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’."
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Photo: Flickr/vince42
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