Re: warm/cold blooded evolution

On Wed, 10 May 2006 23:35:52 -0700, Timberwoof
<timberwoof.spam@xxxxxxxxxxxxxxx> wrote:

In article <1147321576.062282.109750@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"ck19" <ck19bla@xxxxxxx> wrote:

is there a different rate of evolutionary change between warm and cold
blooded animals over the last 500,000 years?



That's probably difficult to answer because the boundary between
"warm-blooded" and "cold-blooded" is hazier than we once thought.
"Endothermic" and "exothermic" were terms used for a while to denote
whether an animal controlled its body temperature internally or depended
on the environment to regulate it, but they get confused with the
chemical meanings of the terms, where endothermic means a reaction that
takes in heat and exothermic means one that gives off heat -- the
opposite of what the mean in biology. But based on the behavior of
certain dinosaurs and the climates where they lived and migrated,
paleontologists have concluded that certain of them were endothermic (in
the biological sense). Nevertheless, it's hard to tell from fossils
alone how an animal regulated its body temperature.

Then there's the meaning of the term "rate of evolutionary change" and
its relevance. Cockroaches, for instance, hare remained pretty much
unchanged for what, a hundred million years? Other species of insects,
however, have changed much, much faster than that.

The words used in physiology are endotherm and ectotherm so there
really is no confusion with exotherm. There are also homeotherms
(animals that regulate their temperature) and poikilotherms (animals
that don't).

There are probably many other confounding factors to consider.
Homeotherms, the warm-blooded regulators tend to be larger than the
poikilotherms, the cold-blooded non-regulators. That is because large
body mass and smaller relative surface area makes it far easier to
generate and retain heat. That is often associated with longer
lifetimes and longer reproductive cycles so they have fewer
generations per thousand years than the smaller poikilotherms. Trying
to compare animals of comparable size is difficult because,
considering terrestrial animals, there are virtually no invertebrates
comparable in size to birds and mammals and many of the larger lizards
do keep their body temperatures at a high level through indirect means
(ectothermal regulation) and can be 'partial regulators' or
heterotherms. It is now generally believed that the larger dinosaurs
did maintain high body temperatures. I guess you would have to
compare fish to birds and mammals but then you are faced with the fact
that the terrestrial climate has changed drastically over the last
half million years while aquatic habitats change much less so.


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