Infant Mortality and Bipedalism. Stage 2.

John Waters (
5 Dec 1996 20:02:34 GMT

In Stage 1. of this series, I put forward a proposition
showing how the conjunction of infant mortality and
bipedalism, could lead to a reduction in body hair, and an
increase in the number of perspiration glands, in the
Hominid species.

However, this evolutionary development could lead to death,
or disablement, due to heat exhaustion. The latter
condition is the result of a loss of body salt due to
excessive perspiration.

The easiest way for a species to replenish lost body salt
is to steal it from other animals, through predation. That
would be fine for adults, and I think we can assume that an
omnivorous species like the Hominids would have been able
to increase
their level of predation if required.

However, there could be problems for the infants. This is
because the younger infants feed solely on their mother€s
milk. They do not eat adult type foods.

In this regard, there would be an evolutionary advantage,
in terms of infant survival, if the effects of body salt
losses could be reduced in some way.

It will be evident that a bipedal nursing female will carry
its infants either in her arms, or on her hips. The latter
carriage may be on one hip, or piggyback on both hips.

Either way, the nursing female will expend more energy when
carrying the infants. If the terrain is hilly, the nursing
female€s main leg muscles will have to work especially
hard. In addition, the abdominal side muscles will work
hard to maintain bipedal

Whenever a muscle works hard, it produces heat. Although
the thermoregulation mechanisms stabilize the mammal body
temperature, the actual temperature of hard working muscles
can rise above the bodily average. Of course, it is only a
temporary condition. The heat is dissipated either directly
through the skin, or conducted into the bloodstream and
dissipated elsewhere.

Nevertheless, the hard working muscles become temporary
*hotspots*, which could be prejudicial to an infant which
was in close proximity to such muscles. As far as a
new-born hominid baby was concerned, the main muscle likely
to effect it would be
the heart. Older infants carried on the hips of the nursing
females, would be affected by the thigh, hip, back and
stomach muscles.

These *hotspots* would generate excess heat which would be
conducted into the infants€ bodies, thus raising the
temperature of the infants. This would be combated by the
infants€ thermo-regulation devices, in particular their
perspiration glands.

So the infants would not die of overheating, but they could
die of heat exhaustion. They would be losing body salt,
which they could not replenish because they would still be
suckling from their mother.

This endemic situation would favour the evolution of
increased subcutaneous fat in the female, in the parts of
her body which came into direct contact with her infant
during bipedal carriage.

The subcutaneous fat would prevent the heat produced by the
nursing female€s muscle *hotspots* from being conducted
into the infants. This would reduce infantile disablement
or death due to heat exhaustion.

Of course, the increase in the level of subcutaneous fat in
the female would increase her own temperature, particularly
where the extra fat was located. This would encourage the
evolution of reduced adult hairiness in these regions.

If at first you don€t succeed....

John Waters

John Waters is the author of "Helpless as a Baby",
a book concerned with general and human
evolution. It may be accessed at URL