Scaling of respiratory variables and the breathing pattern in birds: an allometric and phylogenetic approach.

Allometric equations can be useful in comparative physiology in a number of ways, not the least of which include assessing whether a particular species deviates from the norm for its size and phylogenetic group with respect to some specific physiological process or determining how differences in design among groups may be reflected in differences in function. The allometric equations for respiratory variables in birds were developed 30 yr ago by Lasiewski and Calder and presented as "preliminary" because they were based on a small number of species. With the expanded data base now available to reconstruct these allometries and the call for taking account of the nonindependence of species in this process through a phylogenetically independent contrasts (PIC) approach, we have developed new allometric equations for respiratory variables in birds using both the traditional and PIC approaches. On the whole, the new equations agree with the old ones with only minor changes in the coefficients, and the primary difference between the traditional and PIC approaches is in the broader confidence intervals given by the latter. We confirm the lower VE/VO2 ratio for birds compared to mammals and observe a common scaling of inspiratory flow and oxygen consumption for birds as has been reported for mammals. Use of allometrics and comparisons among avian groups are also discussed.