Photo: flickr/vince42Scientists have long speculated about why humans have so little body hair compared with other primates. (The most widely accepted explanation is that less body hair allowed for more efficient cooling on the savannahs where humans evolved.) However, less attention has been paid to the fine coating of hair that we have retained. In this study, the researchers set out to test their hypothesis that our fine body hair serves to defend us against "ectoparasites"--that is, bloodsucking insects like bedbugs. To do so, they shaved one arm of each subject and then released a bedbug onto each arm (one shaved, one unshaved). The researchers then asked the subjects to count how often they "perceived the presence of something on their arm" (see full methods description below). Turns out that shaving the arms not only made it easier for the bedbugs to bite, but also made them harder to detect. This makes intuitive sense--the tiny hairs on your arm obstruct the bug's passage and make it more likely that you'll feel it moving around. The authors conclude that body hair might have been kept for precisely this reason. Something to consider the next time you head to the waxing salon...
Human fine body hair enhances ectoparasite detection
“Although we are relatively naked in comparison with other primates, the human body is covered in a layer of fine hair (vellus and terminal hair) at a relatively high follicular density. There are relatively few explanations for the evolutionary maintenance of this type of human hair. Here, we experimentally test the hypothesis that human fine body hair plays a defensive function against ectoparasites (bed bugs). Our results show that fine body hair enhances the detection of ectoparasites through the combined effects of (i) increasing the parasite’s search time and (ii) enhancing its detection.” Bonus excerpt from the full text: “(c) Experimental treatment The experimental procedure required that each host was tested on a shaved and unshaved arm (randomized with respect to arm and temporal sequence). The treatment arm was shaved on the upper surface between the wrist and the elbow with a new razor (Gillette Mach 4), while using the same brand of perfume- and colour-free soap (Simple). A rectangle measuring 5 × 10 cm was then drawn on the shaved area with a marker pen and vaseline (a barrier to bed bug locomotion) applied to the marked boundary. This ensured each host experienced the same potential surface contact with the parasites. Testing on the unshaved (control) arm was preceded by washing the unshaven arm with the same soap as the treatment, and generating a vaseline-delimited rectangle as in the treatment. Each host was given a tally-counter and asked to look away as a bed bug was placed within the vaseline rectangle on their arm. Prior to release, all bed bugs experienced the same handling conditions. The volunteer was instructed to use the tally-counter every time they perceived the presence of something on their arm. The experimenter timed the duration of search behaviour of the bed bug on the host’s arm and removed the insect as it extended its proboscis (the stereotyped pre-feeding behaviour). The search time was recorded as the time between placement on the host and extension of the proboscis. Host tally-counts were used as an index of parasite ‘detections’.” Related content: Flashback Friday: Innocent until proven bearded.NCBI ROFL: Frequency of pubic hair transfer during sexual intercourse.NCBI ROFL: What is this I don't even (facial hair edition).
