Re: Terramegathermy in the Time of the Titans (not so long...)

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

Hi David:

So you know, a similar paper on Terramegathermy ... appeared in the 1998 
Modern Geology volume by Greg Paul.  Don't have the ref handy, but could 
send it to you and the list if need be.  I haven't read the 1994 paper you 
quote, but the 1998 one sounds similar.

You said, summarizing Paul and Lehy:
"Very short: Standing is exercise and requires muscle work, the larger the
animal, the more. Leatherbacks are gigantothermic, but ridiculous analogs
for dinosaurs, desert elephants are better. Anaerobic production of power is
impossible for vertebrates to sustain for any reasonable time -- it is an
emergency solution _only_. Big animals require big, tachyaerobic muscles
require big
hearts require more energy than a bradymetabolic animal can produce, and
that 24/7."

Sure.  This is what Paul, Bakker, and others have argued in favor of 
dinosaur endothermy.

Quoting Paul and Lehy (P&L):
"<snip> [HP] Farlow (1990) suggested that large dinosaurs were "damned
good reptiles" with fluctuating metabolic rates (MRs), and in 1993 he argued
that dinosaurs used a combination of rapid reproduction and intermediate
metabolic rates (InMRs) to grow bigger than land mammals."

And I would be inclined to lean toward Farlow's interpretation.  However, I 
think dinosaurs had aerobic muscular metabolism if only to be able to move 
efficiently at such large sizes without paying huge oxygen debts.  There are 
more factors at play than rapid reproduction and intermediate metabolic 
rates for increased dinosaur size, and Farlow et al. (1995) discuss these at 
length, among which are a warmer climate and possibly lower daily food 
requirements.  I have some ideas of my own, but that's for another time (and 
paper).

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.  Plus, African elephants have big, thin and flappy 
ears that help them draw heat away from their brains and countercurrent heat 
exchange in the blood vessels going to their brains to cool themselves.  
Higher core temperatures and bulk alone do not completely explain cooling in 
these giants.

And, elephants evolved from synapids, not diapsids, and are very distantly 
related to sauropods.  True, elephants and sauropods have superficially 
similar column-like limbs, short feet, etc., because they have converged on 
a graviportal body plan.  Both are very big and heavy terrestrial 
vertebrates.

P&L say "Acceptance of [leatherback turtles] as primary models for dinosaurs 
is
therefore surprising - imagine the reaction if whales were used as the
primary analogs for dinosaurs! The structure and hot climates of elephants
are very reminiscent of the dinosaur condition, and it is surprising how
many reject their biology when restoring dinosaur thermodynamics."

And yet sharks and dolphins have similar body plans.  Would it be a fair 
comparison here to say the physiology of a dolphin is a good yardstick for a 
shark or vice versa?  How about ichthyosaurs and sharks, or with dolphins?  
The problem is we don't really know how different sauropods and elephants 
were internally, and thus we are making big leaps to say the similar 
pillar-like legs and big size of elephants and sauropods (or other 
dinosaurs, like P&L more generally argue) point easily to a similar 
physiology.  This doesn't mean that dinosaurs couldn't have been big and 
endothermic like elephants, but it doesn't necessarily follow that easily 
either.

P&L continue:
"... reptile muscles can produce twice as much
anaerobic power as those of mammals and birds (Ruben, 1991), so even small
legged lizards and crocodilians sprint at high speeds. However,
hyperanaerobiosis is an inefficient process (that consumes ten times as much
food as aerobiosis) that works only for a few minutes, and is followed by
toxic effects (Bennett, 1991)."

Weasel words.  Inefficient process in terms of being a mammal or bird, but 
reptiles get by just fine with largely anaerobic muscle metabolism.  Many 
reptiles are sit-and-wait predators who require a few meals a month or less, 
so I would call this very efficient and endotherms very inefficient in this 
respect.

P&L:
"[...] The low capacity and low pressure respiro-circulatory system of
reptiles can deliver only enough oxygen to supply bradyaerobic muscles.
[...] [Large, tachyaerobic muscles require good, large lungs and hearts as
well as high blood pressures.]"

Kinda putting the cart before the horse here.  Crocs and gators have 
four-chambered hearts, like any "good" mammal or bird, with the difference 
that they can control the openings and closing of valves between their two 
aortae (mammals, birds, and presumably dinosaurs, have (had) only one 
aorta).  Having a four-chambered heart allows you to separate high pressure 
from low pressure blood.  Why is this important?  Because, the bigger you 
get, the higher the blood pressure needs to be to get blood to the brain.  
If the same high pressure blood went to the lungs, you would drown in your 
own blood.  Therefore, having a four-chambered heart "preadapts" you to get 
bigger and place your head further from your heart, but it does not 
necessarily make you endothermic -- again, as evidenced by gators and crocs.

Dinosaur metabolism is a fascinating area of study, but the evidence is 
damned equivocal.  We must always remember that dinosaurs are very much 
dead, and what we know about them comes mostly from hard tissues.  While it 
is tempting to look at the size or body form of dinosaurs and argue that 
that size or body form says something solid about physiology, we must 
remember that large terrestrial land mammals are highly derived from, and 
have a very different evolutionary history from, dinosaurs.  By the same 
token, giant leatherback turtles, although gigantotherms in their own right, 
and very derived and live in an extreme environment that no dinosaurs are 
know to have experienced.  Again, fascinating, but yet so equivocal.

Whew ... well, that's a wrap ... for now. =)

Matt Bonnan, Ph.D.*
* (defense one week from Thursday!) =)
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