Body Hair Loss in Aquatic Mammals
Phil Nicholls (pnich@globalone.net)
Sat, 14 Oct 1995 18:00:28 GMT
One of the arguements presented by Elaine Morgan and her supporters is
that the naked skin of humans is a result of an adaptation to an
aquatic or semi-aquatic environment. In 1985 Pete Wheeler published
an article on hair loss which contains a very good critique of the
Aquatic Ape position. I would like to summarize those points here.
Nake skin is advantageous in mammals because it reduces drag and and
thereby lowers the energetic cost of locomotion. Also, since the
purpose of fur is to trap air between the skin and the environment, a
loss of body hair reduces unwanted bouyancy. The drawback is that
the loss of this insulating layer produces a loss of body heat and
this is much more of a problem for aquatic mammals than it is for
terrestrial ones.
The fact is that most species of aquatic mammals have NOT lost their
body hair. Those that have lost body hair reduce heat loss by
increasing their surface to volume ratio and/or have developed a
thick layer of subcutaneous fat or blubber.
Body hair loss in aquatic mammals occurs in mammals with large body
size or mammals that swim very fast and have therefore also developed
extremely fusiform body shapes.
Neither of these is true of early hominids, whose body weighs are
currently estimated at between 25-50kg. There are two species of
Odontoceti (toothed whales) with body sizes that small and they have
very fusiform bodies. Even in tropical waters a primate with a body
size of between 25-50kg would have found aquatic life energetically
expensive.
AAH supporters vary on the amount of time hominid ancestors spent in
the water. Hence we go from an "aquatic ape" to a "wading ape" and
all points in between. However, only fully aquatic mammals have lost
their body fur -- the only exception to this rule are some walruses
and elephant seals and these are rather massive mammals (elephant
seals range from 900-3500kg and walrusus, 700-2200kg).
Hence aquatic convergence does not explain hair loss in hominids.
In her book _Scars of Evolution_, Morgan reviews other ideas put
forward to explain hairlessness. On page 75 she describes something
called the "noonday ape" and from the description it is very obvious
she is describing the position advanced by Pete Wheeler -- that hair
loss is an adaptation to thermal stress. Though she cites an article
by Wheeler in her references she does not mention Wheeler in this
section nor in the entire chapter on hairlessness (the article by
Wheeler she does mention is the same one I have summarized above.
Unfortunately she gives an incorrect citation for that article).
In her description of the noonday ape she writes:
"It hinges on the propsoition, so far unproved, that an ape's brain
may be more suseptible to over-heating than the brains of other
primates like baboons and that Homo's ancestors were therefore forced
to adopt unique strategies to cope with the heat of the savannah.
It's weakness lies in the idea that such an animal would opt to
specialise in forging during the heat of noon, a niche for which it
would seem spectacularly ill suited."
In fact, Wheeler points out and documents the fact well established
fact that the brain in general is sensative to hyperthermia and that
primates in general lack the ability to selectively cool the brain, a
capacity present in most other animals living on the savannah.
Primates must respond behaviorally to prevent their core temperatures
from rising and this places restraints on their ability for forage.
Now apes have bigger brains relative to body size than baboons and
larger brains GENERATE more heat and are therefore a bit more
sensative to hypertherma than baboons. Wheeler's hypothesis does not
hinge on this fact, however. It hinges on the fact that bipedal
posture reduces the amount of body surface exposed (a biped exposes
only 40% of the surface exposed by a quadruped). Bipedalism is then
a pre-adaptation to body hair loss, which occurs to promote
evaporative cooling.
Morgan seems to miss the point of Wheeler's explanation for hair loss
and instead provides an anecdotal account that suggests that the main
advantage to hair loss is preventing "moldy skin" She cites Sokolov
"It has been found that moisture evaporates twice as fast from fur as
from a smooth surface, proving that fur does not prevent the
evaporation of sweat." True enough moisture on fur is spread over a
larger surface and evaporates more quickly. However, moisture on fur
does not carry away body heat as it evaporates. Sweat on the skin of
a hairless biped is more effective in rejecting excess body heat.
Baboons do not forage during the hottest parts of the day. Like most
animals they head for a waterhole or the shade. Protohominds used
erect posture to extend forging times.
Wheeler's hypothesis was recently critiqued by Chaplin et al.(, 1994.
Journal of Human Evolution 27:497) but Wheeler's response to the
critique in the same volume should be reviewed as well.
That difference between Wheeler and Morgan is that Wheeler's scenerio
makes statements about the phyisology of heat rejection systems that
can be tested quantitatively. Morgan's attribution of nakedness to an
aquatic ancestry is inconsistant with observed instances of
hairlessness in aquatic animals. Other than this it provides no
predictions
Phil Nicholls pnich@globalone.net
"To ask a question you must first know most of the answer"
-Robert Sheckley
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