Re: AAH: humans long-distance runners?

Clara N. Fitzgerald (cfitzger@s.psych.uiuc.edu)
2 Dec 1994 22:50:37 GMT

geroldf@sdd.hp.com (Gerold Firl) writes:
>In article <jlittle-3011941644580001@47.69.129.57> jlittle@bnr.ca (Jim Little) writes:

>>I just have to jump in here, all sweating buckets does is remove buckets
>>of essential moisture from the body which must then be replaced.

>True. But that is exactly what humans do; we generate a thin layer of
>moisure all over our body. Of course, some does drip-off, and that is
>wasted, as you say, but most of it evaporates, which results in cooling.
I would lean more toward the buckets rolling off; does anyone have
some reasonable numbers?
>Our cooling system is not optimally *efficient*, just extremely
>*effective*. It rejects huge amounts of heat. A god, or an engineer, would
>try to design a more efficient system; evolution just produces one that is
>effective.
Sweating seems to respond more to activity than internal temperature,
hence the human tendency to heat stroke, etc. It can also dump heat so
fast (when it eventually gets started) as to produce further problems.
This is effective?

>At 100 degrees fahrenheit a dog can cool its entire body by panting, if it
>lays quietly in the shade to minimise the amount of metabolic heat
>produced. A man can run at that temperature.
As long as he has access to drinking water...

>>I can give you a reason for an aquatic hominid to sweat as prodigiously as
>>modern humans do, to eliminate excess salt from the body.

>I haven't read morgan, but from the comments I've read it appears that she
>doesn't mention that the human sweat ducts have a salt-recovery system
>which actually re-absorbs salt from sweat as it is secreted from the
>sub-dermal glands and passes to the surface of the skin. (See Guyton,
>_Human Physiology_).
So, did the recovery system evolve when the gland did, or later?
Rather sounds like a patch-up to me...

>> the human system of sweating is very wasteful and poorly
>>adapted to its function.

<pb> writes: (sorry I missed your name)
- You have fallen into a "logic trap" here. If the human system of
-sweating is very wasteful, as you claim, then it is _also_ a very wasteful
-process if the purpose of the sweat glands is to remove excess salt from the
-body, because essential moisture is lost none-the-less.
Thus the speculation of a water-rich (aquatic?) environment.
- Salt removal glands
More details, please? And what kind of mammals? Seabirds and crocodiles
I seem to remember have some similar mechanism...
-on mammals remove salt from the body in much greater concentrations than
-does sweating in humans, and these glands do it using much less water than
-do humans. Besides, humans have no other form of cooling system other than
-sweating. Therefore, the main purpose of sweating _must_ be for evaporative
-cooling.
[And _you're complaining about logic?]
We can pant, can we not, or we can apply a cool substance to our skin,
like mud, water... And even if that is its purpose now, it does not prove
that it evolved for that reason (our lungs evolved from fish's swim
bladders - what can be made of that?)

- Indeed, sweating in humans _does_ remove salt from the system, but this may
-be just another instance where evolution is killing two birds with one
-stone. Since sweating is the only major thermo-regulatory mechanism in
-humans, what better way to remove a little excess salt at the same time?
Salt is removed from the body in urea. Why develop another, largely
uncontrolled (ie, that does not depend on the concentration in the blood)
pathway to loose an important chemical?
-The plumbing was already installed.
The excretory system was already working.

<pb> 

--

-Clara A. N. Fitzgerald     cfitzger@s.psych.uiuc.edu

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