SCALING OF RESPIRATORY VARIABLES AND THE BREATHING PATTERN IN ADULT MARSUPIALS

Oxygen consumption (V-O2) and a number of components of the breathing pattern were measured in 14 species of non-fossorial marsupials rangin g in mass from 0.008 to 30 kg. All the exponents of the allometric rel ationships for V-O2, ventilation (VE) and breathing pattern scaled as previously determined for eutherians. However, compared to eutherians, marsupials had significantly lower V-O2 and the breathing pattern was deeper (+ 23%) and slower (-31%). While VE was not significantly belo w that reported for eutherians it matched V-O2 such that VE/V-O2 remai ned mass-independent and at a level similar to that observed in the ot her infraclasses of mammals, Thus, it would appear that the increase i n metabolic rate that occurred during the evolution of mammalian homeo thermy was accompanied by parallel changes in VE. It is suggested that these changes in VE were mediated by a shortening of the respiratory cycle, facilitated by the eventual abolition of the end-expiratory pau se (TP), and an increase in respiratory drive (VT/TI). In response to 5%, CO2 all animals in this study increased their VE by increasing bot h tidal volume (VT) and frequency (f), predominantly through the remov al of TP, The increase in VE was less than previously reported for eut herians, suggesting a reduction in chemosensitivity in marsupials. Fur thermore, the similarity in slopes for VE, VT and f between air and 5% CO2 suggests that the gain of the respiratory system is independent o f species size within marsupials.