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Re: Endothermic Crocs in Nature



I haven't read the Nature article yet (it'll have to
wait till tomorrow), but I'm noticing a lot of the
same problems cropping up again. I think the authors
are confusing the results of automatic endothermy with
proposed benefits. Namely, the belief that endothermy
confers the "advantage" of a high basal metabolism. I
don't see how having a metabolism that is always
running, is a good thing. It just means one must eat
more, to keep going. The results of automatic
endothermy are NOT a high activity level and
endurance, both of these have been shown to be
independent of automatic endothermy (see mako sharks,
tuna, bees, ants, and near any autarchoglossan lizard,
for examples). That most automatic endotherms have
higher aerobic costs, does not mean they have higher
endurance than non-automatic endotherms (see cats as a
fine example there).

Just because a creature exhibits high levels of
activity, doesn't mean that it is an automatic
endotherm. Nicholas Hotton, along with Regal & Gans
(both papers are in the book: A Cold look at the Hot
Blooded Dinosaurs) both made this point rather clear.
Aerobic scope is a greater indicator of activity
levels, than thermophysiology.

As such, it would appear that these authors have only
been able to conclude that ancient crocodyliformes had
higher activity levels, compared to their extant
members. This is hardly surprising, as Reilly & Elias
(1998) came to the same basic conclusions when
studying crocodylian limb posture.

Also, considering that varanid hearts become
falcultatively four chambered during periods of high
aerobic activity, would also suggest that this is the
main "drive" behind evolving 4 chambers.

One last thing.

+++++++++++++++++++++++++++++

Since the rate of heat loss is
> greater in water than in air, 
> and since larger animals have higher metabolic
> rates, there would have been 
> selective pressure against endothermy during the
> return to the water.

++++++++++++++++++++++++++++++++++

Is this actually what the authors wrote? Larger
animals do not have higher metabolic rates than
smaller animals. They have LOWER metabolic rates. An
elephant might eat a lot of food per day, but compared
to body size, it is but a tiny fraction of the food
consumed by a gerbil everyday. It's amazing how much
energy can be saved when one isn't just burning up
everything to keep warm.

In turn, one notices that bradymetabolic animals and
automatic endotherms appear to converge on metabolic
rates at larger sizes (e.g. a Rosenberg's monitor has
a resting metabolism 10 times lower than a similar
sized mammal, while a Komodo dragon has a resting
metabolism only ~5 times lower than similar sized
mammal). 

It seems to me like the authors are trying to validate
automatic endotherm crocs so they can make the
possibility of dinosaurian automatic endothermy, the
default case. Again, this is just from what I've read
from this post. I'll have a better idea tomorrow.

As for Chinsamy & Hillenius's chapter in the
Dinosauria. I have no qualms with their final
conclusion, that dinosaurs can be active
bradymetabolic animals. It wouldn't be the first time
such a conclusion was made. Furthermore, considering
that we have active bradymetabolic animals alive
today, I see no reason why this could not be the case
in the past as well.

Jason

"I am impressed by the fact that we know less about many modern [reptile] types 
than we do of many fossil groups." - Alfred S. Romer


                
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