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Re: What is an "active ectotherm"?



Jonathan R. Wagner <znc14@ttacs1.ttu.edu> wrote (quoting me):

>> The temperature of the animal is going to depend on the rate at
>> which heat is produced by the burning and the rate at which heat is
>> being lost to the environment.  The animal's temperature is stable
>> when these two rates are equal.
>
>        So, like us, fish are fighting an uphill battle all the time
> as well.  Don't some of them have special low-temperature enzymes as
> well?

What I wrote was entirely general in terms of the animals.  I did
neglect to mention a couple of very important points, though.  One is
that aside from metabolism, organisms can gain (or lose) heat via
radiation.  Another is that I implicitly assumed that the environment
would be at a lower temperature than the animal.  The environment can
also provide heat.

In any case, the next time you're arguing with a Victorian who's
trying to tell you that endothermy is "better" than ectothermy, remind
them that their enzymes are finely tuned to operate at physiological
temperatures.  Ectotherms must tolerate more slop, so in a sense some
of their machinery is more versatile than ours.  I suspect that's
partly what you're remembering with respect to "special" enzymes.  I'm
not so sure why you think of them as the "special" ones, though.
There are a lot more fish (in terms of species and in terms of
absolute numbers) than there are mammals and birds.  I'd think you
should say that we have "special" enzymes that work at elevated
temperatures.

>> the rate of heat transfer is also proportional to how well the
>> surroundings can take the heat away.  We stay warmer in air of a
>> given temperature than we would in water because the water takes
>> heat away better than air.
>
>        Interesting...  Except I guess the disadvantage for us is
> that that air won't be staying at a "given temperature" for too
> long, even in the tropics.

That's ok.  Our rate of metabolism isn't fixed either.  That's the
major point of these discussions.

>        The greater the specific heat, the greater the heat flow?

Really you need to know also the thermal conductivity.  I just tried
to look back through my physics and heat transfer books to see if
specific heat and thermal conductivity are necessarily related, but I
found no indication that they are.  In electrical circuit analogs,
specific heat is like capacitance and thermal conductivity is like
electrical conductivity.  You really need to know both in order to
analyze the situation fully.  How much heat your immediate environment
can hold is irrelevant if the conductivity is so low that you can't
get the heat into it.  And how fast you can get heat into your
immediate environment is irrelevant if the environment can't take much
heat.  However, you can look at it this way: If you're in a medium
with a high specific heat, then by definition it's temperature won't
change much as it takes heat from you.  That means that the driving
force that's pulling out your heat (the temperature difference between
you and your surroundings) stays high.  If instead the environment has
a small heat capacity then it will come up to your temperature rather
than your temperature dropping to its.  The air immediately around you
is generally warmer than the air farther away.

> I am aware that many fish have insulation, but does their lack of
> marine-mammal-like levels of fat mean that, swimming efficency or
> no, they generate more than enough heat to keep their little bodies
> going?

I think the answer is trivially yes.  Otherwise they'd stop :-)

In other news, Nick Longrich wrote of feathers on a dinosaur:

] Everyone realizes, of course, that this is the most ironclad
] evidence of endothermy (at the very least, in one large branch of
] Dinosauria) you could ever ask for.

What feathers do in all of this is reduce the rate of heat transfer
between the organism and its environment.  That's only one of a suite
of characters associated with endothermy.  While it's important, I
wouldn't go around saying we can all go home now.  Just because the
animal was trying to keep in its heat doesn't mean that it was capable
of stoking its metabolic fires to maintain its temperature.

--
Mickey Rowe     (mrowe@indiana.edu)