Re: Warm-Blooded debate

[Dinogeorge@aol.com]

In a message dated 97-06-20 14:25:04 EDT, tons@ccs.netside.com (Michael)
writes (responding to me):

<< >   Nevertheless, can you name me a fully erect, fully
four-chambered-hearted,
 > ectothermic animal? >>
 > 
 > Any adult sauropod will do, I'd say.
 
 While you may be correct about the four-chambered heart(I suspect you 
 are for several reasons), AFAIK no one really knows if dinosaurs had 
 four chambered hearts.  I would be interested in discussing this with 
 the list if anyone is interested. >>

Four-chambered hearts are >necessary< for any tetrapod to have long legs and
an erect gait, because it separates the pulmonary circulation from the
systemic circulation. This allows an elevated blood pressure in the systemic
circulation while keeping the blood pressure lower in the lungs. A high
pulmonary pressure would all too easily flood the lungs with blood and result
in the premature death of the animal, which is why the pulmonary circulation
must be kept quite separate from the systemic in erect animals.

Elevated systemic blood pressure allows the evolution of "verticality." All
known terrestrial vertebrates, large and small, with three-chambered hearts
or incompletely four-chambered hearts are sprawlers or slitherers, simply
because they cannot muster the blood pressure required to bring blood back to
the heart from erect limbs. I think the emergence of semi-erect therapsids in
the Permian and archosaurs in the Early Triassic correlates well,
phylogenetically as well as structurally, with the (independent) development
of the four-chambered heart in those groups. For me, the
quadricameral-heart/erect stance argument is a clincher.

<< > Indications are that adult fully endothermic sauropods would have
overheated,
 > particularly since they have no known anatomical structures with which to
 > dump excess body heat, so full adult endothermy >likely< developed in the
 > aforementioned lineage leading to birds >after< the sauropods diverged.
 
 I still wonder about this.  Animals as large as elephants do not 
 overheat, and their main source of cooling other than getting in the 
 shade is radiation via the ears.  Of course they radiate heat back 
 to the environment at night as large animals do as well...>>

Some sauropods were much larger than elephants, and as far as we know they
didn't have anything like those big ears. Of course, today's Indian elephants
have smaller ears (because they live in shadier forests?). And cooling would
not have been a problem for ice-age mammoths and mastodons(!). Like I said,
every argument has counterexamples...

By the way, a few sauropods seem to have evolved structures for controlling
excess heat anyway: the sails of _Amargasaurus_ and the tall dorsal vertebrae
of _Rebbachisaurus garasbae_.
 
<< > It is possible, for example, that endothermy developed initially to
allow
 > >juvenile< dinosaurs to grow to half-adult size quickly. Then, as they
 > attained maturity and giant size, endothermy gradually reverted to
ectothermy
 > (or gigantothermy) as the no-longer-needed growth rate slowed. I
 
 I think this would be very unlikely.  Any animal that had an 
 endothemic metabolism would probably be stuck with it.  There are 
 too many cellular, organ, and endocrine differences for me to imagine 
 this all turning off although it is an interesting concept and I 
 guess possible.  Are there any extant organisms that would qualify?
 >  >>

We have no idea what kind of metabolism "partially endothermic" vertebrates
might have had, because modern vertebrates are either ectothermic or
endothermic--although hibernating mammals (for example) do turn down their
endothermy and are not endothermic all the time. Since I cannot imagine the
evolution of complete endothermy in one jump from complete ectothermy, I have
to admit the certainty of various kinds of intermediate metabolisms, whose
features we can only guess at because animals with such metabolisms are by
and large no longer extant. Growth rates of juvenile dinosaurs of many kinds,
as observed from their bone growth patterns, were fast, but they slowed down
with the onset of adulthood and maturity to the more typical reptilian
slow-but-perpetual growth pattern. This suggests a concomitant gradual
slowdown in their metabolism--an effect that gigantothermy and the
quadricameral heart might have countered with regard to their diurnal
activities. With slower metabolisms, for example, giant adult plant-eating
dinosaurs would not have needed the tremendous amounts of food that have been
calculated they would need as full endotherms, so the local plant life could
have supported great herds of them without being completely devastated. Here
again, note that juvenile endothermy would not necessarily have "turned off"
all at once but over a period of several years, from, say, advanced
subadulthood through the onset of full maturity. There would be plenty of
time to accommodate the requisite hormonal and cellular changes.

If you like, you can also imagine the juveniles covered with insulatory down
that would be lost in adults...