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tracheal diameter and endothermy

Comparing tracheal diameter alone and endothermy may be somewhat
misleading.  Tracheal resistance to air flow is a function not only of
cross-sectional area but also of tracheal length.  Birds generally have
longer tracheae than mammals of the same body weight, and their
tracheal diameter is also larger; the result is a similar level of tracheal
resistance in both.  Since soft tissue is so rarely preserved, we actually
may be able to estimate tracheal length better than tracheal diameter in
dinosaurs and other extinct vertebrates by measuring neck lengths.  By
making some assumptions about tracheal resistance, we may be able to
calculate some mean tracheal diameters, but the results would be
questionable, and I wouldn't want to use them as a basis for a statement
about endothermy.

James M. Norton