# Re: Brains size and sweat

Glenn Morton (grmorton@gnn.com)
Wed, 08 Jan 1997 19:54:11

>Greetings!
> To continue an on-going discussion:
>
>First, I became fascinated by this proposition so have spent the
>morning at the library examining comparative physiology and anatomy
>books.
>
>(1) The unanimous consensus of the books (about 12 I read) was that
>sweating constitutes on 25% of the cooling system in human beings;
>60% of heat loss in humans is from radiation.
>
If this is an average, over most of the time, then I have no doubt about it.
At temperatures above 80 F or so, the rate of heat loss from the body by
radiation is insufficient to relieve the body of enough heat. At 90 F humans
sweat fairly easily and by 98.9 radiation removes almost no heat. Sweat must
remove nearly all of it.

>(2) According to the authors, _all_ mammals sweat, even dogs (there
>are sweat glands in the skin and on the pads of dogs). Skin sweating
>in dogs has been observed and can be induced by medication similar to
>humans. In addition, according to the authors, the cooling systems
>in dogs is "as efficient as in humans."
>

This cannot be true at all. Efficiency is defined as an output/cost ratio.
The cost can be time, energy etc. As I showed last night Mankind's ability to
move water through the skin in the form of sweat exceeds all animals in
quantity per time or per kg. This means that the total evaporation can remove
more heat from a man than for an animal. If as you mentioned earlier that man
can sweat 13.5 liters per day, this is 7830 big calories per day or per 70 kg.
A camel can only sweat 8.8 liters per day per 100 kg. This is 5104 big
calories per day per 100 kg. Mankind is more efficient either per day or per
kg.

A furry animal simply cannot have as efficient a sweating mechanism as a
hairless one. I will agree that a kg of water evaporated will remove the same
amount of heat whether under fur or not. But fur will seriously hinder
evaporation. I do not have the data for dogs, but I do have it for camels.
Consider this:

"Their total water expenditure per day was 3 liters per 100 kilograms body
weight in the shorn animal and only 2 liters in the unshorn camel. Thus a
shorn camel with fur 0.5-1 centimeters long would evaporate 50 percent more
water than would an animal with unshorn fur. The temperature at the fur
surface was as high as 70-80 C, so the temperature gradient through the fur
was more than 30 C."~Hilde Gauthier-Pilters and Anne Innis Dagg, The Camel,
(Chicago: University of Chicago Press, 1981), p. 73

Think about the physics and you will see why fur hinders the efficiency of
sweating.

1. Fur hinders the free flow of air. The air under the fur becomes saturated
and must be replaced by dry air for heat to be removed.

2. Water on the skin under the fur is cooler than water would be on the open
skin. Cooler temperature means slower evaporation and less heat removed per
unit time.

3. Some of the sweat under the fur will be stuck to the hair itself. When it
evaporates, it cools the hair, which does not have the same effect as cooling
the skin, under which flows the blood. It is the blood which needs to be
cooled so that it can cool the interior of the body.

>(3) In terms of heat rejection (as opposed to mere sweating, see item
>#1 above), the human is below the rat and the rabbit in terms of
>thermal loss per kg of body weight.

This is a size effect. Larger animals have a smaller surface area/mass ratio.
This is why Eskimo bodies are short and stout--that shape minimizes surface
area and retains heat. African bodies, like the Watusi, are long and lean,
maximizing surface area and minimizing heat retention.

>(5) Most texts ignore the distinction between apocrine and eccrine
>sweat glands. However, the eccrine glands are much fewer in number
>and smaller in size than the apocrine glands (the latter being found
>in the arm pits, groin and anal regions), which, in my experience,
>are the primary sources of sweat. According to my reading,
>"apocrine" basically means the glands empty onto hair follicles,
>whereas "eccrine" means the glands empty directly on the skin. This
>means the eccrine glands are predominantly found on the palms of the
>hands and bottoms of the feet. The argument that we have an
>increased capacity of sweat _and_ that we have an essentially unique
>type of sweat mechanism (eccrinic) appears to be in conflict. I
>repeat an earlier email: the pig is predominantly "naked" (i.e.,
>devoid of hair), hence should have mostly eccrinic sweat systems.
>
Pigs have hair, it is sparse. Appearances may deceive you. Humans have as
many hair folicles and hair as chimpanzees and gorillas:

"Interestingly, the number of hair follicles in humans is similar to that in
chimpanzees and gorillas, but the much reduced size of hair shafts in humans
gives a hairless appearance."~Adrienne L. Zihlman and B. A. Cohn, "Responses
of Hominid Skin to the Savanna," South African Journal of Science, 82:2,
(1986), p. 307-308, p. 308

Because of this I would like to see numbers on the hair of pigs vs. wild
boars.

>(6) Since some have argued that apocrine glands are not, per se,
>sweat glands but are special modifications (possibly for sexual
>attraction), and since the previous emails are proposed the theory
>that "our form of sweating (i.e., eccrinic)" is different from that
>of animal (i.e., apocrinic), arguments about the volume of sweat
>secreted (?) by humans need to be subdivided into volumes of
>apocrinic secretions versus eccrinic secretions. We would probably
>find that the volume of eccrinic (or "true sweat") per kg of body
>weight is of a more comparable

At least compared to Elands and Gazelles, if their sweat comes from apocrine
glands, the amount sweated is not comparable per kg of body mass
>
>(7) Regarding camels, according to one study a camel can sweat 30%
>of its body weight away without complications. Ten percent loss in
>humans results in major complications. This constitutes, to me, a
>much greater sweating capacity in camels than humans.
>

This is an entirely different issue. This measures how much dehydration men
and camels can undergo, not necessarily how much sweat can be produced. A
camel deprived of water for days at a temperature of 10 C probably doesn't
sweat very much but he can still lose 40% of his body weight through
dehydration. Man can only suffer a 12% loss. This capacity says nothing