Carina

["Patrick Norton" <ptnorton@email.msn.com>]

Pat Shipman, in her recent book "Taking wing; Archaeopteryx and the
evolution of flight" mentions almost in passing that the function of the
keeled sternum in birds is not to provide increased area for the attachment
of flight muscles but to prevent ventilatory air channels in the pectoralis
muscle from collapsing when the muscle contracts during the downstroke. She
says that respiratory air sacs within the furcula branch out into a network
of air cavities that spread into the tissue of the pectoralis muscle and
function to dissipate heat generated by the muscle. Heat is collected in air
circulating through these "cooling coils", passed to the air sacs and
exhaled.  This is a physiological explanation for the presence of a keeled
sternum in birds that appears to explain why other vertebrate flyers can fly
without such a structure.  For example, she says that bats don't have a
carina because heat is removed from their flight muscles by blood and
carried to the wings where it is disposed of by convection.

Unfortunately, Shipman does not cite any published work on this, although
she includes quotes by Colin Pennycuick  who made a similar claim in his
1972 book, "Animal Flight".

This seems like an important concept to me, and one that has apparently
received little attention.  Does anyone have a reference for empirical
studies on this?  And if Shipman and Pennycuick are correct, what does it
suggest (if anything) anything about the physiology of other vertebrates
with keeled sternum such as Pterosaurs (flyers) or Mononykus (a non-flyer)?