Human body hair ‘protects against insect bites’
MedWire News: Body hair affords protection against biting insects, say UK researchers, who propose this as one reason why humans have retained body hair throughout evolution.
Although humans are relatively naked in comparison to other primates, the human body is covered in a dense layer of fine hair. The hair is of two types: the almost invisible "vellus" hair and the longer, darker "terminal" hair.
"There are relatively few explanations for the evolutionary maintenance of this type of human hair," note study authors Michael Siva-Jothy and Isabelle Dean, both from the University of Sheffield, in the Royal Society journal Biology Letters.
To investigate their hypothesis, Professors Siva-Jothy and Dean recruited 29 volunteers who agreed to have a patch of hair on one arm shaved off. They were then told to close their eyes while a single bedbug was placed on their arm.
Importantly, the bedbugs had not been fed before the study so were hungry. The bugs were left on the volunteers' arms for 5 minutes and watched closely by the researchers for signs of trying to feed (eg, extending their proboscis). The volunteers were also asked to signal if they felt something on their arm.
The same experiment was conducted with each of the 29 volunteers and on each of their arms (unshaven and shaven), 1 week apart.
The researchers also graded each of the volunteers for levels of "hairiness," based on the density of arm hair (how many follicles per unit of skin area) and the average length of hairs.
The study had two main findings, say Professors Siva-Jothy and Dean.
First, the amount of time the bedbugs spent looking for food was lowest on the shaved arms and highest on the most hairy, unshaven arms.
Second, the ability of volunteers to detect bugs on their arms was lowest on the shaved arms and highest on the most hairy, unshaven arms.
Taken together, these findings suggest that hair on the human skin helps protect against biting insects. "Our proposal is that we retain the fine covering because it aids detection and if we lost all hair, even the relatively invisible fine hair, our detection ability goes right down," say the researchers.
They explain further: "The hairs have nerves attached to them and provide us with the ability to detect displacement. By forming a barrier and providing detection, these hairs prolong search time and make detection more likely because the bug has to spend more time clambering over them."
The team is now investigating the feeding behavior of bedbugs in more detail with the aim of findings more effective ways of controlling parasites and the diseases they spread.
MedWire (http://www.medwire-news.md/) is an independent clinical news service provided by Springer Healthcare Limited. © Springer Healthcare Ltd; 2012
By Joanna Lyford