On Mon, 27 Sep 2004 04:59:31 +0000 (UTC), matt999_999{at}yahoo.com (Matt
Lewis) wrote:

>I've been wondering for a while about this and it seems that the main
>advantage to maintaining a constant body temperature is that it allows
>mammals and similar animals to maintain a high rate of cellular
>respiration even in cold conditions.  This in turn allows the animal
>to perform activities requiring lots of power, e.g. hunting, even in
>very cold conditions.  However, animals such as sharks are apparently
>able to produce high enough levels of power to hunt even in arctic
>waters.  This seems to imply that they are able to respire rapidly
>regardless of their internal temperature.  If this is the case, why
>are mammals not able to perform the same trick and if they are why do
>they waste energy maintaining body temperature?
>
>Thanks for any help,
>Matt

The fact is that sharks are not commonly found in arctic waters. Of
the several hundred species, only a handful are found in arctic
waters.  The large Greenland Shark is also considered to be
exceptionally slow.
For details, see
  http://www.elasmo-research.org/education/topics/d_arctic.htm

Your point is that there is a rich life of invertebrates in icy cold
arctic waters.  However the water never drops below just a few degrees
below freezing (salt water freezes below 0 C).  Mammals are
terrestrial animals and  air temperatures regular get far colder than
freezing. An air temperature just below freezing,  in the high 20's F,
is considered extremely warm for winters in much of the northern
United States!

Birds and mammals maintain their temperature for a variety of reasons.
Being able to maintain independence from the environment is a major
one.  Preventing freezing is also very significant -- terrestrial
ectotherms can't function in temperate or arctic winter and must find
some way to overwinter, burrowing below the frost line perhaps,  or
else die leaving behind some life stage -- eggs perhaps -- capable of
surviving freezing.

Having a large body size is a very important feature of birds and
mammals that helps them maintain their temperature in the cold.  Of
course, the presence of hair and feathers is the other feature that
helps them.  Sharks, like tuna, are large enough that they can
maintain core body temperatures greater than that of the environment
-- they can be "regional homeotherms" especially during periods of
very active activity (temporal homeothermy). Most birds and mammals
are significantly larger than their poikilothermic terrestrial
counterparts, mainly  the reptiles and insects.  Large reptiles tend
to be tropical; dinosaurs were likely homeothermic. The smaller birds
and mammals have a great deal of difficulty during cold winters.  Body
size tends to increase in colder climates (Bergmann's rule).


The problem is not respiration, but heat exchange.  Cold water, in
particular, holds more oxygen than warm water fish have an easier time
getting oxygen in conditions than in warm.
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