Monday, May 6, 2013

Benefits of body hair: it won't let the bedbugs bite

Beach season approaching!

Time for women (and some men) to break out the razors, hot wax, and even lasers to say goodbye to unwanted hair in strategic parts of their bodies, so I figured body hair might make for an interesting post.

We’re mammals, and mammals have hair – it’s one of our defining characteristics. Even if we sometimes try to fight it, body hair plays a key role in our health and comfort.


Why do mammals have hair?


Fur protects mammals from cold, strong sun, and possibly biting insects. It can also distinguish one species from another and camouflage prey and predators from each other.

sparse hair doesn't slow down common dolphins
photo: Shane Anderson, NOAA
Any mammals running around today with sparse hair must thus have a very good evolutionary reason for losing it.

Actually, some of these are swimming -- think of whales, dolphins, and seals – since less body hair means less resistance as they swim through the ocean. Their thick layer of blubber keeps them warm or cool as needed.

While elephants and rhinos can afford to invest little energy in maintaining body hair because they don’t lose much heat at night due to their large size and thick skins, most other land mammals have substantial hair, even in the warm tropics.

The coats of 3 monkey species (squirrel, saki, & tamarin) living in the same forest
distinguish them, protect them from the elements, and even help camouflage them.


So what happened with humans? How did loss of body hair improve our biological fitness enough for us to colonize almost every type of habitat on Earth?


Why did humans lose our body hair?


the bald-faced saki monkey's long
shaggy fur helps to camouflage
it in the forest canopy
photo: Edgard Collado
Truth is, we don’t know. Suggestions proposed for the loss of thick hair in humans include:

-- losing thick fur helped our ancestors stay cooler as they moved from the African forest to the savannah, and barer skin cools faster when sweating; however, this is countered by the speed with which bare skin heats up (daytime) and cools down (nighttime), compared to fur-covered skin.

-- removable clothing would give the wearer the chance to regulate his/her temperature easily, possibly to move between elevations or vegetation types; however, humans apparently developed clothing long after we lost thick hair. Hmmm.

-- our evolution included some type of “semi-aquatic phase”, as suggested by several characteristics, such as our ability to dive and swim underwater and the slight webbing in our hands (this one is under debate).

-- thick fur can harbor ecto-parasites, such as fleas, lice and ticks, which can spread disease that weaken humans, decreasing their chances of both survival and reproduction (“yuck”). Seriously, "that guy with the ticks might catch Lyme disease and get too ill to provide and care for the family. And I don't want them either."

If the more naked early humans contracted and transmitted fewer diseases, then it’s likely they would have survived longer and bred more than the furry ones, and the genes associated with body hair loss would have been passed on to the subsequent generations.

evolution of body hair loss?
image: Belgravia Centre

Given the benefits associated with hairlessness, humans at the time might have caught on and associated it with higher fitness. They would have sought out mates with bare skin, enabling sexual selection to further speed the spread of this feature through the population.


So, why do we still have some body hair, sometimes known as peach fuzz?


Curiously, humans didn’t lose all their body hair. We kept thick hair on our heads, of course, as well as in our reproductive areas, chests, arms, legs, and feet (OK, some of us). Our fierce and extensive use of razors, creams, wax treatments, and even lasers to get rid of this thick hair, called androgenic hair, suggests that sexual selection for nearly naked skin has been around for many generations and all across the globe.

Then there is that thin soft hair, sometimes known as peach fuzz but technically called vellus hair, that still covers most of our bodies and that we tend to leave alone. It isn’t thick enough to protect against sun or distinguish us as a species or as individuals, so why does it remain a part of us and is, in fact, as dense as the hair on a chimpanzee, just a whole lot shorter and finer?

For one, the fine hairs act like wicks that allow us to sweat, which as many of you know, cools the body and is what allows us humans to walk and run long distances at a time, relative to other mammals.

It also turns out that the fine vellus hairs function as motion detectors that alert us to insects, such as fleas and lice or even mosquitos, on our skin before they can bite us.

bedbug avoiding hair while
searching for a good bite spot
A 2011 study showed how people’s fine body hair improved their ability to detect bedbugs, a type of ectoparasite, while also increasing the search time needed by the bugs to find a good spot to start biting.

The researchers found this out by placing a bedbug (removed before it actually bit people) on shaved and unshaved portions of the arms of test subjects. Then they measured the time it took for the person to detect the bug and for the bug to start to bite the person. (Any more volunteers? Maybe the pay was good...)

recommended: unless you are competing,
hold on to that protective fine vellus hair!
photo: Alexis Brown, U.S. Navy
This parasite-defense theory is supported by other research, which found that batbugs and birdbugs (you guessed it, cousins of bedbugs) prefer to bite the hairless areas of bats and the featherless areas of birds. There is also evidence that mosquito bites occur primarily on the relatively hairless underside of wrists and ankles.

So our covering of short, fine, nearly invisible hair harbors fewer skin parasites, makes it harder for any remaining bugs to actually bite and transmit disease, and helps people to detect parasites. These advantages were even greater thousands of years ago, before modern hygiene and antibiotics, than they are now.

Even so, “Don’t let the bedbugs bite” still rings true today.

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To learn more about the bedbug detection experiment, here's the reference to the original paper: Dean & Siva-Jothy. 2011. Human fine body hair enhances ectoparasite detection. Biology Letters. http://dx.doi.org/10.1098/rsbl.2011.0987.

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