RBH

Having grown up in Minnesota (where, contrary to published reports, the mosquito is not the state bird!), my general hypothesis, uncontaminated by systematically gathered field data or experimental tests of any kind whatsoever, is that bipedalism and hairlessness co-evolved, with the (relative) hairlessness secondary to bipedalism's freeing of the hands to swat skeeters and deer flies.

RBH

Peez

No, it does not ignore that fact. Humans differ from other primates in a number of ways, and it is possible (at least in principle) that some of these differences made either putative parasite advantage greater, or the cost to hair loss lesser, for humans than for those other primates.If all else was equal, then that would be arguable. However, all else is not equal.It is quite possible that our hairlessness was not favoured by natural selection, but if it was favoured by natural selection then it was almost certainly a complex interaction involving the various ways that hairlessness would impact on our survival/reproduction. The advent of clothing would, for example, radically change the functionality of hairlessness (though I tend to doubt that this was what happened), but it is potentially much more complex than that. Perhaps hairlessness would increase the efficiency of parasite removal in other primates, but that it simultaneously increased the energy required to survive cool nights. Perhaps it allowed superior cooling on the open savannah, but increased the chances of getting cuts and scrapes. If it evolved as an adaptation, then we would have to look at the balance among these various factors.

Note that the AAT is not only entirely conjectural, but it is also contradicted by a number of simple observations (nose shape and ability to close, circulatory system, etc.). I would regard the "parasite hypothesis" as being much better, but even so I never objected to the AAT being presented. It is, in fact, a good thought experiment, even though hopelessly flawed. As of yet the parasite hypothesis does not seem to be so flawed, though it is nothing more than an interesting idea for the time being.

Peez

MrDarwin

Hairlessness is uncommon among mammals (whales, hippos, rhinos, humans, manatees, elephants, naked mole rats, etc.) and has arisen independently in several different lineages which, having quite different lifestyles, probably evolved hairlessness for different reasons.

In the case of humans, I think it's reasonable to hypothesize that our hairlessness with respect to all other living primates is tied to some characteristic of humans that other primates do not have--but that's a lot of characteristics (although they tend to be cultural rather than biological). Whether that characteristic is fire, clothing, tool use, language, art, sexual selection, a combination of two or more of the above, or something else entirely (like a complete accident), seems to be very much an open question and I don't see how one could possibly test any one of several hypotheses that could be formulated, especially in the absence of any clue as to when humans lost their hair. Heck, maybe humans lost their hair first, and that drove them to harness fire and wear clothes!

I find the ectoparasite hypothesis weak for several reasons: it doesn't explain why humans have retained dense hair in several very discrete places (genitals, eyebrows, head, beard) nor why there is a distinct sexually dimorphic pattern to human hair (human males are generally hairier than females, and tend to grow beards).

But geez, if we can't speculate, what fun is science?

Peez

If you don't want to hear what ideas are out there, then there is no problem simply ignoring those ideas. However, many of us are interested in hearing about new ideas. Note that this was published in The Royal Society Proceedings B: Biology Letters, which (according to the website) provides a forum for, among other things, speculation:I agree that speculation must be presented as speculation, and papers that imply empirical evidence when there is nothing more than speculation should be rigorously rejected. On the other hand, I see a place for sharing useful speculation. Various scientists may disagree on what constitutes "useful," but I believe that rejecting speculation just because it is speculation is not particularly useful. A hypothesis with some potential for explanatory power is worth publishing as such.

Peez

KC

No, it is still missing my point, if I read your post correctly. I'm talking about the selection pressure to be hairless due to parasites, NOT the disadvantages to being hairless that man's culture makes up for. That only that brings up the question of what disadvantages---if any--there are to man's relative hairlessness in his original environment.

What concerns me is, if parasitic infection was the driving force behind hairlessness, then the fact we do NOT see more of it in our primate cousins makes me question what is so compelling about the hypothesis in the first place. Bringing in additional factors which favor hairlessness, like man's use of clothing, only weakens the justification for the original hypothesis, in my opinion.

KC

ps418

That's a perfectly reasonable question, and it was just answered. Ectoparasites are a salient problem for all furry animals. They bite, irritate, open wounds for infection and also act as vectors for viral and bacterial pathogens. After thinking about it for a little while last night, I was struck for the first time by how much time furry animals spend biting their fur and skin, rubbing against things, and grooming each other in response to ectoparasites. Humans may still get ectoparasite infestations on their heads and groins, but that's far better than having such crabs and lice covering your entire body. Any feature or quantitative trait that reduces the load of these critters without incurring a greater conteracting cost, would be selected for, and it is hypothesized that hairlessness is just such a feature.

That's a perfectly reasonable question as well. Some potential costs of hairlessness include skin cancer and greater heat loss of heat in low-ambient-temperature environments. Pagel and Bodmer offer some other interesting comments as to why human 'hairlessness' needs any explanation in the first place. For instance, we are the only hairless primate, and of the 5000 or so known mammals, only a handful of those are essentially hairless, such as elephants, rhinos, hippos, naked rat moles, humans, walruses, and whales. Many of these have an aquatic or semi-aquatic existence that may explain their relative hairlessness. Others are hairless but develop a tough hide. Humans and naked rat moles are neither aquatic nor have thick hides. Of course, the AAT hypothesizes that hairlessness was adaptive during a phase of semi-aquatic existence 6-8 million years ago. Even if that were true, and there's no reason to think that it is, there is no reason to suppose that hairlessness would have been subsequently retained, as indeed it had been, even in populations inhabiting relatively cold regions.

"The amount of body hair changes rapidly in evolutionary time as seen from comparing mammoths with extant savanah-dwelling and relatively hairless elephants, or domestic pigs and some dog breeds with their closely related and hair wild cousins. Among modern humans there in variation in the degree of body hair, suggesting substantial genetic variance for this trait"

This is especially notable as I said in light of the potential costs of hairlessness include skin cancer and greater heat loss of heat in low-ambient-temperature environments. I'd say its a reasonable inference, though far from proven, that hairlessness is confering some net benefit and is selected for.

I directly addressed this. There are obvious reasons why hairlessness may have fitness advantages in humans but not in any of our closest relatives. Hairlessnes would have an adavantage in lots of mammals, except that these would not outweigh the high costs.

Those were never offered as additional factors favoring hairlessness. They were factors that served to ameliorate the potential costs of being hairless. Likewise, the fact that naked mole rats live underground where temperature fluctuations are relatively minor is not a factor favoring hairlessness, but it does
ameliorate the potential costs of being hairless.

You're right. However, the authors do clarify what they mean:

I disagree that that is a problem with respect to the hypothesis. Since as you say we don't know, this argues neither for nor against the hypothesis.

That's a good point. I'm not sure it actually weakens the ectoparasite hypothesis though it may indeed suggest ectoparasite pressure alone does not explain evey facet of the distribution of hair on human bodies. After all, there could well be competing selective pressures (e.g. sexual selection for head and/or facial hair --such hair does in fact play a role in attractiveness and mate choice) that explain the retention of hair in these areas but not on the bulk of the body. Or maybe not.

This is an even better point, and raises suspicions that sexual selection may be involved, either in addition to or to the exclusion of other selective pressures. It could be, perhaps, that relative hairlessness confers a natural selection advantage that was subsequently exaggerated by sexual selection. Pagel and Bodmer speculate:


I say speculation, which is basically synonomous with hypothesizing, is a indespensible and highly desirable component of science. So I guess I have a Popperian view of science as an endless series of conjectures and refutations. Someone speculates about something, someone else suspects the speculation is wrong-headed, and both parties keep their eyes out in the future for evidence supporting or falsifying the speculation. There can be no better example here than Darwin himself, whose writings are literally overflowing with speculations on every topic that other naturalists went out to confirm and refute.

Another way to look at the question is: is there an ecological correlation between ectoparasite concentrations and body hair cover? If there is not, then that would be a strike against against the hypothesis. Another question to consider -- I don't know if I'd call it a test-- is: are ectoparasite loads loads greater on the hairier parts of the body, such as the groin and head? They are, obviously.

On my next trip to the library I plan on looking for information on ectoparasite-host interactions. I have a few refs I plan on looking at, but if anyone has more recommendations to share, I'd be much obliged, particularly review articles and particularly with a focus on mammals.

Hebb et al, 2000. Bird-ectoparasite interactions, nest humidity, and ectoparasite community structure. Ecology 81, 958-968.

Hurtrez-Boussès, S., J. Blondel, P. Perret, and F. Renaud. 1997. Relationship between intensity of blowfly infestation and reproductive success in a Corsican population of Blue Tits. p. 267 in Journal of Avian Biology 28, no. 3

Lehman, T., 1993. Ectoparasites: Direct impact on host fitness. Parasitology Today 9(1): 8-13.

Mataxas. K.M. and O.J. Pung. 1999. Effect of blood parasites and hematophagous ectoparasites on Eastern bluebird (Sialia sialis) reproductive success. The Oriole. 64: 41-46.

Pung, O.J., L. A. Durden, M. J. Patrick, T. Conyers, and L. R. Mitchell. 2000. Ectoparasites and gastrointestinal helminths of southern flying squirrels in southeast Georgia. Journal of Parasitology. 86: 1051-1055.


Patrick

KC

Hi Patrick,

After reading your post, I still think we are talking past each over a crucial point. So, rather than quote and respond, I'll just summarize what I'm trying to say here.

If ectoparasites are enough of a problem to provide strong selection for hairlessness, then I fail to see why such selection would not be seen in man's close relatives as well. I know you pointed out there may be disadvantages to hairlessness in other primates, but I dont recall seeing any compelling evidence presented FOR that proposition, at least on this thread. Without any, what reason do we have to even make the ectoparasite/hairlessness selection connection in the first place, other than the observation that ectoparasites are a bummer?

I agree that man's hairlessness needs explanation--especially in the light of the hirsuteness of his closest relatives-- but I think at this stage the ectoparasite explanation is weak, and at the level of the AAT in adaptive explanatory power.

However, I'd definitely change my mind if I saw some evidence for disadvantages to non-human primate hairlessness. I'll try and look for some papers on that.

Regards,

KC

Oolon Colluphid

Well let's see... the important point is that these places retain hair. It's not like these places have something unusual for a mammal, it's the rest of the body that's unusual. If there is an adaptive reason behind hairlessness, it is a case of a shift in the balance between advantages-of-hairy vs advantages-of-hairless: between picking up scratches and having to scratch less .

So one could view it like this. Anything that reduces the parasitic load is an advantage. Being hairless (as defined ) might do so, by leaving fewer places for the critters to hide on our hide. Note, fewer: it doesn't have to be none, if there are reasons for keeping hair in some places.

So the ectoparasite hypothesis does not need to explain why hair is retained in some places, for there are advantages to having hair too.

So, why in these other places? Well, eyelashes serve to emphasise our eyes, which are so used on non-verbal communication. Note that the whites of our eyes are visible, whereas a chimp's are not. And at the risk of being supercilious, our eyebrows too are rather important aides-communique (or whatever ).

Head? Well there's that big hot thing in those African skies to consider; conversely, it is said by those who'd have us wear hats in the winter that we lose some significant percentage of our body heat through our heads. So head hair is a useful thermoregulator.

Armpits and genitals? Well, iirc we produce different sweat from these areas... containing pheromones etc? Sweat-soaked hair acts to maintain the smell rather than it blow away with evaporation: these areas are neat little scent-puffs. To be honest, I'm at a loss as to why armpits particularly, but with genital hair, being where it is, the connection is rather obvious.

And beards: they grow rather excessively, unchecked; they are not just a normal bit of mammalian pelt, but will grow positively shaggy. Together with the sex difference (ie women not having them), this looks like sexual selection at work.

In short, what I'm suggesting is that if parasites were part of the selection pressure towards hairlessness, then we lost hair from everywhere except those places where it was still of use, where to lose it from became too much of a loss. That we are relatively hairless is all that it might take to have relatively fewer parasites, and so a relatively lower drain on our resources.

As an aside, though, it's curious that it is the male hormones that are responsible for secondary sexual characteristics -- such as significantly more hair, and especially beards -- that are also, I gather, linked to hair loss in male pattern baldness. (I know it's not a case of 'testosterone makes hair grow', but it's still odd, don't you think?)

Cheers, Oolon

Doubting Didymus

This is a very interesting topic. I'm finding it very hard to know what to think.

Obviously, adaptations are a part of evolution somewhere. What I would like to know is this: are there any specifically adaptationist theories that we are fairly confident of? What does the confirming evidence look like in those cases?

Nic Tamzek

There was an ARN thread on this awhile ago:

http://www.arn.org/cgi-bin/ubb/ultim...;f=13;t=000540

I think that one important point about the hairlessness issue is that at the moment, we have no particularly strong reasons to consider human hairlessness an adaptation at all. It is a perfect example of something that might have happened for non-selective reasons, or selection for something unrelated.

Here is my post from that older thread:

I think that that relative hairlessness of various primates point would be well worth investigating in this regard. Chimps seem to be somewhat on the low-hair end of the scale. Other primates have various large bare spots for various signalling and display purposes.

But keep in mind, it could be that hairlessness has *no* particular function and is a by-product of something else.