Metabolism and temperature regulation in Marsupials with particular reference to the potoroo, potorous tridactylus apicalis Gould.

In an attempt to clarify aspects of thermoregulation and metabolism in marsupials a detailed investigation was made of the potoroo. Measurements of activity, oxygen consumption, creatinine excretion and total urinary nitrogen excretion of potoroos were made over 24 hour periods at a range of environmental temperatures. Basal oxygen consumption was in agreement with previous observations which have been made on this and other marsupial species, while the rate of creatinine excretion appeared to be higher than in other marsupials. Nitrogen metabolism did not appear to be related to ambient temperature or total oxygen consumed. Total serum thyroxine (TT4) and effective thyroxine ratio (ETR) were measured in 12 potoroos. Although TT4 was low compared with eutherians, the ETR suggested that free thyroxine concentrations were equivalent to normal human levels. Using a barometric technique tidal volume (VT), minute volume (VE), respiratory frequency (f) and respiratory evaporative heat loss (Eex) were measured from potoroos, barred bandicoots and laboratory ex rabbits at temperatures in and above the thermoneutral zone (TNZ). Rectal temperature (Tre) and oxygen consumption (V02) were also measured. VT initially decreased with rising ambient temperature (Ta) but in the potoroo and rabbit it then increased past the resting level. VE increased much more in the marsupials than in the rabbit and higher Eex maxima were also found for the marsupials. The marsupials had a mean panting efficiency of 80% while the rabbits had a panting efficiency of 100%. Non-shivering thermogenesis (NST) was investigated in potoroos by the injection of catecholamines. Calorigenic potencies at thermoneutrality were isoprenaline >adrenaline> noradrenaline; the increase in vo2 produced by these drugs being blocked by propranolol but not by phenoxybenzamine. As the rise in vo2 due to injection of noractrenaline was significantly less at 17°C than at 29°c the potoroo must utilize NST in the cold. To further investigate the metabolic status of marsupials water turnover rates of Tasmanian devils were measured under standardised conditions using tritiated water. Total body water of lactating females was lower than in non-lactating animals, while water turnover rates per kg were not significantly different due to a higher rate constant for lactating animals. Mean water turnover rates were considerably higher than predicted from other marsupial studies; statistical analysis of this plus data published from 13 species of marsupial and 27 eutherian species showed habitat to have a far greater effect on standard water turnover rate than phylogenetic differences. A similar statistical analysis was performed on vo2 and Tre data from 30 marsupial species and 34 eutherian species to investigate whether differences between the two groups might be attributable to habitat differences. This showed that the BMR of marsupials as a group is significantly lower than that of eutherians even when animals from similar habitats are compared.