Re: Endothermy speculation

["Matthew Bonnan" <mbonnan@hotmail.com>]

Graeme:

You ask many good questions for which in some cases there are no good 
answers.  I will try to address one or two of your questions.

You said:
"Given that body temp needs to be maintained relatively constant at least in
> part because critical enzymes in metabolic pathways have a fairly narrow
> operating range, I wondered why this should be so. The ideal situation in
> theory would seem to be metabolic systems that work most efficiently at
> ambient temperature, as this removes any need for drastic temp regulation."

Well, "reptiles" are ectotherms which for the most part (with some 
exceptions) regulate their body temperature externally, using the sun, or 
heated rocks, etc.  They spend a good deal of time travelling between sun 
and shade to control their body temps usually within a surprisingly narrow 
range (although the range is far broader than that of homeothermic 
endotherms like mammals and birds).

This would be oversimplifying things a bit, but to avoid long, drawn-out 
physiology lectures, the point is that "reptiles" are increasing or 
decreasing their enzyme efficiency when getting warmer or colder, 
respectively.  Most reptiles, for instance, cannot digest a meal when their 
core body temp is below 20 degree Celcius, and this is in part due to 
lowered enzyme efficiency.

Therefore, most "reptiles" are small animals that live within 
microenvironments in which they can closely regulate their body temperatures 
during the day and when needed for digestion.  There are few big "reptiles," 
and these animals can take advantage of their "bulk" to keep their 
temperature range within an ideal area longer.  However, many, like the 
alligators and crocs, live in warmer regions and usually near water, a 
natural, quick and effective heat-dump.

This apparent "inefficiency" actually works quite well for "reptiles" since 
they do not need to keep a constant body temp for constant enzyme efficiency 
and thus they do not "burn" as many calories, and can get by on far less 
food.

Remember that ectothermy was the primitive condition in vertebrates, and it 
would seem to "work best" in water where temperature fluctuations tend to be 
slow and constant.  In the air on land, the drastic temperature changes that 
can happen make ectothermy less than ideal compared with a fish, for 
instance, but in most cases ectothermic animals are very good and efficient 
at what they do.

For your second question:
"The
> question for dinosaurs becomes something like - did they exist in a
> situation where their metabolic processes did function best at the then
> ambient temp? THis would presumably have a large impact on the amount of
> energy (and hence food intake/oxygen intake) required for them to maintain
> homeothermy."

This is a bit more sticky, since we cannot test dinosaurs directly for ecto- 
or endothermy.  We do know that the Mesozoic climate tended to be warmer 
than the present (in fact, we know of no glacial depositis or evidence of 
ice caps during this time), but we do find dinosaurs distributed worldwide, 
and some within the Antarctic circle, which at the very least would have 
been six months of darkness followed by six months of light.

We have a problem too with our living outgroups.  Alligators and crocs both 
have four-chambered hearts and take care of their young, but neither is a 
homeothermic endotherm.  Birds, on the other hand, have core temperatures 
approaching 42 degrees Celsius.  Nor is it as simple as saying dinosaurs 
were or were not endothermic.

Monotreme mammals (the platypus, the echidna) tend to have core temperatures 
around 32 to 30 degrees Celsius; Marsupials fall in the range of 35-37 C; 
and placentals in the 38-40 C range.  Even among homeothermic endotherms you 
get a wide variety of metabolic strategies.

Many have argued that dinosaurs could have relied on their sheer bulk to 
stay at a relatively stable core temperature, but this ignores a number of 
factors.  First, we know of dinosaurs that lived in cooler regions.  But 
more importantly, being an endotherm is not just a matter of body 
tempearture and its constance.  Endothermy (or ectothermy) is tied into 
every system.

The muscle of "reptiles" tends to be anaerobic with regard to burning 
energy.  That is, "reptiles" essentially have a stored amount of energy in 
their muscles which, when burned, quickly fatigues the animal.  This is why 
we have no long-distance running "reptiles."  This is unlike endotherms, 
which have aerobic muscles which also have stored energy but also can 
acquire and burn new energy as the situation permits.  Therefore, endotherms 
have more "stamina" than "reptiles."

If something the size of a sauropod dinosaur were an ectotherm, I would be 
interested in knowing how a high constant body temperature would get around 
this muscle-fatigue problem.  It would seem to me that such an animal would 
take a few steps and then wait for a long time while its muscles 
recouperated.

On the other hand, an endothermic sauropod might be in danger of 
over-heating.  We know of no sweat glands in dinosaurs, and unless the long 
neck and tail of sauropods could be used as heat-radiating devices, how such 
a problem was overcome is difficult to imagine now.

As you can see, the problem is a bit complicated.  My personal suspicion is 
that dinosaurs were endothermic BUT perhaps in way that was different from 
our extant examples.  Endothermy does not automatically imply a very high 
body temperature, and I stress that I mean endothermy only in the sense of 
the muscle stamina and homeothermy issues, and do not mean to imply 
sauropods or other large dinos bounded over the hills like rabbits. =)

Anyways, hope this helps.

Matt Bonnan
Dept Biological Sciences
Northern Illinois University



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