“Finding hairs in your food can be disgusting, and it seems that blood-sucking insects feel just the same,” the Daily Mail has claimed. The newspaper has somewhat misrepresented research that offers clues as to why humans have only fine hair on most of their bodies. While a stray curly hair in your food might turn your stomach, bedbugs were not quizzed over their disgust at human hair in this study.
This story, also slightly over-interpreted in the BBC and Daily Mirror , is based on a laboratory study which tested the theory that humans’ fine body hair might have been retained to act as an early warning system against skin parasites. The researchers did this by testing whether the hair on our arms aids our detection of bed bugs by comparing the ability of student volunteers to detect bed bugs on shaved and unshaved arms.
The study found that people were more likely to detect bed bugs on their hairy (unshaved) arm than their hairless (shaved) arm. The bed bugs also took longer to choose a place to feed from on the hairy arm.
The results suggest that having fine hair on our bodies may have given us an evolutionary advantage in dealing with skin parasites. In real life, bed bugs have more time in which to bite people than they did in this lab-based study, so having arm hair may not be sufficient to stop them biting.
The study was carried out by researchers from the University of Sheffield. No source of funding was reported. It was published in the peer-reviewed journal, Biology Letters .
This research was generally accurately covered by the Daily Mail and by the BBC, whose report included quotes from other academics, explaining how these findings tally with other studies on human evolution.
This laboratory based study aimed to test whether fine human body hair plays a defensive role against parasites that live on the surface of human bodies (called ectoparasites). The researchers used bed bugs as an example of this type of parasite. Although humans seem relatively hairless, we have the same density of hair follicles as apes, but our body hair is much finer. Researchers wanted to know if there is an evolutionary advantage in retaining fine hair on our bodies.
This type of research can help us to understand why certain body characteristics might have been retained over time, rather than being lost.
The researchers wanted to see whether body hair affected the time taken for bed bugs to select where to bite (called the search time), and whether body hair helped or hindered people’s ability to detect the bed bug on their skin (it is already known that fine human body hair plays a role in sensing mechanical pressure or distortion).
The bed bugs had been fed a week before the trial, making them ready to feed and likely to engage in feeding behaviour. Only female bed bugs were used.
Researchers recruited 29 student volunteers through Facebook. Although this is a novel use of the social networking site, it is normal for researchers to ask for volunteers for this type of study. Each volunteer had one arm shaved, while the other arm remained unshaved. An area of a same size was marked out on each arm using vaseline, which prevents bed bug movement. The researchers then placed five bed bugs onto the test area on the volunteer’s arm. Volunteers (who the researchers also referred to as hosts) were prevented from seeing their arm during the test.
The researchers defined search time as the time between the bug being placed on a volunteer’s arm and it extending its proboscis to feed (bite). The researchers determined the search time and the volunteers were asked to press a button when felt something on their arm. The bed bugs were removed just before they bit (or after five minutes if they didn’t extend their proboscises).
The same five bed bugs were used on each arm, and the volunteers’ arms were tested a week apart. The order of testing (shaved or unshaved) and whether the right or left arm was shaved, were selected at random. The researchers checked that their results were not affected by whether the left or right arm was shaved, or whether the bed bugs were exposed to a shaved or hairy arm first.
The researchers also looked at whether individual host hairiness had an effect. To do this they calculated the ‘hair index’ (the number of follicles per cm2 of skin area multiplied by the average length of the hair). Male volunteers had a higher hair index than the female volunteers. The researchers did not differentiate between fine ‘vellus’ hairs and longer ‘terminal’ hairs.
The bed bugs took significantly longer to select a site to bite on hairy arms than on shaved arms in men, but not in women. On unshaved arms, search time increased with increasing hairiness in both men and women. Both women and men recorded feeling something on their arm more times per second on their ‘hairy’ arms.
The researchers concluded that the presence of fine body hair prolongs the time taken for bed bugs to select a site to bite, and enhances the ability of the host to detect them. They discuss the theory that there is a balance between being hairy, which improves your chances of detecting skin parasites and being less hairy, which gives ectoparasites fewer opportunities to hide. They suggest that this balance has resulted in the maintenance of fine human hair. They suggest that, ‘reduced body hair in humans functions, at least partly, as a defence against ectoparasites’.
This laboratory based study helps us understand why humans have retained fine body hair, by testing whether the fine hair aids our detection of bed bugs. They found that people were more likely to detect bed bugs on their hairy arms, which could act as an early warning system for parasites. The bugs took longer to select a site to bite on hairy arms. The results may explain why parasites choose relatively hairless parts of host bodies to feed from.
The study does have limitations in that it was small and may not have recreated the behaviour of bed bugs outside of the laboratory. As bed bugs tend to have more than five minutes in which to bite people in real life, having arm hair probably won’t stop them biting.