[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Terramegathermy in the Time of the Titans (not so long...)
I had said, quoting your quote:
> Continuing (P&L):
> "Rather than going belly up when it gets hot, [elephants] use high body
> temperatures of 37°C and bulk to thermoregulate in extreme heat."
> What is interesting here is that P&L reject gigantothermy as a hypothesis
> for dinosaur thermoregulation, but use bulk in elephants as a means of
> arguing thermoregulation.
"Gigantothermy is understood here as the combination of being
bradyaerobic, endothermic, homeothermic, and insulated by size, shape and
fat. In this sense, elephants are not gigantothermic..."
Yes, but I think you're missing the point. P&L say bulk cannot explain the
thermoregulatory strategies proposed by Spotila, Paladino, etc., and then
turn around and use it to explain heat resistance in elephants. The point I
was making is that elephants do much more than use bulk and a high body
temperature to thermoregulate, not that elephants are gigantothermic.
Furthermore, the point was that P&L seem to be arguing that bulk
homeothermy, gigantothermy, etc., is too simplistic of a model for dinosaur
metabolism, and yet use the size and body temps of elephants to justify
endothermy in dinosaurs.
"I wouldn't consider "hyperanaerobiosis [...] works only for a few minutes,
and is followed by toxic effects" weasel words: they say quite clearly that
while hyperanaerobiosis is perfectly fine for sit-and-wait activities, it
can't be sustained for e. g. migrating."
But as pointed out by Darren Naish recently on the list, many "ectothermic"
organisms do migrate vast distances. I'm not arguing for hyperanaerobosis
in dinosaur muscles, just that P&L are setting up a false dichotomy: if
dinosaurs were not like modern reptiles, they had to be like mammals and
birds. Not necessarily so. And even then, WHICH mammals and WHICH birds?
Placental mammals? Marsupial mammals? Montremes? Ratites? Non-ratites?
All of these tachymetabolic endotherms have a wide metabolic scope.
"But being big requires a heart that is not only four-chambered but
also _big_ and therefore can't be financed by bradymetaboly (see Table 1),
therefore, being big requires being tachymetabolic, P & L write."
Yes, I understand, but do you understand that the large, four-chambered
heart probably came BEFORE endothermy? Again, the intial "reason" for
having a four-chambered heart seems to be related to separating high
pressure from low pressure blood. It does not necessarily follow that
therefore if you have a big, four-chambered heart you are automatically a
tachymetabolic endotherm. Besides, things are not so black and white. Reid
(1997, in the Complete Dinosaur) has even suggested that dinosaurs had an
intermediate metabolism and were "failed" endotherms. Dinosaur physiology
is a very complicated area of study because so much of the evidence is
indirect, and we must be very careful about the conclusions we draw.
Yes, the upright limb carriage and large body size of many dinosaurs
suggests (does not prove, but suggests) that they had a metabolic strategy
different from that of extant reptiles. Beyond that, it gets harder to pin
these sorts of things down.
If you haven't, I recommend reading the articles and books of Dr. Knut
Schimdt-Nielsen. He writes fascinating discussions about animal size,
metabolism, and physiology. I have found his discourses on animal
physiology absolutely fascinating, and I have found over time that the issue
of dinosaur metabolism is not so black and white as it may at first appear.
I encourage you to read his works, those of R. McNeill Alexander, Steven
Vogel, and Chris McGowan, to name a few popular but serious sources on
animal metabolism, biomechanics, etc., to appreciate the very grey world of
vertebrate mechanics and physiology.
"Good luck [on your Ph.D. defense next week]! What's the topic?"
Sauropod locomotion. =)
Get your FREE download of MSN Explorer at http://explorer.msn.com