Re: What makes mammals warm blooded, and other animals not?

Hi Jerry!  Thanks for the excellent question!

"Warm-blooded," or endothermic animals, maintain their own internal body 
temperature.  "Cold-blooded," or ectothermic animals, have internal body 
temperatures near or at their surrounding environment.

Birds and mammals, both endothermic, maintain a relatively constant 
internal temperature by oxidizing food.  The specific process, Cellular 
Respiration, involves two phases:
1. Glycolysis (occurs in the cytosol of the cell):  Glucose (food 
molecules) --> Pyruvic Acid
2. Oxidation of Pyruvic Acid, also called the Citric Acid Cycle (occurs in 
the mitochondria)into Carbon Dioxide, Water, and ATP.

The ATP that is produced is then used by the body for various 
energy-consuming functions, including muscle contractions (shivering), 
vessel constriction/relaxation, eating, panting, sweating...

Cold-blooded animals, such as lizards, fish, and amphibians, have much 
slower metabolic rates and are at the mercy of their environment for 
maintenance of their internal body temperature.  This sounds bad:  If I put 
my American Chameleon on the South Pole and it's -20'F outside, pretty soon 
his little body would have an internal temperature of -20'F.   So why be 
cold-blooded?  There must be some advantages, otherwise these animals would 
not have existed as long as they have.  Let's see:

One advantage to being cold-blooded is that since their metabolic processes 
are slower than warm-blooded animals, you can get by on fewer meals, albeit 
with less activity.  A lizard does not have to spend every minute grazing 
in pastures or chasing down gazelles.  My chameleon can eat one cricket 
every few days, and then lounge around, metabolizing. 

Another advantage is the ability to lounge around in an efficient manner. 
If I put put my chameleon outside in the snow, he will not start shivering. 
He'll utilize the environment for sources of heat to warm his body.  He'd 
sit on a rock warmed by the sun, and bask in the sunlight to increase his 
temperature.  If no sun is out, he'd drop into a state of anesthesia and 
become physically mentally inactive, awakened only by an increase in his 
body temperature if the sun came out. Likewise, if I put him in Hawaii, 
he'd hide in the shade of a plant or a rock in attempt to cool off.  In 
Knut Schmidt-Nielsen's Animal Physiology text, he mentions the Peruvian 
lizard Lioaemus, that had a measured internal body temperature of 91.4'F, 
while outside it was only 34.7"F! The lizard adjusted his body to face the 
sun side-on (to expose maximum surface area), and used the solar energy to 
generate heat.  Very economical!  He didn't have to use up his own body 
fuel for  shivering or curling tightly into a little ball to stay warm.   

So, if being cold-blooded is so economical (as solar energy is free), why 
be warm-blooded?  It seems that a warm-blooded animal, in order to maintain 
its own internal temperature, must eat and eat and eat all day to 
continuously "feed" the mitochondria in its cells to generate more ATP for 
more energy in order to eat to feed the mitochondria to make ATP 
to…..ARRRGH!  An endless cycle!  Let's see:

The large majority of birds and mammals exist at regions on the planet 
where days are warm and nights are cool.  Homeotherms have evolved their 
metabolic processes to function most optimally when they are awake and 
using energy to feed --- during the day when surrounding temperatures are 
warmest.  This poses a problem during nights, as the temperature drops.  
Bulking up on food to store as fat is one strategy used by larger animals, 
but small animals would have to eat non-stop to function like this.  
Evolving a means of maintaining a constant internal temperature allowed 
them to not only exist at cooler temperatures, but to be night-active, 
physically and mentally.  

Great, you think.  So now homeotherms can function 24 hours a day, 7 days a 
week!  They can now eat, hunt, mate, or evade predators at all hours!   
They'll get tired and die!  Yes, they would, but along with the evolution 
of warm-bloodedness came another evolution --- sleep.  Sleep is unique to 
homeotherms in that it is a state of semi-inactivity which can be 
immediately interrupted.  Unlike the anesthetized state that cold-blooded 
animals experience and become very vulnerable in, sleep allows a homeotherm 
to suddenly awaken with the ability to fight or run away as necessary.

Hope this helps to answer your question! 
G. Monreal

For more information, please consult the following: http://home13.inet.tele.dk/palm/warmweb.htm http://www.dinosauria.com/jdp/misc/blood.htm http://www.insect-world.com/mammals/warm.html
Schmidt-Nielsen, Knut.  (1993) Animal Physiology:  Adaptation and 
Environment.  Cambridge University Press.