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Sweating: AAT Critique
Phil Nicholls (pn8886@csc.albany.edu)
5 Jan 1995 14:54:30 GMT
Note: This article is the first of three I would like to
write and submit as a contribution to the FAQ for this
newsgroup. In them I will focus on the three main arguments
of the Aquatic Ape Theory: Sweating, Hairlessness and
Bipedalism.
The aquatic ape theory is about why humans differ so greatly
from the living great apes.
". . . the chief mystery does not lie in any one of these
ANOMALIES [emphasis added], not even the woderful brain or the
dexterous hands or the miracle of speech. It lies in the
sheer number and variety of the ways in which we differ from
out closest relatives in the animal kingdom." (Morgan, 1990)
One of the most important contributions made by Charles Darwin
was the proposition that the differences between us and the
apes are differences of degree rather than kind. Morgan's
reference to morphological "anomalies" expresses the contrary
position held by may psychologists, cultural anthropologists
and even some neurobiologists. Are the differences that
Morgan cites as evidence of aquatic ancestry really anomalies?
Let's start with a quote from Pat Dooley, one of Morgan's
supporters here on sci.anthropology.paleo. In response to one
of my messages, she wrote:
>You make the fundamental mistake of assuming that eccrine
>glands are sweat glands. In every sweating mammalian species,
>except one, the acropine glands are used for sweating. No
>prizes for guessing the odd one out. So far as I know, no
>other ape sweats, even when subjected to extreme heat under
>laboratory conditions. Humans, as you well know, have a 99/1
>distribution of eccrine/acropine glands that puts them well
>off the ape norm. If they had gone directly from the forest to
>the savannah with your 50/50 distribution, then 100 million
>years of mammalian evolution says that humans would have
>evolved acropine sweating if they needed to improve their heat
>dissipation mechanism. The fact they evolved a new sweating
>lost the other 98% of their acropine glands BEFORE sweating
>evolved. As you said, in as many words, nature doesn't evolve
>new pathways when it can build on old ones.
That statement contains a number of errors. One of these was
originally supplied by me. A "50/50" or 1:1 ratio of sweat
glands does not characterize the great apes. This ratio is
actually characteristic of larger Old World Monkeys.
As a matter of fact, if we look at primates as a whole, we
find that in Prosimians and New World monkeys the apocrine
glands are more numerous but that in Old World Monkeys there
distribution is at a ratio of 1:1, 1:2 or 1:3 depending on the
body region sampled (Sokolov, 1982:160-169). For the rhesus
monkeys, Sokolov remarks that "Except for the lips and ischila
callosities the eccrine glands are plentiful." (Sokolov, 1982,
p.165. Johnson and Elozondo (1974) note that the distribution
of eccrine glands in the rhesus monkey is identical to that
observed in humans.
For the chimpanzee, the Sokolov notes "All of the features of
chimpanzee eccrine glands are similar to those of humans. In
the immature female the apocrine sweat glands are much smaller
in size than in the male. These are fewer in number than the
eccrine glands." (Sokolov, 1982, p.169).
It is interesting to note that we are always hearing about the
RATIO of apocrine to eccrine glands. Since apocrine glands
open onto hair follicles, sometimes two or three glands to a
follicle while eccrine glands open directly onto the epidermis,
(Bligh, 1967) it is reasonable to expect that a higher ratio
in modern humans may have more to do with a reduction in hair
follicles than any dramatic increase in the number of eccrine
glands.
The second error in Pat's statement is that no other mammalian
species use eccrine glands as sweat glands. This is, as far as
we know, correct for non-primates. However, in primates any
sweating that occurs is eccrine sweat (see Robertshaw, 1985
for an overview of sweating in primates vs non-primates. For
a look at the research on sweating in primates, see Hiley,
1976; Johnson and Elizondo, 1974 and Newman et. al., 1970).
The first primates were probably nocturnal, as are many of the
living prosimians today. Nocturnal primates do not really
need to worry about overheating. They discharge excess body
heat by panting. As a result, the apocrine glands in
Prosimians did not develop a thermoregulatory role. As
primates evolved and anthropoids appeared and moved into
diurnal niches, they continued to pant until two evolutionary
pressures forced a change. Increase in body size and increase
in relative brain size are well known trends in anthropoid
evolution. As body size increases the number of eccrine sweat
glands also seems to increase (Robertshaw, 1985). As brain
size increases, the size of the nasal sinuses is reduced.
Since in closed-mouth panters this is the place where most of
the heat exchange takes place, the increase in brain size
produced a need for an alternative heat rejection system.
Now the question, as Pat asks, is why eccrine glands when
apocrine glands are the gland of choice in other mammals. The
answer, I believe, lies in the neurophysiology of sweating.
Apocrine glands are controlled by the sympathetic nervous
system. The neurons which control apocrine glands use
noradrenline as their neurotransmitter. Eccrine glands are
also controled by the sympathetic nervous system but unlike
apocrine glands they are cholinergic, i.e. use acetylcholine
as a neurotransmitter. The difference may be compared to
playing a piano. Noradrenline works like the pedals,
affecting the action of all the tones being played.
Acetylcholine is like the individual piano keys. Cholinergic
neurons are employed where fine control over the effector
organs is required. Anthropoid primates need a greater degree
of control over their heat rejection systems because of their
larger brains, which are very sensative to temperature
changes. This is particularly important in Homo sapiens.
Apocrine sweat occurs in bursts which saturate the skin
quickly. The amount of sweat they produce cannot be regulated
nor can their action be sustained for any period of time.
(Robertshaw, 1985). This is ideal for an animal that needs to
cool off quickly after a period of brief intense activity but
are not suited to the task of regulating body temperature over
an extended period of time.
It would appear then that eccrine sweating in humans is not
anomalous at all. It is consistant with the trend observed in
anthropoids and was therefore established long before the the
hominid/pongid split.
-------------------------------
Bligh, J (1967) A thesis concerning the process of secretion
and discharge of sweat. Environmental Search 1:28-51.
Hiley DA (1976) The thermoregulatory responses of the galago
(Galago crassicaudatus), the baboon (Papio cynocephalus) and
the chimpanzee (Pan satyrus) to heat stress. Journal of
Physiology, 254:657-670.
Johnson, GS and Elizondo, R (1974) Eccrine sweat gland in
Macaca mulatta: physiology, histochemistry and distribution.
Journal of Applied Physiology 37:814-820
Morgan, E. (1990) Scars of Evolution. New York: Oxford
University Press.
Newman, CM; Cummings, EG; Miller; Wright, H (1970)
Thermoregulatory responses of the baboon to heat stress and
scopolamine. Physiologists 13:271-285.
Robertshaw, D (1985) Sweat and heat exchange in man and other
mammals. Journal of Human Evolution 14: 63-73.
Sokolov, VE (1982) Mammal Skin. Berkeley, CA: University of
California Press.
--
Philip "Chris" Nicholls Department of Anthropology
Institute for Hydrohominoid Studies SUNY Albany
University of Ediacara pn8886@cnsunix.albany.edu
"Semper Alouatta"
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