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Avian and dinosaur air sacs



Your post on April 19th was very thought provoking! 

John K. Terres, in his _Audubon Society's Encyclopedia of North American
Birds_, 1980, states that the air going through the air sacs and lungs
provide more ventilation than needed for a bird's use in flight.  He
feels the extra ventilation helps regulate a bird's body heat.  Birds
have no sweat glands, so can decrease body heat by internal radiation and
vaporization, according to Terres.  So these serve the same function as
perspiration.

You state "if air sacs evolved in terrestrial non-avian archosaurs, then
they should have been expected to have functioned differently than those
of any modern bird and had less capacity than in flying birds."  Well,
heat exchange could be the different function, but maybe the capacity was
not necessarily less than a flying bird.  

If the Mesozoic was warm, and the dinosaurs active, their muscles may
have generated excess heat that needed to be dealt with.  As dinosaurs
got bigger, this may have been more crucial, since a large body can end
up being a heat trap.  Air sacs could have made a competitive difference,
and could have helped dinosaurs be as active as warm-blooded mammals
without overheating.  They didn't even have to be warm blooded for this
effect to work.  If they were warm-blooded, then air sacs may have been
more crucial.

Would a very active predator, like a Utahraptor, use as much oxygen
pursing and dispatching prey as a modern bird does by flying?  Wouldn't
air sacs give that predator greater stamina in the chase?  Maybe the
predators developed the air sacs, first for heat dissipation from their
active life styles, then to increase stamina in the chase.  Can
warm-bloodedness or flight be far behind?

This is all speculation, of course, but it was your posting, and the
question about whether air sacs contribute to gas exchange by Michael
Teuton, that got me thinking.

Which may have come first, air sacs or warm-bloodedness?

Judy Molnar
Education Associate, Virginia Living Museum
vlmed@juno.com
jamolnar@juno.com
All questions are valid; all answers are tentative.