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Re: What is an "active ectotherm"?

In a message dated 96-10-08 14:44:00 EDT, ajhowey@ix.netcom.com (Andrew
Howey) writes:

>  Could somebody please give an example of such an animal?  I'm
> having trouble understanding what is meant here.  DinoGeorge said
> that such a creature would have/need "just a good circulation with a
> quadricameral heart".  The only ectotherms I know of with
> four-chambered hearts are the crocodilians, and they're not all that
> active, spending much of their time basking.  Are there any
> present-day examples of "active ectotherms"?  Sorry for specifying
> you, DinoGeorge -- you just happened to give some of the
> physiological requirements for such a critter.  I posed the question
> so that I can better understand the varying points-of-view.

Do fish count? Water has a much higher specific heat than air and is a much
more effective cooling agent. Yet many fish can contrive to swim perfectly
well through a medium whose temperature would kill an unprotected human in
minutes. They're mainly ectothermic, but they generally keep moving around
their fish tanks. How about sharks? How about insects? These are all examples
of active ectotherms. Why couldn't dinosaurs be likewise?

Crocs are not a terribly good example, because they diverged from the
archosaur-to-bird lineage much earlier than did the dinosaurs, and it might
be too much to expect a fully active lifestyle from them. But let me also
point out that the earliest crocs had long, slender, upright limbs like those
of greyhounds, and may have been active quadrupedal foragers. Present-day
crocs could well exemplify reversal to a prior, less active lifestyle.

It seems perfectly clear to me that endothermy did not develop overnight.
Since it seems to be an intricate kind of physiology, it surely evolved in
stages, over quite a few millions of years. Perhaps active ectothermic
dinosaurs represent such an intermediate stage or stages in the development
of true avian endothermy. Perhaps the question we should be asking is, Why is
there endothermy at all?