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Re: Warm- vs. Cold-blooded Dinos



>I would like to ask for an opinion from the group on whether dinosaurs
>were endo or ectothermic. 

Everyone so far seems to be in favour of endothermy. While I think it very
likely that some dinosaurs were endothermic, I am not at all convinced that
all, or even most were.

Dave's evidance does apply very well to small theropods. They would probably
need an endothermic metabolism to maintain their active lifestyle.

Large plant eaters, however wouldn't lead such an active lifestyle. Also,
predator-prey ratios (if they can be accurately determined) only tell you
about the metabolism of the predator - the prey could still be endothermic
without affecting the ratio. I'm not trying to disprove Dave's argument,
though - all his points are valid.

The main reason I think most dinosaurs were exothermic is a very simple one:
energy. A typical endotherm eats about ten times as much as an exotherm of
similar mass (the usual example is of lions and crocodiles). This is a big
problem for very large animals, as mass is a cube function of length (i.e.
if you double the length, you increase mass by a factor of eight). It is
difficult to imagine a huge sauropod being able to stuff enough low-grade
mesozoic plants through its tiny (relatively speaking) mouth unless it was
exothermic.

But that is not to say they were warm-blooded.

Sea turtles manage to keep an almost constant body temperture, not through
endothermy, but "gigantothermy". I don't know of this is a real term (its
used The Complete T. rex).

I think that most of the large dinosaurs could have stayed relatively active
simply by being big. Since heat-loss is a function of surface area, a
doubling in size would quadruple the surface area - considerably less than
the increase in mass. So large animals cool down much slower than small ones.

Below is a rough guide to which dinosaurs I think were endo- and exothermic.
The ones at the top are the most likely endotherms.

Probable endotherms:
        Archaeopetryx
        Ornithomimids
        Deinonychosaurs
        Ceolurosaurs
Possible endotherms:
        Carnosaurs
        Ceratopsians
        Pachycephalosaurs
Probable exotherms:
        Herrerasaurs
        Ornithopods
        Ankylosaurs
        Stegosaurs
        Sauropods

The thing I love about this argument is that we have very few clues to go
on, and when it comes down to it we're just guessing. So us amateurs have
almost as much chance of being right as the experts. ;-)

James Shields  -  jshields@iol.ie  -  http://www.iol.ie/~jshields
------------------------------------------------------------------------------
And when the ark was finished Noah said unto Elvis, "What do you reckin?"
And Elvis checked out his own cabin and shook his head saying "poky".
And so did they knock several walls through and install a jaccuzzi.
And when it was all done Noah scratched his beard and said, "We don't have
room for all the animals now."
And Elvis perused the livestock list and in his wisdom said, "Lose the
dinosaurs."
        -Robert Rankin, The Suburban Book of the Dead