body hair enhances “ectoparasite” (bed bug) detection and, consequently, that this is evolutionary reason for the existence of such hair.

In their study they test this hypothesis by seeing if subjects were better at tactile detection of the presence of a bed bug on a shaven or unshaven arm. Their results showed that subjects with an unshaven arm were more likely to detect the movements of a bed bug. Moreover, the more arm hair the subject had (a higher “hair index”), the more likely they were to sense the bed bug on their skin.

This, they argue, might be because the presence of body hairs makes it harder (and slower) for the bed bug to move around, therefore giving the subject more time to notice the parasite.

One difficulty with this research is that bed bugs are nocturnal and, consequently, their human hosts are normally asleep when these parasites are searching and feeding. But the subjects in the experiment were awake. It is highly questionable whether the presence of body hair would aid with bed bug detection in a sleeping subject.

Problems also appear when we consider that males have a higher hair index than females. As Dean and Siva-Jothy point out, this implies males will be better at detecting ectoparasites than females will.

But what evolutionary purpose could this serve? Why should males have evolved to be better at detecting ectoparasites than females? Dean and Siva-Jothy quickly deal with this problem by suggesting that the increased male ability here might have been driven by female mate selection and a preference for parasite-free males “and/or” a sexual dimorphism – a difference between male and females investment in immune function.

But why should a parasite-free sexual partner have been more of a concern for females than for males? A parasite-ridden or sickly female would seem to be just as unattractive to a male as a parasite-ridden or sickly male would be to a female.

The high sexual value that males tend to put on smooth, unblemished female skin suggests females who were able to rid themselves of ectoparasites would be selected for. The presence of ectoparasites and their bite marks would leave female skin looking blemished and quite unattractive to the male.

Further, it seems that this importance males place on the smooth and unblemished appearance of their sexual partner’s skin is not fully reciprocated by females. That is, females are not so fully concerned about imperfections in male skin. With females, sexual attraction would seem to be based more on other features, such as strength, social status, kindness, and so on.

Consequently, it looks like the reverse of what Dean and Siva-Jothy suggest might even be the case. They suggest males place a higher value on ectoparasite-free partner than do females. If Dean and Siva-Jothy’s view is correct, females should be hairier than males.

Further, it seems questionable that ancestral females, many of whom were continually pregnant or breast-feeding, were able to exercise much sexual selection.

The other suggestion – of a dimorphism in immune function – seems equally questionable. Since it is females who bear and nurse offspring, and males who are evolutionarily more dispensable than females it seems likely that female investment in immune function would be higher than male investment.

This is not to say Dean and Siva-Jothy have not located an important function of human body hair. But it does suggest their account is not the whole story.

I have argued elsewhere that the loss of human body fur had its origin in the ancestral mother-infant relation. The loss of body hair gave rise to the pleasure of skin-to-skin contact with the infant, which in turned encourage the mother to bear the infant.

This would explain why females and children are more hairless than adult males. This maternal selection for hairless infants was then reinforced through primarily male sexual selection for a hairless partner.

Because of this, I think it likely that the sexual dimorphism in fine body hairs would have also been under the pressures of maternal and sexual selection. On this account it would then be females who evolved a lower hair index, not males who evolved a higher hair index.

That is, females simply lost more body hair than males did.

See also:

• Shave tight? Don’t let the bed bugs bite by Mike Elgar