Body Temperature

[Djefoo]

Hello,

Why is the human body at 37 degrees celcius?

Thanks,

Jeff

[Griffin]

On 2004-06-28 15:37:12 -0400, taiwanjeff2002@yahoo.com (djefoo) said:

> Why is the human body at 37 degrees celcius?

Evolutionarily, that is the "set point" enabling homeostasis
throughout the range of habitable climates found on Earth.
Or so the leading theory goes.

[bae]

In article <2004062818174016807%nospam@herenet>,
Griffin <nospam@here.net> wrote:
>On 2004-06-28 15:37:12 -0400, taiwanjeff2002@yahoo.com
>(djefoo) said:
>
>> Why is the human body at 37 degrees celcius?
>
>Evolutionarily, that is the "set point" enabling
>homeostasis throughout the range of habitable climates
>found on Earth. Or so the leading theory goes.

But for most other mammals, from cats to cattle, it's about
39C or 102F. And birds run at about 41C, IIRC. (paging Dr.
Hillary...)

The question of why normal human body temperature is
significantly lower than that of other mammals is an
interesting one. Perhaps it has something to do with our
remarkable longevity, several times longer than most other
large mammals.

[Carey Gregory]

bae@cs.toronto.no-uce.edu.yyz wrote:

>The question of why normal human body temperature is
>significantly lower than that of other mammals is an
>interesting one. Perhaps it has something to do with our
>remarkable longevity, several times longer than most other
>large mammals.

Thought #1- During the vast majority of human evolution,
life spans weren't several times longer.

Thought #2 - Okay, interesting thought, so what's the normal
temp of elephants and other long-lived mammals?

I'm too lazy to look it up myself. And besides, it's your
hypothesis, not mine.... ;-)

[Mxsmanic]

Carey Gregory writes:

> Thought #1- During the vast majority of human evolution,
> life spans weren't several times longer.

Not true. Human beings have always had extremely long
lifespans compared to other animals. In the absence of
disease, they routinely live for six or seven decades or
longer, and this has been true since time immemorial.

--
Transpose hotmail and mxsmanic in my e-mail address to reach
me directly.

[bae]

In article <7e74e0p51188j1vg02lmvb15oemd4teiol@4ax.com>,
Carey Gregory <tiredofspam123@comcast.net> wrote:
>bae@cs.toronto.no-uce.edu.yyz wrote:
>
>>The question of why normal human body temperature is
>>significantly lower than that of other mammals is an
>>interesting one. Perhaps it has something to do with our
>>remarkable longevity, several times longer than most other
>>large mammals.
>
>Thought #1- During the vast majority of human evolution,
>life spans weren't several times longer.

I wasn't suggesting that our relatively low body
temperature derives from selection for longevity. I was
wondering (not even speculating) whether the two phenomena
might be related.

Most animals have very short life expectancies in the wild,
mainly due to predation, especially on the young and the
sick, weak and aging.

I should have qualified "lifespan". I mean maximum lifespan
under ideal conditions. That would be humans with good
nutrition and good medical care, no predation or injuries,
animals in well-run zoos or well cared for pets. Under these
conditions, most people will live to 80 or so, and the
record seems to be 122. House cats are now living into their
early twenties, and although I don't know the maximum, IIRC
cattle and horses can often survive to thirty or more under
good conditions.

Still, we seem to routinely live several times longer than
other mammals of comparable size under comparably good
conditions.

>Thought #2 - Okay, interesting thought, so what's the
>normal temp of elephants and other long-lived mammals?
>
>I'm too lazy to look it up myself. And besides, it's your
>hypothesis, not mine.... ;-)

Okay, I did some Actual Research, and it appears that while
humans (and some of their primate relatives) are near the
lower end of the range for placental mammals, down there
with bats and the whales, I was deceived by my limited
knowledge -- ruminants (cows, goats, sheep) and house cats
appear to be nearer the upper end of the range. Sorry, I've
never taken the temperature of any other mammals, AFAICR.

Placental mammals have normal body temps between about 36-
40C, and this is unrelated to body size. Exceptions are
animals that are hibernating, bats which conserve energy in
the daytime by letting their body temps drop to the ambient,
and the eland, a large antelope that survives high temps
while conserving moisture by letting its body temp rise as
high as 42C, cooling only its brain by a special heat
exchange network of blood vessels in its mouth and nose.

Elephants have a body temp of about 36.9. Their ears have a
heat exchange network of blood vessels and the temp
difference between blood entering and leaving the ear may be
as much as 1C. They flap their ears or orient them relative
to wind to cool down. Elephants have a lot of weird
anatomical features - worth reading about.

So I guess I'll have to abort this potentially active usenet
discussion, harpooned by facts. I retract my hypothesis and
cease to speculate, lest I turn into an AP clone.

[Carey Gregory]

Mxsmanic <mxsmanic@hotmail.com> wrote:

>Carey Gregory writes:
>
>> Thought #1- During the vast majority of human evolution,
>> life spans weren't several times longer.
>
>Not true. Human beings have always had extremely long
>lifespans compared to other animals. In the absence of
>disease, they routinely live for six or seven decades or
>longer, and this has been true since time immemorial.

Gee, I almost hate to ask after our last chat <g>, but...

Got a cite for that?

Six or seven decades (or even close to that) for primitive
hungter-gatherers doesn't fit with my understanding at all.
Am I wrong?

[Tech27]

"Mxsmanic" <mxsmanic@hotmail.com> wrote in message
news:ntd4e0pa2ae0u17o8l8etie89j3hee96pj@4ax.com...
> Carey Gregory writes:
>
> > Thought #1- During the vast majority of human evolution,
> > life spans
weren't
> > several times longer.
>
> Not true. Human beings have always had extremely long
> lifespans compared to other animals. In the absence of
> disease, they routinely live for six or seven decades or
> longer, and this has been true since time immemorial.
>
> --
> Transpose hotmail and mxsmanic in my e-mail address to
> reach me directly.

Not entirely correct. Human lifespans (actuarial vs some
"potential" measurement), have been steadily rising over
time. Although humans from way way back may have had the
physical potential to live as long as they do now, in the
real world they rarely did.

[Steven Bornfeld]

Mxsmanic wrote:
> Carey Gregory writes:
>
>
>>Thought #1- During the vast majority of human evolution,
>>life spans weren't several times longer.
>
>
> Not true. Human beings have always had extremely long
> lifespans compared to other animals. In the absence of
> disease, they routinely live for six or seven decades or
> longer, and this has been true since time immemorial.

Are you distinguishing potential life span from life
expectency? As recently as the turn of the last
century, average life expectency was in the 50s.

Steve

[Carey Gregory]

bae@cs.toronto.no-uce.edu.yyz wrote:

>So I guess I'll have to abort this potentially active
>usenet discussion, harpooned by facts. I retract my
>hypothesis and cease to speculate, lest I turn into an
>AP clone.

Hey, you did good. It was one of the few threads actually
relevant to, and worthy of, sci.med -- or, at least, as
sci.med ought to be.

Now, back to convincing some random, anonymous person why
they really ought to see a doctor about that nasty boil on
their private parts, or finding more cases of fulminant
stupidity and/or psychosis to killfile..... ;-)

[Steve Harris Sb]

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

[Mxsmanic]

Carey Gregory writes:

> Got a cite for that?

Look at the ages at death of many historical figures. It's
surprising how many lived into their sixties, seventies, or
eighties. After excluding accidents, infectious and
treatable diseases, and poor nutrition, lifespans for human
beings have always been quite long--essentially the same as
they are today, which is several times that of most other
animals, both proportionately and in absolute terms.

> Six or seven decades (or even close to that) for primitive
> hungter-gatherers doesn't fit with my understanding at
> all. Am I wrong?

They often were killed or died of infectious or (currently)
treatable diseases at an early age, because it was a hard
life. But their genetic lifespan was the same as ours. It
wasn't that long ago. And their lifespans (and ours) are
much longer than those of many other animals.

--
Transpose hotmail and mxsmanic in my e-mail address to reach
me directly.

[Mxsmanic]

tech27 writes:

>
> "Mxsmanic" <mxsmanic@hotmail.com> wrote in message
> news:ntd4e0pa2ae0u17o8l8etie89j3hee96pj@4ax.com...
> > Carey Gregory writes:
> >
> > > Thought #1- During the vast majority of human
> > > evolution, life spans
> weren't
> > > several times longer.
> >
> > Not true. Human beings have always had extremely long
> > lifespans compared to other animals. In the absence of
> > disease, they routinely live for six or seven decades or
> > longer, and this has been true since time immemorial.
> >
> > --
> > Transpose hotmail and mxsmanic in my e-mail address to
> > reach me directly.
>
> Not entirely correct. Human lifespans (actuarial vs some
> "potential" measurement), have been steadily rising over
> time. Although humans from way way back may have had the
> physical potential to live as long as they do now, in the
> real world they rarely did.
>
>

--
Transpose hotmail and mxsmanic in my e-mail address to reach
me directly.

[Mxsmanic]

tech27 writes:

> Although humans from way way back may have had the
> physical potential to live as long as they do now, in the
> real world they rarely did.

Animals in the wild rarely live as long as those in
captivity, too.

--
Transpose hotmail and mxsmanic in my e-mail address to reach
me directly.

[Mxsmanic]

Steven Bornfeld writes:

> Are you distinguishing potential life span from life
> expectency?

I'm talking about built-in lifespan, which is the only
relevant variable in this discussion of body temperature.

--
Transpose hotmail and mxsmanic in my e-mail address to reach
me directly.

[Tech27]

"Mxsmanic" <mxsmanic@hotmail.com> wrote in message
news:hiv5e011tlg538d0ng1dpp8dgdfhtukmsf@4ax.com...
> tech27 writes:
>
> Animals in the wild rarely live as long as those in
> captivity, too.

Interesting comment. I bet you could find examples of
animals in captivity having shorter lifespans too.
Environmental stuff plays such an important role. We are
all aware of how many species will not breed in captivity.
But I think we have lost the original premise, which was
that animals, or man IIRC, had undergone any evolutionary
changes which would have altered their base longevity, not
taking into account any external forces such as disease,
predators, etc.