Re: Brains size and sweat

Glenn Morton (
Tue, 07 Jan 1997 22:25:48

Hi Rob,

you wrote:

>I was interested in Glenn's discussion about brain size, sweat and hair, but
>I had some questions from a scientific point of view.
I was planning on making a couple of clarifications in a couple of days
anyway. I have been engaged in some fascinating private e-mail discussions
with two people. One said that the brain did not create a lot of heat (which
is your first question. The other criticized me for implying that animals do
not sweat. Re-reading my statement tonight:
>Concerning the Dmanisi, Georgia Homo erectus dated at around 1.6 myr ago, and
> his need for clothing, I had better head off a possible counter argument.
> There is some good evidence that Homo erectus had sweat glands like we do.
> The importance of this is due to the fact that humans have a unique sweating
> system and it seems to preclude hairyness SIC?. It will take a couple of
>days for me to put that argument together. But if H.erectus was not hairy and
> was relatively hairless as we are, he would need clothing.

I am not sure I really said anything about animals not sweating, but if anyone
else read what I wrote to say that animals don't sweat, that is wrong. They
do sweat but not nearly as much as we do and it would appear that the type of
sweat glands they have is different, they have apocrine glands under their
fur. I have been challenged on this, but all the sources I have consulted
agree with this and no one has been able to come up with a source which says

> - Does the brain actually create more heat than active muscles? I had
> never heard that the brain creates a significant amount of
> heat. How can we calculate the heat generated by the brain and
> how does that compare to the heat generated by the muscle
> system, because if we can't demonstrate that the brain generates
> a significant portion of the heat generated by the body, then
> that speculation is nonsense.

Yes. According to Pinker and Johanson and Shreeve (Steven Pinker, The Language
Instinct, (New York: Harper/Perennial, 1994), p. 295; Johanson and Shreve,
Lucy's Child, p. 263) state that the brain uses 20% of the total metabolic
calories. Even the gentleman who challenged me that the brain was not a huge
user of energy said it only used 15%. Let's use the 15% figure. The average
European male weighs 70 kg. At a density of 1 g/cc the average brain 1350 cc
weighs 1.35 kg. Assuming a 2000 calorie diet for simple calculations, the
brain represents 1.9% of the body mass and uses 300 calories. This is 222
calories per kg. The rest of the body, mass 68.65 kg, uses 1700 calories.
This is 24 calories/kg. The brain is a greedy energy pig and creates this
much heat. Even if you have the skeleton (which does have some living cells
represent half the weight of the body, the muscles then produce 48
calories/kg. This is still much less than a kg of brain.

> - Why aren't non-sweating primates brain damaged from heat, or are they?

First, their brains are smaller. Chimpanzee brains are 400 cc Brace and
Montagu, Human Evolution 1965. p. 146

the encyclopedia Britannica V 16 (1982) different place from the
other day, says:

"The two categories of sweat glands are those called apocrine, which are
usually associated with hair follicles, and eccrine, which are not.
"Most other mammals have numerous apocrine glands on the hairy skin; eccrine
glands are usually absent from the hairy skin and limited to friction
surfaces. In nonhuman primates there is a tendency for the number of eccrine
sweat glands over the body to increase in progressively advanced animals at
the same time that the nubmer of apocrine glands become reduced. Prosimians
have only apocrine glands in the hairy skin; eccrine glands begin to appear in
some of the higher forms. The great apes either have equal nubmers or have
more eccrine glands, with apocrine glands restriced to specific areas.
"Strictly speaking, apocrine glands have nothing to do with sweating."
[by this them mean sweating like us--grm]
"In spit of their large size, apocrine glands secrete only small amounts
of a milky, viscid, pale gray, whitish yellow, or reddish fluid that
contributes very little to axillary sweat. If eccrine glands were not there,
the axilla would be relativley dry." p. 843


"Sweat glands on the hairy skin of subhuman primates probably function
subliminally or not at all, although they are structurally similr to those of
man. The skin of monkeys and apes remains dry even in a hot environment.
Profuse thermal sweating in man, then, seems to be a new function." p. 843
> - Does the fact that horses sweat suggest anything?
>While the discussion is tantalizing, I would really like to see more facts
>and figures to support it.

The data for the below is from a Jan.1969 Scientific American Article on the
Eland and Oryx by Taylor. Desert animals like these should be expected to
sweat a lot but they don't.

sweat eland
uses 5.5 liters of water per 100 kg body weight when hot p. 94-95
60 % lost via evaporation. 20% via feces 20 percent via urine. p. 89

5.5*5*.6=16.5 liters per day via evaporation. or 3.3 liters per day per 100 kg
body weight. In human terms they sweat 2.268 liters/day (a human weighs 70 kg
so there are 7.14 human body masses per eland.

~C.R. Taylor, "The Eland and the Oryx",Scientific American January 1969

Humans are better. humans weigh 70 kg thus there are 7.14 humans in an eland.
"Sweating can deliver two litres of water to the skin surface in two hours and
carry off almost 600 calories of heat. "~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

A human supplied with sufficient water and salt can theoretically sweat up to
24 liters over a 12 hour period. This shows the difference between our
sweating efficiency and the eland.


Oryx weighs 202 kg Grolier's Computer encyclopedia
uses 3 liters water per 100 kg on hot day. p. 95
62% lost through evaporation p.89
2.02*3*.62=3.75 liters per day or 1.87 liters per day per 100 kg body weight.
In human terms they sweat 1.299 liters of water per day. (there are 2.88
humans per oryx body mass.)

Sweating efficiency.

The oryx sweats 15 liters/202kg over a 24 hour period. This is
.074 liters per kg. At 580 large calories per liter evaporated, this is the
removal of 42 calories per kg. Mankind on the other hand can sweat .34
liters/ kg over a 12 hour period (2 liters per hour and 70 kg mass). This is
the removal of 197 calories/kg. If we do this in time then the oryx removes
362 calories per hour and man removes 1160 calories per hour. Human sweat is
tremendously more efficient by either measure!

As to the need for cooling I took my cannibalistic hamster and measured her.
(This vile, nasty little rodent, I call Hannibal after the guy in the movie
The Silence of the Lambs raises ethical problems which need to be discussed )
She can be represented by a cylinder 2.6 cm radius and 15
cm long. My cat, aptly named "Butterball" can be represented by a cylinder
8.4 cm radius and 35 cm long. The hamster has an approximate surface area of
.0287 square meters. The cat has .2289 square meters. Their respective
volumes are 318 cc for the hamster and 7735 cc for the cat. Lets try to
maintain a constant temperature of 25 degrees C in a cylinder of water with
these dimensions. Room temperature lets assume is 20 C. Radiation laws say
that heat is exchanged is area x sigma *(To-Tr)^4. Sigma=5.67 x 10^-8 watts
per meter squared.

Plugging in these values we find using 1 gm/cc:

the cat loses .0037 microjoules per hour per gram.

Hannibal the cannibal loses .0115 microjoules per hour per gram.

The cat has more difficulty losing heat than the hamster on a per gram basis.

Thus, since the oryx is bigger than mankind, he should have more trouble
eliminating heat (even if he were naked) and so should need a more efficient
heat removal mechanism but he has a less efficient one.


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