Re: Bipedalism and endurance (Re: AAH - enough already)
Gerold Firl (geroldf@sdd.hp.com)
16 Dec 1994 13:03:09 -0800
In article <3co8m1$17j@phakt.usc.edu> bharring@phakt.usc.edu (Bryce Harrington) writes:
>geroldf@sdd.hp.com (Gerold Firl) writes:
>>Fur will wick sweat away from the
>>skin, where heat provided by the flow of the surrounding air will
>>evaporate it. This is nearly as bad as sweat which drips off the body, to
>>evaporate on the ground. It doesn't help the animal to cool. The sweat must
>>be in thermal contact with the skin as it evaporates, or else it is
>>useless.
>Not true. Fluid evaporating off of a surface will directly result in
>good heat loss, but fluid evaporating off of a surface near the skin
>will also give some heat loss.
True, but the key word is *some*. *Some* heat loss. Whereas water
evaporating from the skin gives maximum heat loss.
>Think about it this way. When you
>step out of the shower, the water on your body evaporates quickly
>and makes you colder than the water on your head, but after 10 minutes,
>the waterng you (relatively) cooler than the remainder of your body.
Good point. Wet hair does help cool the body, especially if it is long
enough, and flexible enough, to come in and out of contact with the skin as
the animal moves around. You shower example is well-taken, and pertains
directly to the question of human cooling.
>Plus, the water in your hair is remains longer and helps cool better
>than the rest of your body.
Not at all. Much of the heat which evaporates the water in hair (especially
if the hair is short and bristly, standing up off the surface of the skin)
comes from the surrounding warm air, rather than from the animal. Sweat
which is wicked away from the skin by hair is much less effective at
cooling than sweat which evaporates from the surface of the skin. One of
the reasons your hair stays wet longer than your skin is the fact that you
don't have direct thermal contact between your body and the wet part of
your hair. This does not allow you to get the full benefit of the heat of
vaporisation of water.
>Also, evaporation does not care about
>water quantity as much as about surface area, so when you compare
>water evaporating off skin with water evaporating off skin+hair,
>you can see that one will give better results than the other.
No, to understand about the relative merits of naked vs. furry skin you
need to account for where the heat comes from; is the water being
evaporated as the result of heat provided by your body, or heat provided by
the surrounding air? The total surface area limits the rate of evaporation,
but high rates of evaporation don't do you any good unless you are
providing the heat.
>And that's not all the story, either. You also have heat reflection/
>absorbsion going on. As the AAH people have constantly pointed out,
>the presence of hair will serve to reflect more heat than bare skin.
>This alone could help a great deal in an environment proposed by
>the SAH.
That is why we still have hair on our heads, of course.
>Second, the SAH doesn't explain *why* this kind of adaptation was
>required. The only thing I've seen is that it helps when chasing down
>horses and so forth.
No, the ability to operate on the ground, at a particular time of day when
the large carnivores are quiescent, is advantageous whether you are
hunting, scavenging, foraging or gathering. Particularly so during the
early transition from arboreal to terrestrial, when the hominids were still
slow, small, and low-tech. These creatures were not chasing down horses.
They were opportunistically getting whatever they could, just like hominids
always do. But during the dangerous period of human evolution when we
weren't particularly smart, and still incompletely bipedal, our cooling-
system advantage made it possible to move out of the trees and at least
start thinking about those juicy horses.
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