Re: Homo amphibius and Hypothermia

Phillip Bigelow (
Sun, 25 Dec 1994 17:50:09 GMT writes:

> A swimmer once told me that humans can suffer from hypothermia
> if many hours are spent in the water -- even fairly warm water
> (less than body temp, I guess).
> Does anyone know if this is true?
> -APB

Yes, it is, _particularly_ in animals that weigh less than 500 lbs. There
are no small, hairless, semi-aquatic mammals or small hairless fully
aquatic mammals living today. All small endotherms need protective fir.
Large endotherms such as the elephant and the whales, lack hair, because
they keep warm as a result of their great mass (low surface area/volume
ratio). This feature in large mammals is called "endothermic
mass-homeothermy". In low-mass animals (less than 500 lbs), water saps heat
from the core of the body extremely fast--much faster than does air. That is
why low-mass endothermic aquatic mammals _always_ have a protective fur for
insulation in the water. There are no exceptions to this rule.
Lucy (A. afarensis) weighed only 25-50 KG (less than an adult human). At
that mass, the presence of hair is a necessity, and a near-certainty. Any
earlier ancestors of Lucy were her size or smaller, and, by the limitations
imposed by their small mass, would also have to have abundant body hair...
even if they lived on land all of the time. Small mammals _need_ hair.
It's a rule of thermo-regulation in the Mammalia.
My reference for the necessity of hair on small mammals is:

Schmidt-Nielsen, K. 1975. Animal Physiology. Adaptation and
Environment. Cambridge University Press, Cambridge.

My reference on the body mass of Lucy (A. afarensis) is:

Carroll, R.L. 1988. Vertebrate Paleontology and Evolution.
Freeman Press, 698 pages.