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Re: Avian and dinosaur air sacs



J. Molnar suggests that the air-sac system of theropods evolved in order to
cool off the high heat production associated with high activity levels,
rather than to oxygenate the high level of oxygen consumption associated with
"warm-bloodedness". 

Molnar appears to be making a mistake that is very common and that I have
commented upon before to the point I am getting rather tired of it. This is
the idea that an animal can somehow be "highly active" without having rates
of oxygen consumption. It just ain't so. Think about it. In order to produce
lots of heat you have to burn lots of oxygen (this is true even when
anaerobiosis is initially used to generate heat). The maximum oxygen
consumption of all and any reptile living and extinct is so extremely low
that they cannot sustain levels of activity high enough to require a
sophisticated cooling system. Ergo, no reptile has air-sacs, sweat glands
etc. Activity levels above those achieved by reptiles, especially vigorous
exercise (including walking at a few mph) for a substantial period of time
requires that the aerobioc exercise capacity be boosted into at least the
lower mammalian level. Period. 

Perhaps Molnar was suggesting that dinosaurs had a higher AEC than reptiles,
but that their resting metabolic rate remained low. The problem with this is
that it probably is not possible for vertebrates to have a low resting MR and
a high AEC (see K Hammond & J Diamond 1997 Max. sustained energy budgets in
humans and animals NATURE 386:457).  

The presence of air-sac lung ventilation is therefore strongly indicative of
high AEC and probably of high resting MR as well. High estimated trackway
walking speeds certainly show that theropods and sauropods were walking much
faster than reptiles and therefore both using more oxygen and generating more
heat. 

Perhaps air-sacs evolved for cooling as well as enhanced lung ventilation
during exercise in the very highly active theropods, but one could not happen
without the other. In sauropods lung ventilation was probably the critical
factor. They needed the air-sacs mainly to overcome the dead space of their
long airways. Since they were rather slow, like elephants, high AEC was less
critical. As I have already explained too many times on this list, giant
animals are actually very hard to overheat (low surface area keeps out
intense midday heat, low mass specific MR and enormous heat storage capacity
allow internal heat to be stored until night, etc.), and respiratory cooling
becomes less effective as breathing frequency declines with size, so air-sac
cooling was probably not important to sauropods. 

GSPaul