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Warm- vs Cold-Blood (was Re: komodos)

At 01:34 PM 12/9/96 -0500, Barry Williams wrote:

>Pardon me for asking, but doesn't 'bloodedness' of reptiles v mammals
>etc refer to the ability to maintain internal body temperature, and
>has nothing to do with blood heat per se.


>ie mammals, birds etc maintain a fairly constant body temperature
>regardless of ambient, while a reptile's blood temperature rises and
>falls with ambient temperatures - thus reptiles need a bit of a
>'warm-up' in the morning, after a cold night, before they get going.

There are actually several different ways of looking at the
"warm-blooded"/"cold-blooded" division.  In terms of energy source,
warm-bloods are endotherms: they have internal, cellular mechanisms that
generate heat.  Cold-bloods are ectotherms: their heat comes from external
sources (the sun, a hot rock, etc.), as well as non-cellular internal
processes (i.e., digestion).

In terms of rate of nutrient consumption, warm-bloods are tachymetabolic:
they use up a LOT of nutrients at a rapid rate (in a large part, to support
the cellular heat generation).  Cold-bloods are bradymetabolic: they use up
nutrients much more slowly.  To a large part, the use of predator-prey
ratios and Haversian bone texture have been attempts to find tachymetabolism
in the fossil record, rather than endothermy per se.

Additionally, warm-blooded animals tend to be homeothermic: they have
relatively constant body temperatures.  In comparsion, cold-bloods are
poikilothermic: their temperature fluctuates, in general as a response to
external changes.

However, although you can stereotype warm-bloods as endothermic
tachymetabolic homeotherms and cold-bloods as ectothermic bradymetabolic
poikilotherms, there is a lot of variation out there.  Many lizards, for
example, are behavioral homeotherms: by moving between sun and shade they
retain a relatively constant temperature.

Hope this helps,

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661

"To trace that life in its manifold changes through past ages to the present
is a ... difficult task, but one from which modern science does not shrink.
In this wide field, every earnest effort will meet with some degree of
success; every year will add new and important facts; and every generation
will bring to light some law, in accordance with which ancient life has been
changed into life as we see it around us to-day."
        --O.C. Marsh, Vice Presidential Address, AAAS, August 30, 1877