Ticked off

14 05 2007

Every once in a while something happens that causes little flurries of thought in my head, and such was the case when I decided to take a hike through a section of the Appalachian Trail that runs through the Delaware Water Gap this past Saturday. As I made my way up the hillside, keeping my eyes open for interesting arthoropods and (if I was lucky) mammals, I felt something crawling up the back of my leg. When I stopped I discovered that it was an American Dog Tick (Dermacentor variabilis), trying to make its way to warmer regions. I promptly flicked it off (another one attached itself to me moments later and received the same send-off), but after doing so it struck me that I detected its presence because it was moving the hairs on my legs as it crawled along. I certainly didn’t feel it on my skin, but as it climbed over and pushed aside the hairs on my legs I detected the presence of the little arachnid.

There is some debate as to why we humans have rather sparse body hair. Some believe that it’s in response to the warm climate in which we originated (the Savanna hypothesis), others regard it as a sign of human ancestors once living in water (the Aquatic Ape hypothesis), yet others proposing that it was part of Fisherian runaway sexual selection. The first two are especially contentious and the issue will likely still be debated for some time, but struck me is that if there was some reason that we should lose most of our hair, why didn’t that trend continue? What advantage would there be to being at least partially hairy? Hair serves many functions in keeping unwanted foreign objects like dirt, sand, etc. out of important sensory organs and orifices, but what about our legs? Could it be that we still have relatively hairy extremities because of selection caused by insects that spread disease? To test this we could find some participants of varying hairiness and put ticks on their arms/legs (watching that the little blighters didn’t get away and tuck in, of course) and see if some people are better at detecting them than others. Someone who shaved arms/legs would have to be tested as well, of course.

I could be entirely wrong in my hypothesis and it could be entirely baseless, but I think the role of disease-spreading insects has been largely overlooked when it comes to human evolution (there are exceptions of course, like this recent research). A lot of attention is paid to giant hyena or saber-toothed cats, but I doubt they killed as many humans as disease did, and maybe hair had something to do with who survived and who did not, perhaps even mitigating runaway sexual selection. As I stated before, this is merely blind hypothesis based upon a walk through the woods, but it does make me wonder about the intricacies of our own evolutionary history.


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5 responses

14 05 2007
TheBrummell

As a Hairy Northern Barbarian (TM), I like your hypothesis, but not as much as I like the Fisherian Sexual Selection hypothesis (as the latter implies my current, er, dry spell is an abberation).

I’d happily participate as a subject in any research designed to test either or both hypotheses.

14 05 2007
laelaps

Haha, anything for science eh? I just have a feeling that if I ever conducted said study, someone would reply “All you’ve proven is that there are people who don’t mind having ticks put on them.”

I think both hypotheses could work together, though; whomever has the right “hair density” to allow them to detect ticks/insects might get sick less and be more desirable, so at least then you can say “Hey baby, I’m 50% less likely to catch tick-borne diseases. Why don’t we get together and better the species…” [insert 70’s porno riff here]

14 05 2007
utenzi

It would seem to me that having less hair would discourage not encourge pests like the ticks. The pelts of our more densely haired pets certainly aren’t acting to discourage ticks and fleas.

14 05 2007
laelaps

Thanks for the comment utenzi. I’m not suggesting that the denseness of hair would encourage/discourage ticks or other pests; at least for ticks, they’re attracted to body heat and will hop on no matter how much hair there is. What I’m suggesting is that if you have too much hair, you might not detect ticks or other insects, and perhaps if we didn’t have any we wouldn’t detect them either. Because of our rather sparse hair covering, we can feel it when something is crawling up our leg because they move hairs as they go, and so people with an amount of hair that was “just right” might have been better able to detect (and then remove) disease-carrying pests.

I may be entirely wrong, but maybe there is an advantage to being only sparsely covered in hair.

15 05 2007
utenzi

That seems reasonable. There’s lots of places on our body where our skin isn’t very senstive–but the hairs with their proprioceptors are.

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