Matt Lewis  wrote or quoted:

> 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?

``There are many advantages to being warm-blooded. Warm-blooded animals 
  can remain active in cold environments in which cold-blooded animals 
  can hardly move. Warm-blooded animals can live in almost any surface 
  environment on Earth, like in arctic regions or on high mountains where 
  most cold-blooded animals would have difficulty surviving. Warm-blooded 
  animals can remain active, seek food, and defend themselves in a wide 
  range of outdoor temperatures. Cold-blooded animals can only do this 
  when they are warm enough. A cold-blooded animal's level of activity 
  depends upon the temperature of its surroundings. A reptile, for 
  example, will increase its body temperature before hunting and is 
  better able to escape predators when it is warm. Cold-blooded animals 
  also need to be warm and active to find a mate and reproduce.''

 - http://coolcosmos.ipac.caltech.edu/image_galleries/ir_zoo/coldwarm.html
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