Re: Body Temperature
From: Steve Harris sbharris_at_ROMAN9.netcom.com (sbharris_at_ix.netcom.com)
Date: 07/03/04
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Date: 2 Jul 2004 23:05:06 -0700
bae@cs.toronto.no-uce.edu.yyz wrote in message news:<2004Jun30.111210.13021@jarvis.cs.toronto.edu>...
> Still, we seem to routinely live several times longer than other mammals
> of comparable size under comparably good conditions.
COMMENT:
Yes. The number you're really looking WRT aging is specific metabolic
rate multiplied by maximum life span. That gives you a
"calories/joules per gram per lifetime" number. More than a hundred
years ago Perls noticed that this number is (very) roughly the same
for all mammals. It's pretty close to the same for mice and elephants,
for example-- mice have 20 times the specific metabolic rate (specific
means "per gram") that elephants do, but they live 3 years max instead
of 60, so it works out the same number for each. An elephant-load of
mice burn an elephant lifetime of food in only 3 years. Mice run very
hot, because they need to keep warm and have a poor surface volume
ratio, like everything small.
Not surprisingly, except in shrews (which have hit the heartrate
limit), mammalian heartrate scales according to specific metabolic
rate. So mice have 20 times the elephant's 30 bpm heart rate, and that
gives both species the same number of heartbeats per lifetime.
A few species are way off this heartbeat calorie burned per life span
curve. Humans get up to 3 times the number of calories per gram and
heartbeats that elephants and mice do. Capuchin monkeys do nearly as
well has humans.
Clearly, metabolic rate itself in placental mammals is a
surface/volume thing, so it scales roughly as the 2/3 power of body
weight (actually more like 3/4 for some reason-- probably having to do
with nature economizing on calories by fooling around with hair
length). So the max calorie per lifetime limit generally makes large
mammals live longer. [Specific metabolic rate is divided by weight, so
it generally scales as 3/4 -1 = -1/4 power of size. A mouse weighs 30
g and a human 60 kg, with the ratio 1/2000. Raise that to the -.25
power and you get 6.7, which is about the right ratio of specific
metabolic rates].
Big exceptions to the rule are primates like capuchins and humans, and
we both have very large brain/body wt ratios. So evidentally large
brains are such a good evolutionary trick against predation that it's
worth it for evolution to spend time repairing us, and thus we age
more slowly metabolically and get 3 billion heartbeats in a lifetime,
instead of the standard billion for mice, cats, cows, etc.
An even better trick is wings. Birds and bats both do several times
better than even primates, including humans. Some bats don't cool in
the day, and live 20 years anyway. This in an animal with twice the
metabolic rate of the similar sized mouse which gets 3 years. Do the
math.
Shells are a good trick against predation, and turtles do the best
amoung reptiles. But if you do the heartbeat/heat calculation, even
turtles at 200 years don't do better than primates/humans.
Body temp in placental mammals (naked mole rats excepted!) is almost
completely independent of all this, except that critters with the need
for high metabolic scope (really high aerobic capacity) tend to run
higher temps. Birds all run very hot by mammal standards, and so do
active bats. It's easy to see why dogs run 39-40 C. What's not so easy
to see is why cats run almost as hot. As noted, the correlation isn't
perfect. A better correlation is that carnivores/hunters, which have
the need for really high metabolic bursts, then high rates of
digestion afterwards, tend to have slight higher temps. So that gets
cats in. We humans and omnivores, and in just a few milllion years
haven't gotten our temps much above those of our vegetarian primate
ancestors. We have great aerobic capacity-- as good as anything except
canids. But we learned to run on the really hot savannah, where the
higher body temps weren't really needed. When we were running, we
generated them for ourselves. Even now, your average marathoner gets
up to at least 38 C and often 39.
Steve
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