Regional Intraspecific Differences of Thermal Biology in a Marsupial Hibernator.

AbstractDuring periods of torpor, hibernators can reduce metabolic rate (MR) and body temperature () substantially. However, to avoid physiological dysfunction at low temperatures, they defend at a critical minimum, often between ~0°C and 10°C via an increase in MR. Because thermoregulation during torpor requires extra energy, individuals with lower 's and thus minimal MR during torpor should be selected in colder climates. Such inter- and intraspecific variations occur in some placental mammals, but for the evolutionary separate marsupials, available information is scarce. Marsupial eastern pygmy possums (; ~22 g body mass), widely distributed along the Australian southeastern coast including subtropical to alpine areas, were used to test the hypothesis that the defended of torpid individuals is related to the climate of their habitat. Possums were captured from five regions, 1,515 km apart, with midwinter (July) minimum environmental temperatures (min 's) ranging from -3.9°C to 6.6°C. Captive possums in deep torpor were slowly cooled with ambient temperature (), while their MR was measured to determine the minimum torpor metabolic rate (TMR), the at which their MR increased for thermoregulation (min ), and the corresponding minimum (min ). Partial least squares regression analysis revealed that and were the strongest explanatory variables for the min . The min and were also correlated with latitude but not elevation of the capture sites. However, the best correlations were observed between the min and the min and for individuals experiencing min ; these individuals thermoconformed to min 's between -0.8°C and 3.7°C, and their min ranged from 0.5°C to 6.0°C and was 0.5°C-2.6°C below the min at the capture site. In contrast, individuals experiencing a min of -3.9°C regulated at or 4.5°C above the . The minimum TMR of all possums did not differ with and thus did not differ among populations and was 2.6% of the basal MR. These data provide new evidence that thermal variables of marsupials are subject to regional intraspecific variation. It suggests that min is a function of the min but only above 0°C, perhaps because the differential for torpid possums in the wild, at a min of -3.9°C, remains small enough to be compensated by a small increase in MR and does not require the physiological capability for a reduction of below 0°C.