Is body hair a functionless vestige?

Hi, Ken.

Welcome to EvC!

Ken Fabos writes: It seems to be widely held that human body hair, being too thin to provide thermal insulation, is essentially functionless.

I've never really thought of this paradigm as being particularly pervasive.
I don’t know of many people who deny that human hair has functions.
But, in terms of thermoregulation, it’s perfectly reasonable to say human hair has essentially lost its function. That it retains some other functions that it probably also performs in other organisms doesn’t affect this assessment at all.

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Ken Fabos writes: [Hair] may have a role in carrying pheremones, perspiration as well as water away from the skin...

But, hair would tend to hamper the movement of things like volatiles and perspiration away from the body. Pheromones would be less effective with hair covering the glands then with bare skin. Perspiration is also less effective when there is hair interfering with convective heat transfer.

If anything, these are reasons for the reduction of hair, not for the persistance of hair.

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Ken Fabos writes: Is it possible to make informed speculation about the evolution of our body hair without consideration of it's sensory function?

Simple comparisons to other organisms reveal that our pelage is relatively impoverished.
So, even if you’re correct, the primary question remains: why is our pelage less extensive than other organisms’ pelage?

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-Bluejay (a.k.a. Mantis, Thylacosmilus)

Darwin loves you.

Hi, Ken.

Ken Fabos writes: By slowing but not blocking air flow close to the skin a layer of cooler air could develop.

Heat radiating from the body (conductive heat loss) becomes trapped in the boundary layer and keeps the air immediately around the skin warm. Blocking air flow would only increase the amount of heat accumulating at the skin's surface.

Air currents moving across the skin are what move the heat away from the body (convective heat loss) and result in cooling of the skin. Hair and fur function to prevent air currents from moving across the skin, and thereby hamper convective heat loss. Without convective heat loss, we only shed excess heat by conduction, which is slow and generally cannot outpace the production of heat that comes from working our muscles.

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Ken Fabos writes: what are the functional advantages and disadvantages of the variable pelage we currently possess? Sensory function belongs somewhere in that answer.

Why?
Do you think our pelage is better at sensory perception than the thicker pelages of other animals?

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-Bluejay (a.k.a. Mantis, Thylacosmilus)

Darwin loves you.

Hi, RAZD.

RAZD writes: I've heard this argument before, often as part of a "why humans lost hair" argument, and it seems somewhat logical at first ... but it doesn't explain horses. They sweat, and they have lots of horse scents ...

Play on words: I like it.

In terms of sweating, surface cooling isn't the only thing it does: heat is removed from the core in sweat too, so, even if it doesn't evaporate away, it's still helping dissipate heat.

About pheromones and other scents, I admit less understanding. It may be that the slower dispersal of scents caused by hair is advantageous in some cases, and can actually concentrate the scent more.

I know that some moths actually collect pheromones in their hairs, then spread the hairs and fan them in the wind to disperse them, so hair can serve a pheromonal purpose.

The thing that bugs me about the way Ken presented this suggests that our body hair is superior to both total nakedness and heavier pelage in terms of perspiration and pheromonal dispersal, which doesn't make a lot of sense to me.

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-Bluejay (a.k.a. Mantis, Thylacosmilus)

Darwin loves you.

Hi, Mr Jack.

RAZD and I had this debate awhile back on why is the lack of "fur" positive Progression for humans?, starting around Message 156.

Mr Jack writes: African men are considerably more hairless than european men.

I argued this, as well, but it turns out to not be entirely correct. I even looked up the most basal groups of humans and found them to be the least hairy of the Africans, but it turns out that they've got admixture with Asian populations, so hairlessness may not be a plesiomorphic trait (I don't think there's any evidence on this at the present time, though).

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Mr Jack writes: RAZD writes: This also explains why other cursorial hunters, like wild dogs and wolves, are naked ... ? Those aren't persistance hunters. Humans are the only species in the world known to persistant hunt, AFAIK.

Actually, this isn't true. All the big Canidae are built for endurance hunting: they hunt in long chases until their prey are exhausted. Other examples are hyenas (particularly the extinct "hunting hyenas"), stoats and possibly even the thylacine.

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Since Ken Fabos wants to focus on sensory functions of hair, maybe we could continue this discussion on why is the lack of "fur" positive Progression for humans?: I think there's still potentially a lot to be said for thermoregulatory functions of hairlessness.

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-Bluejay (a.k.a. Mantis, Thylacosmilus)

Darwin loves you.

Hi, Ken.

Ken Fabos writes: I think I was referring more to the hairy pits and crevices for pheromones - and pointing out that hairs, generally, are part of the mechanism of such dispersal and therefore functional.

I think you’ve not been reading us very carefully. None of us has argued that hair is not functional. I’m not sure why you think any of us would be arguing that hair is non-functional.

We have only been pointing out that, in order for the functions you ascribe to our relative “hairlessness” to have evolutionary significance, you need to show that functionality is greater under the our situation than under other situations.

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Ken Fabos writes: This function, to wick away moisture - with and without pheromones - is also known in the scientific literature - David Wolfgang-Kimball's "Pheromones in Humans - Myth or reality" for example. This has to impact temperature regulation... Thinned out hair might actually be superior in this regard than a thick, heavy pelage.

This would be of very little benefit for evaporative cooling.

Sweat works in two ways.

First, the liquid carries some excess heat away from the core and deposits it outside of the body, thus reducing core temperature. The heat shed in this way is not enough to cause the liquid to evaporate, which is why it is exuded as a liquid.

Second, when exuded onto the skin surface, the liquid absorbs heat from the surface, which causes it to evaporate. The net effect is reduced surface temperature. Sweat beads that are wicked away from the skin surface will absorb less (if any) heat from the skin surface, and will thus be less effective at fulfilling this second function than sweat beads that are not wicked away from the surface.

You can test this by putting a drop of water on an arm hair, then blowing it away; and then putting a drop of water on your skin, then blowing it away. Sweat wicked away from the body is of less benefit to evaporative cooling than sweat retained on the surface of the body.

So, completely bare skin should still outperform a thin pelage in terms of sweating.

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-Bluejay (a.k.a. Mantis, Thylacosmilus)

Darwin loves you.

Hi, Ken.

Ken Fabos writes: I would point out again that very fine vellus hairs retain a lot of sensitivity and I would not be surprised if they can detect impulses smaller and finer than thick heavy hairs of a thick pelage.

For what purpose?

I don’t see a really good reason why detection of parasites requires hairs that are sensitive to even the subtlest breezes when thicker hairs are sufficiently sensitive to detect the movement of parasites on the skin. Fine hairs also result in wide gaps between them, which creates pockets of sensory void that animals with thicker hairs don’t have. I just don’t see how our sparser hair really makes sense as a sensory adaptation.

Furthermore, I think the heightened sensitivity to touch on your face can be explained at least as well by the increase in the density of nerves in the skin of the face as by the better sensitivity of fine hairs.

Certainly, hairs have important sensory functions for humans. I don’t deny this.
But, hairs also have important sensory functions for other mammals. You haven’t given any good reasons to think that our hairs are better than other hairs at sensory perception, and you haven’t given any good reasons to think that humans needed or benefited from such an adaptation.

And, finally, how does your sensory hypothesis explain ethnic variation in hair-shaft coarseness?
Do Caucasians have a stronger need for this sensory adaptation than Africans do?

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-Bluejay (a.k.a. Mantis, Thylacosmilus)

Darwin loves you.

Hi, Robert.

Robert Byers writes: This is rock solid analysis upon data.

Please spend some time looking up and studying the words "analysis" and "data," with particular emphasis on how they are used in science. Then, do some careful thinking about what the phrase "rock-solid" means.

If, after you've done this, you still believe that the quoted sentence accurately represents the crap you keeping putting out here, please tell us again, so that we can all know that engaging you in further conversation is completely futile.

But, make sure you post it publicly so that we have the documentation. Thanks.

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-Bluejay (a.k.a. Mantis, Thylacosmilus)

Darwin loves you.