Re: thermoregulation in hominids

Phillip Bigelow (
Tue, 17 Dec 1996 18:27:01 -0800

Newington Reference Library wrote:

> Is it just me or does anyone else have a problem with the application of
> Allen€s rule to hominids? The rule states that mammals adapt to the
> temperature of their environment through variations in limb length.
> Longer limbs give the body a larger surface area from which heat can be
> dissipated more rapidly.

This rule is actually derived (borrowed) from physiology and zoology.

> It should be possible to measure the surface area of people from
> different populations around the world together with their weight.

Not weight. Total *volume*.

> This
> would be the best way to assess the theory. The brachial index (the ratio
> of the length of the upper arm to that of the forearm) and the crural
> index (the ratio of the length of the thigh to that of the lower leg) are
> used instead, presumably because they are easier to measure. But it seems
> very contrived to me, almost as if someone has juggled data to see what
> fits the theory best. Another measure is €relative body breadth€. That
> does not seem to make any sense either.

In terms of biophysics, which I assume we are discussing here,
the only accurate comparison of this is the following:

Relative heat loss comparison between any two taxa (assuming the
same sub-cutaneous fat thickness, and same type and thickness
of integument, or lack there-of, for both taxa)
= total surfacearea/total volume (a ratio)

Note that internal heat *production* isn't a factor in this
ratio. Internal heat ("core heat") can be measured by direct
means, anyway.

This ratio gives a good approximation of the relative degree of
susceptability of an animal to thermic *dis*-equalibrium.
High surface area-low volume animals lose heat faster into the
ambient surroundings than do low surface area-high volume animals
(assuming the integument and sub-cut' fat are the same between the
two animals being compared).

After this ratio has been calculated, then one must take into account
the normal life-position of the taxa under comparative study.
By this, I mean that heat loss is achieved in two main ways
in an endothermic animal that is at thermal equilibrium with
it's surroundings:

1) by radiative cooling (orientation of the animal is unimportant).
2) by convective cooling (orientation of the animal is very important).

A knuckle-walking primate is less effective at convective cooling
than is an upright hominid. This is because, in hominids,
the body is oriented to enhance the convection process of air around
the body. Note, also, that water, even warm water, greatly
exascerbates the convective heat loss from an animal. This is because
water is a much more effective absorbing medium than is air.

A knuckle-walking or palm-walking primate is also more prone
to absorbing both radiative heat from the ground, and from reflected
heat from the ground than is an upright hominid, because the
animal's torso and brain are both closer to the hot ground than
is a hominid's. These are just a few of the myriad of things to
take into consideration when assessing the thermal stability of
any particular animal. I'm sure there must be more issues to
take into consideration.