Is body hair a functionless vestige?

It seems to be widely held that human body hair, being too thin to provide thermal insulation, is essentially functionless. It's my contention that it's probably multifunctional and that it's primary function is sensory. It may still have a role in thermal regulation for those who retain a lot of it and may have a role in carrying pheremones, perspiration as well as water away from the skin - with some impact on how well perspiration cools - however I want to focus specifically on hair's sensory function. I believe it's widely overlooked and I'm interested in how so many people, who live constantly with the sensory input these humble mechano-receptors provide, can fail to consider that function relevant to discussions on the evolution that led to our current (relative and variable) state of sparse hair cover, rather than fur covered, skin.

Thread copied here from the Is body hair a functionless vestige? thread in the Proposed New Topics forum.

A vestige,certainly, but perhaps not entirely without function.But it's hard to say. Your own post say perhaps this and perhaps that. And then again, perhaps not.Perhaps it serves some minor function. How would we find out? We could forcibly shave off all the body hair of a million men, and compare them to ... what WOULD be the control group? Double-blind testing is right out.

Body hairs are very sensitive to movement (perceived as touch). Whilst not strictly good science, brushing a fine hair across areas of my own skin that are truly hairless was imperceptible but brushing it across the hairs on skin nearby was clearly perceptibe. ie the threshold sensitivity for hairs is lower than for direct skin touch. I tried shaving a small area just in case those hairless areas - back of knuckles, inside wrist, inside of fingers and palm, hairless areas of feet - were naturally less sensitive. Close shaving was required because even short stubble transmits sensory information, but it convinced me that body hairs do actually provide greater sensitivity to low threshold tactile sensations than hairless skin. Not scientific evidence perhaps, but cause for someone to conduct some experiments. I did find an abstract from a single paper in the obscure field of Haptics that tried to measure the trheshold sensitivity of hairs on the back of a finger to vibrations - paywalled unfortunately. Otherwise it's conspicuous by it's absence.
I would point out that body (vellus) hairs provide sensitivity to touch that extends beyond the surface of the skin - during moments of fright or arousal goose bumps cause them to stand out from the skin and extend that sensitivity even further. 30mm and more on my legs and arms with an occaision hair that extends as far as 50mm. Further on my chest, belly and back.
Eyelashes may be a special case but like all hairs, they are sensitive to touch - extremely sensitive in this case - and detect and sweep away foreign particles and insects. Our outer ears have hairs and they also act to alert us to intrusions by insects. General body hair ... again, personally anecdotal but...
I recently felt a paralysis tick (ixodes holocyclus) making it's way up my leg only because it brushed against the hairs - I've watched insects cross a hairless area on my foot and felt nothing until the moment they reached an area with hairs. Finding that tick before it attached itself saved me from a painful, itchy lump and headaches. For people more sensitive to these creatures the results can be seriously debilitating. I can't consider that to be a function so unimportant as to be irrelevant.
Is it possible to make informed speculation about the evolution of our body hair without consideration of it's sensory function?Edited by Ken Fabos, : fixed typos and some rephrasing

Hair on the head can serve as a sexual cue.Hair around the genitals and joints (such as armpits) can serve to prevent chafing. That sort of skin irritation can lead to infection and other difficulties.Thin body hair can still act as a partial barrier against insects, or again prevent chafing with clothing. (more modern clothing won't chafe even without hair, but older clothing probably would.)

Hi Phage & Ken Fabos, I can verify this, as when I lost all my body hair (chemo) I had to use talc powder to lubricate the skin or it got raw. ... as a means to detect insects (mosquitos that carry diseases?) as a survival benefit.Enjoy.

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It should be pointed out that there are some ethnic groups of Africans who don't have any body hair at all, not even the fine fuzz that most people have (that is, they have the normal concentrations of hair on the head, armpits, and pubic area, but none on their arms, legs, etc). I write this from memory, so I can't give a reference. I've also seen it for myself.They seem to get by without it.Edited by Dr Adequate, : No reason given.

I think it is certainly vestigial, but there could be some functionality with it that nature has co-opted, like chaffing. I don't think the role is as pronounced as you theorize for the simple fact that certain humans have less hair than others. Are Asians, Africans and women then less adapted for sensory input than exceptionally hairy people, like many caucasians?No doubt the evolutionary trend is that each successive generation is likely to be statistically less hairy than the one before it. If it served a vital and highly advantageous function it wouldn't be deleted from the gene pool and yet evidently it is. Edited by Hyroglyphx, : No reason given.

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"Men never do evil so completely and cheerfully as when they do it from mistaken conviction." — Blaise Pascal

I think perhaps your confounding cause and effect: that is, the reason that these regions chafed was because they had been so long protected.After all, you didn't chafe in these areas when you were prepubescent, did you?The fact that such hair is acquired at puberty suggests to me that it has something to do with sex; if it had any other purpose it would develop earlier.

Hi, Ken.Welcome to EvC! 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.----- 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.----- 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.

But heat transfer through evaporation might be more efficient with more-or-less isolated human hairs wicking sweat away from the skin.Research is needed, but I really, really don't want to be the one doing it. (And it's likely already been done.)And welcome to EvC, Ken!!

Hyroglyphx - My understanding is that it's not the number of hairs that's widely variable, just their size - but I could be wrong. Anyone know? Being very fine and short doesn't make them insensitive and may make them more sensitive to even lower threshold stimuli - although within a zone closer to the skin. From personal experience the very fine hairs high on cheeks near my eyes are barely visible yet are extremely sensitive to touch. Seems to me such sensitivity could be variable according to location as well as by the thickness, length and stiffness of the hairs themselves; there'd be advantage to such fine sensitivity around eyes and ear canal over heavier hairs that may not pick up the presence of very small insects.
I'm not sure I'd agree that there's an ongoing across the board trend towards less body hair, either by numbers, by skin area or by size. Seems to be more variable across our species than that. I'm not sure how the results would play out with competitive beard and chest displays amongst males seeking to force away rivals from bare skinned beauties - my understanding of the mechanics of such evolution is a bit thin and I'd hoped to tap some people here who've studied it.

Hi Dr Adequate, I thought about this, and considered that perhaps I didn't chafe because I was a scrawny kid, or that I lost whatever tough skin conditioning I had acquired as a child, once the hair was in place, or I never noticed as a kid, not having anything to compare it to. Plus any explanation for why hair is useful (or not) needs to explain the sexual dichotomy, just as any explanation for hair loss needs to explain the sexual dichotomy (as sexual selection does).Enjoy

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bluejay, the pervasiveness of this paradigm is something I am interested in. It seems that general body hair - rather than that of more location specific hair like the pits and crevices, the eyebrows and eyelashes, beard and chest hair - does get widely considered functionless and the sensory function in particular is conspicuous for failing to even rate a mention. From Wikipedia, to the recent SciAm article "The Naked Truth" by Dr Jablonski, the sensory function of body hair fails to get any consideration. Rather than explicitly stating that it's essentially functionless, that paradigm seems implicit and Jablonski even extends it to creatures like mole rats - which very much would be advantaged in their dark tunnels by touch sensitive body hairs. Likewise M. Rantala's paper "The evolution of Nakedness in Homo Sapiens" gives it no consideration.
On whether the hair we retain functions to inhibit or improve evaporation of sweat, I tend to agree with Coragyps; the outer surface of hairs themselves are well suited to carry moisture by capilliary action which would increase the overall surface area. By slowing but not blocking air flow close to the skin a layer of cooler air could develop. Just speculating of course and my focus remains the much neglected sensory function.
To rephrase your final question - what are the functional advantages and disadvantages of the variable pelage we currently possess? Sensory function belongs somewhere in that answer.Edited by Ken Fabos, : rephrase for clarity

Hi Bluejay, 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 ...Enjoy.

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