Hairy skin helps stop bed bugs biting, according to new research from the University of Sheffield in the UK. Apparently, not only does the fine hair that covers our bodies help us feel the presence of parasitic insects on our skin, it also acts as a barrier to stop them biting us. The findings of the study appeared in an online before print issue of the Royal Society journal Biology Letters on 14 December.
Although humans seem relatively naked compared to other primates, our bodies are covered in a layer of two types of fine hair. One type is called vellus, which is short and nearly invisible, and the other is called terminal hair which is longer and more visible. The follicles are also quite close together on the skin.
The researchers note there are “relatively few explanations for the evolutionary maintenance of this type of human hair,” so they wanted to test the idea that perhaps it acts as a defense against ectoparasites or bed bugs.
First author and Sheffield Zoology graduate Isabelle Dean picked the study as the subject of her honours project, which she carried out under the supervision of co-author Professor Michael Siva-Jothy, of the University´s Department of Animal and Plant Sciences.
For the study they recruited 29 healthy volunteers who had one arm shaved and one arm left with hairs on, and then allowed hungry bed bugs to be placed on the skin of both arms.
The results show that the fine body hair acts to our advantage because it helps us detect the presence of bed bugs in two ways: by increasing the time it takes for the parasite to find a suitable site to start sucking blood, and by helping us feel their presence on our skin.
They also showed that this advantage was greater for those participants who had more layers of hair: the insects took longer to find an ideal feeding site on their arms.
The study helps explain why bed bugs and other parasites such as mosquitoes, midges, ticks, and leeches, seek out the less hairy body sites such as wrists and ankles.
Siva-Jothy explained to the press:
“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 authors suggest the findings also help explain why we have lost the thick coat of our primate cousins, but still retained some body hair.
“For example, if you have a heavy coat of long thick hairs it is easier for parasites to hide, even if you can detect them. 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,” said Siva-Jothy.
Siva-Jothy leads a team that is investigating the biology of blood-sucking insects, how they reproduce and retain immunity. They want to discover new ways to control these parasites, which can help us develop better ways to reduce transmission of insect-borne diseases.
He said men have more body hair than women, a result of their increased testosterone at puberty. But this does not mean women are more likely to be bitten:
“Blood-sucking insects are likely to have been selected to prefer to bite hosts in relatively hairless areas,” said Siva- Jothy.
Written by Catharine Paddock PhD