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Tiny human hairs beat back bugs

Hair for a reason Human body hair turns out to be quite useful, enhancing our ability to detect parasites and can even prevent pests from biting.

A study, published in the latest Royal Society Biology Letters, helps to explain why the human body looks relatively hairless compared to that of other furrier apes, but yet still has the same density of hair follicles as would be expected of a chimp or gorilla of the same size.

The fine hair consists of types known as vellus and terminal. The former is sometimes referred to as 'peach fuzz', while the latter refers to head hair as well as to hair that develops in the armpits and around the genitals.

"All these hairs have nerves attached to them and provide us with the ability to detect displacement of the hair," says co-author Professor Michael Siva-Jothy. "By simultaneously 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."

Siva-Jothy, a researcher at the University of Sheffield and colleague Isabelle Dean came to these conclusions after conducting an experiment that was both figuratively and literally hair-raising.

The researchers placed bed bugs on shaved and unshaved arm areas belonging to 10 women and 19 men who were student volunteers from the university. Each bed bug was hungry and ready to feast on its potential victim.

The volunteers had to remain still throughout the experiments, so participation in the study was not suitable for potentially bug-squeamish people.

The scientists took note of how hairy the students were and how long it took for the bed bug to get ready to eat. While the volunteers were warned that they might be bitten during the experiment, no one was.

"Just before it begins to feed, the bed bug swings its proboscis from a 'stowed' position to a 'ready for action' position," says Siva-Jothy. "We stopped the trial as soon as that second position was adopted."

The bugs aren't looking for veins, but instead disrupt the capillaries and subsequently feed on the pool of blood under the skin.

The hairier, the better?

Host detections of the parasite were more frequent on unshaved arms of both men and women. Bed bug search times took significantly longer on the unshaved arms of men. Men simply have a lot more body hair, so their bodies present challenging obstacle courses for bugs.

"Men tend to have more terminal hairs because of the effects of testosterone," says Siva-Jothy.

The researchers believe human body hair foils not only bed bugs, but also other parasites, such as mosquitoes, ticks and leeches.

Humans have retained this useful peach fuzz and other body hair, but have lost their heavy, thick coat of fur over evolutionary time. Thick fur provides warmth and protection, but it can also aid parasites, by giving them a place to hide and making it harder to remove and crush them.

Prior research has determined that human body hair aids in sweat gland maintenance. Other as-of-yet undiscovered functions could also exist.

Professor Kenneth Haynes, a researcher in the Department of Entomology at the University of Kentucky, says, "The idea that fine body hair might improve detection of an ectoparasite seems very reasonable, and was demonstrated convincingly in this research with bed bugs."

"These results are consistent with a fascinating hypothesis that ectoparasites had an impact on the evolution on our hairiness or apparent hairlessness," he adds. "The authors list other hypothesis that could play a role too."

"I guess that humans should be known as the apparently naked ape."