reopening the can

["Colette H. Adams" <cadams@hh.gpz.org>]

I have been quiet for a while now on the bioenergetics issue, but with
Dann's post on ectothermy/endothermy and Archaeopteryx it seems like an
opportune moment to make a few remarks.  Dann has his facts straight as far
as I can tell, but the specific examples cited may tend to mislead.

The basic difference between an ectotherm and an endotherm is
physiologically quite a profound one, a fact not always appreciated by
those who discuss the subject.  It is true that the metabolic rates of
endotherms are generally much higher than those of ectotherms.  But resting
metabolic rate varies greatly with body size, and this must always be
factored in when discussing same.

The critical difference is that when the temperature drops, an ectotherm's
metabolic rate decreases.  An ectotherm's core temperature can fluctuate
tremendously without doing it harm because ectotherm proteins are designed
to withstand these fluctuations without denaturing.  An endotherm is in a
sense just the opposite.  When its thermostatic mechanism detects a
temperature drop, its metabolic rate INCREASES.  Its core temperature stays
relatively constant, as it must.  Otherwise its proteins, which are highly
temperature sensitive, will denature and it will be very dead.  

Of course there are animals that have "hybrid" systems (although not that
many), and there are variations on the two basic themes.  But the
commitment to endothermy is a major one, necessitating the abandonment of
the highly energy-efficient ectothermic life style.  This is probably why
no crocodilian, lizard, or turtle that we know of has ever made the switch,
despite a few million years of opportunity.  Similarly, no endotherm that
we know of has ever switched back to ectothermy.  

Generally, endotherms have much higher resting metabolic rates than
ectotherms of similar size.  This is because endotherm body temperatures
tend to be much higher than the surrounding environment, and maintaining
the thermal gradient requires a lot of energy.  I suspect that the reason
endotherms have such high body temperatures is probably related to the
function of their proteins, although I cannot claim to have seen any
literature on the subject.

Any time we see evidence of specialized insulation, we should be instantly
suspicious.  No living ectotherm has blubber.  No living ectotherm has
feathers.  No living ectotherm has fur.  Why?  Because cover a snake with a
blankie and you have not increased its body temperature one iota.  You
can't conserve heat that isn't there.  

Archaeopteryx was an endotherm.  This hypothesis is one that I am quite
confident of, for all kinds of reasons.  Rather than launch into a long
list of them, I will simply point out that all living ectothermic tetrapods
have sprawling or partially erect postures.  Far from developing a fully
erect posture, many such animals have evolved smaller legs, even to the
point of leglessness.  Even fast, relatively active ectotherms, such as
whiptail lizards, have relatively short legs that sprawl to the side.  Why?
 Because ectothermy by its very nature involves long periods of immobility.
 The most common question we get from people in the zoo about reptiles is
"Are they alive?  Why don't they move?"  Ectotherms are energy conservers.
Why evolve an erect posture if you're going to spend most of your time
sitting on your bum?  If you have a sprawling posture and break a leg bone,
no big deal.  You can still walk and even run.  But if you have an erect
posture and break a leg, you're in trouble.

It may well be asked, how will endothermy ever evolve from ectothermy?  I
believe it does require an unusual set of circumstances, which is why it
has happened only two or perhaps three times to my knowledge in the whole
history of evolution, all of which occurred at more or less the same time,
the Triassic.  What was so special about the Triassic?  I don't know, but I
suspect that it has something to do with four-chambered hearts and stamina.
 Something must push the animal to develop a partially erect posture, which
preadapts it to some degree to endothermy.  I think crocodilians might have
evolved endothermy in time, except that they became aquatic.  It is
interesting that most have maintained a partially erect posture in spite of
their aquatic habits.

Best regards,

Dave