Flexibility in thermoregulatory physiology of two dunnarts, Sminthopsis macroura and Sminthopsis ooldea (Marsupialia; Dasyuridae)

Stripe-faced dunnarts (Sminthopsis macroura) and Ooldea dunnarts (S. ooldea) were acclimated for 2 weeks to ambient temperature (T-a) regimes of 12-22 degrees C, 18-28 degrees C and 25-35 degrees C, and then measured for standard, basal (BMR) and maximum (MMR) metabolic rate using flow-through respirometry. Sminthopsis macroura maintained a stable body temperature under all experimental T-a and acclimation regimes. Although its BMR was not statistically different between the three acclimation regimes, the lower end of the thermoneutral zone (TNZ) shifted from 30 degrees C under the 18-28 degrees C and 12-22 degrees C acclimation regimes to 35 degrees C under the 25-35 degrees C acclimation regime. MMR increased significantly at the cooler acclimation regimes. EWL increased at T-a=35 degrees C, compared with lower T-a, in all acclimation regimes, but an increase in evaporative water loss (EWL) at T-a=10 degrees C observed in cool acclimations did not occur at the 25-35 degrees C regime. In contrast, S. ooldea had variable body temperature between experimental T-a in all acclimation regimes, but no acclimational shift in TNZ, which was between 30 and 35 degrees C. Neither BMR nor MMR was affected by exposure to the three acclimation regimes. EWL did not change across T-a or with acclimation regime. Sminthopsis macroura was flexible in many aspects of its thermoregulation (involving energy and water balance) in response to thermal acclimation, presumably allowing it to balance its energy and water requirements over a broad range of climatic conditions. Sminthopsis ooldea seems to have an inflexible energetic and water balance in response to thermal acclimation, but has low nominal expenditure of either resource on thermoregulation because it thermoregulates less precisely than S. macroura. It seems that S. ooldea is adapted to a more narrow, stable climate.