Infant Mortality and Bipedalism. Stage 1.

John Waters (
4 Dec 1996 11:24:29 GMT

Infant mortality in mammal species is an unfortunate fact
of life. The death rate in infancy due to various diseases
is much higher than in adulthood. This is an endemic
condition affecting all species of mammals. A common cause
of death is overheating due to a fever.

The question arises as to whether the interaction of this
endemic situation, and a species adaptation to bipedalism,
would lead to any unusual consequences.

Let us compare Chimpanzees and Hominids. I think it is
reasonable to assume that Chimpanzees are as hairy as they
are for good reason. Although it can get hot in the day
time in equatorial conditions, it can also get quite cool
at night. Furthermore, when it is wet and windy, a
*hairless* Chimpanzee could die of exposure.

However, when infants have a high fever, their condition
can be improved if their temperature can be brought down in
some way. In this context, there would be an evolutionary
advantage accruing to any infant which had less body hair,
or more
perspiration glands. These could help to lower the body
temperature at times of high fever, and enable the infant
to survive.

Clearly, this evolutionary advantage would apply to both
Chimpanzee and Hominid infants. This raises the question as
to why the Chimpanzee species has not taken this
evolutionary road to improve the survival chances of its
infants. The answer
is clear. *Hairless* Chimpanzee infants could die of
exposure in cold or wet

Now let us look at hominids. The endemic conditions are the
same. The evolutionary advantages are the same. But the
hominids are different from the Chimpanzees in one way.
Hominids are bipedal, whereas Chimpanzees are quadrupedal.

As a result, if a hairless Chimpanzee baby hung on to its
mother, either ventrally or dorsally, it would be
relatively exposed to wind and rain. In these conditions,
it could die of exposure. Hominid infants by contrast,
would be carried in their mothers arms. Nice and snug and
warm. They would be protected from the worst effects of
exposure by their mother's arms, or body.

Of course, infants grow up to become adults, and a
hairlessness condition could have an adverse effect on an
adult. However, adults are larger than infants, so their
surface to volume ratio tends to make them less susceptible
to the effects of cold
weather. Nevertheless, in really cold conditions, the
adults could huddle together, like male Emperor Penguins
wintering out in the Antarctic.

Alternatively, just as the rate of hair growth could be
slowed down during the period of infancy, it could be
increased in the adult stage. Indeed, this is what happens
in the Human species today.

Is this a reasonable proposition? Could the conjunction of
bipedalism and infant mortality have led to the
hairlessness of the hominid species?