VENTILATORY ACCOMMODATION OF CHANGING OXYGEN-CONSUMPTION IN DASYURID MARSUPIALS

Little is known about how the mammalian ventilatory system accommodates the large change in rates of O-2 consumption (Vo(2)) between minimal and maximal aerobic metabolism. These changes are expected to be particularly large in marsupials, which have a substantially greater aerobic scope than similar-sized placental mammals. We measured ventilation and Vo(2) at metabolic rates between basal metabolism (BMR) and maximal O-2 consumption during thermogenesis (Vo(2max)) in three dasyurid marsupials: Planigale tenuirostrus (5.7 g), Sminthopsis crassicaudata (14.2 g), and Antechinus swainsoni (66.9 g). Comparative data on a fourth dasyurid species, Dasyuroides byrnei (144 g), were obtained from the literature. Thermogenic aerobic scopes (Vo(2max)/BMR) were 9.2 9.0, 8.7, and 10.4, respectively. Ventilatory accommodation of changing Vo(2) was qualitatively similar in the four species, with most accommodation resulting from changes in minute volume (V-t). However, there were differences in the relative importance of frequency (f), tidal volume (V-t), and O-2 extraction (Eo(2)) in accommodation. In all species, f and V-t were strongly correlated with Vo(2). Tidal volume was significantly correlated with lie, in S. crassicaudata, A, swainsoni, and D. byrnei, while Eo(2) was significantly correlated with Vo(2), only in A. swainsoni. Analysis of factorial ventilation changes across a standardized eightfold change in Vo(2) revealed overall similarity between the four species in V-t and V-t, with significant differences in f and Eo(2). For the latter two parameters, there was no difference between P. tenuirostrus and S. crassicaudata, whereas A. swainsoni and D. byrnei differed significantly from other species. As size increases in dasyurids, Eo(2) increases, mass-specific V-t decreases, and the importance of V-t, changes in accommodation increases. When compared with a generalized allometry for ventilation in resting placental mammals, dasyurids had much lower f and greater V-t than predicted; in the two smaller species, resting Eo(2) was lower than predicted. However, when the dasyurid data were compared with data obtained from 10 placental mammals measured without the use of restraints or invasive procedures, there were no differences between the two taxa. We found no indication that ventilation constrained thermogenic Vo(2max).