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Re: sauropods: homotherm,heterotherm or gigantotherm?



Well, it WOULD gather food much more quickly. It would
have to, since it would use the food up much faster. I
don't know of any evidence to suggest any
physiological increase in food gathering ability,

Well, higher endurance should lead to the ability to walk around all day...

...and then there's the metabolic work needed for just _standing_, which possibly requires tachymetaboly above some threshold body size.

being coupled with a switch to automatic endothermy.

Is that how you call endothermy produced by tachymetaboly?

I think there's a misconception prevalent here. A 10
degree change in temperature will change digestion
speed, NOT digestive efficiency. A reptile that is
digesting a meal at 10 degrees less than the optimum
temperature, will still digest that meal much more
effectively than most mammals (possibly most automatic
endotherms in general). It will just go slower.

OK, but this limits the _eating_ speed of poikilothermic animals, doesn't it? (I mean that you can't eat faster than your digestion can keep up.) I wouldn't recommend such a limit to a growing sauropod...


If dinosaurs had digestive systems more in line with
other reptiles, than mammals (at least. I'm still not
sure about the birds), then we would expect them to
require less food than a similar sized mammal. Judging
by the relative efficiency suggested by the
_Tyrannosaurus_ coprolite, it would appear to be that
way.

What do you mean? That coprolite contains plenty of bone fragments that don't occur in crocodile droppings.


The biggest problem with that argument, is that all
the examples we have of "ectotherms" growing to such
large sizes, are fossil examples.

Yep.

Furthermore it always seems that these fossil examples happen
to then become the subjects of questions on metabolism.

For instance, rauisuchians reached _Allosaurus_ sized
levels.

Rauisuchians are very close to crocodiles, and there's evidence for... elevated metabolism... around the base of Crocodylomorpha...


In the water we had dozens of large mosasaurs,
ichthyosaurs and plesiosaurs.

In the water. And of course at least the ichthyosaurs have repeatedly been suggested to have been warm-blooded, though I can't remember any data...


We also had all the many
(seem to be coming out of the woodwork lately)
terrestrial crocodylomorphs that were, at least, on
par with the largest terrestrial cats today.

See above, and we don't seem to approach 1 t here...

So it becomes a catch 22.

Yes.

I suppose one could ask why we don't have any extant
multitonne reptiles, but then one could ask the same
thing about all those "hyper-endothermic" birds people
are so fond of.

Too many mammals around -- which applies in both cases...

Some of that material has been recently published, and
can be read about in Ralph Molnar's recent book on the
subject (Dragons in the Dust: The Paleobiology of the
Giant Monitor Lizard _Megalania_). He calculated that
the largest material suggested a 2.1 tonne (1.9 metric
tonne) animal.

I understand that the recent publication of a weight of mere 160 kg is now considered too low, but... how can estimates differ by a factor of 10...?


I do believe that falls in line with estimated growth
rates for the giant squid (_Architeuthis dux_),

Oh. Yes, that one must grow very fast to die that young...

Young altricial birds grow at twice the rate of a
proportional sized sauropod (Erickson, et al 2001).
Yet this is also the time of their lives, when these
young birds are known to be ectothermic (see Chinsamy
& Hillenius [2004] for a substantial list of
references backing this up).

Fine, they are ectotherms, but _very_ tachymetabolic -- and actually we're talking about metabolism here, not about thermoregulation. Said birdies are just too small (and to little insulated) to keep a constant body temperature, so they rely on their parents instead.


It would appear that food availability, and nutrient
partitioning (rather than general metabolic rate) are
the prime factors in fast growth.

Yeah... and body temperature.