VENTILATORY ACCOMMODATION OF CHANGING OXYGEN-CONSUMPTION IN DASYURID MARSUPIALS

Little is known about how the mammalian ventilatory system accommodate s the large change in rates of O-2 consumption (Vo(2)) between minimal and maximal aerobic metabolism. These changes are expected to be part icularly large in marsupials, which have a substantially greater aerob ic scope than similar-sized placental mammals. We measured ventilation and Vo(2) at metabolic rates between basal metabolism (BMR) and maxim al O-2 consumption during thermogenesis (Vo(2max)) in three dasyurid m arsupials: Planigale tenuirostrus (5.7 g), Sminthopsis crassicaudata ( 14.2 g), and Antechinus swainsoni (66.9 g). Comparative data on a four th dasyurid species, Dasyuroides byrnei (144 g), were obtained from th e 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 accommodat ion 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 signifi cantly 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 standa rdized eightfold change in Vo(2) revealed overall similarity between t he 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. byrn ei differed significantly from other species. As size increases in das yurids, Eo(2) increases, mass-specific V-t decreases, and the importan ce of V-t, changes in accommodation increases. When compared with a ge neralized allometry for ventilation in resting placental mammals, dasy urids had much lower f and greater V-t than predicted; in the two smal ler species, resting Eo(2) was lower than predicted. However, when the dasyurid data were compared with data obtained from 10 placental mamm als measured without the use of restraints or invasive procedures, the re were no differences between the two taxa. We found no indication th at ventilation constrained thermogenic Vo(2max).