Re: Bipedalism and endurance (Re: AAH - enough already)

Braxton (
Fri, 16 Dec 1994 21:57:24 -0500

In article <>, (Gerold
Firl) wrote:

> In article <3co8m1$> (Bryce Harrington) writes:
> > (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.

I've been following this discussion as a simple-minded general science
teacher and there seems to be a few basic science dogs that aren't barking.

1. A major factor in heat loss is the ratio of surface area to volume.
The larger your volume, the smaller the ratio of surface area to volume. It
gets harder to cool off as your volmue increases. Humans or pre-humans with
a smaller surface area to volume ratio would need to adapt other ways to
cool themselves. Not having fur might be one of those. In the same vein,
limbs have a large surface area to volume ration so they radiate heat
quickly with fingers and toes being great radiators of heat. Apes have
longer arms-good radiators-and small volumes which gives them a good
surface area to volume ratio for cooling.

2. Since evaporation depends on the relative humidity of the ambient air,
fur tends to reduce evaporation by restricting the airflow and sweating
under the fur raises the humidity of the air near the skin reducing
evaporation. If you have a bigger volume, you had better have less hair and
or it should be much shorter. Case in point elephants. Elephants are also
an argument for point one in that they have paper thin ears which increase
their surface volume and allow better heat radiation.

3. The bad news for human cooling is the fact that hot air rises. When
they stood up, the warm air from their body rose along their body keeping
it enveloped and reducing the heat; whereas in apes, the semi-erect
position keeps a cool air flowing along the body. By the way, I've heard
this called the human plume. If it gets too hot, the plume causes problems,
so you put on a long flowing garb. It makes you and your arab robe act like
a chimney and increases the upward velocity of the plume causing, in
effect, a wind chill.

4. The answer to hair on the head is simple. The head loses a large amount
of heat-- due to the blood flow I think. When you sleep, you're not
standing and the plume is no help to keep you warm. Hair on the head then
could be an advantage.

Bill Sheaffer