Regional Intraspecific Differences of Thermal Biology in a Marsupial Hibernator.

AbstractDuring periods of torpor, hibernators can reduce metabolic rate (MR) and body temperature (T_b) substantially. However, to avoid physiological dysfunction at low temperatures, they defend T_b 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 T_b'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 (Cercartetus nanus; ~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 T_b 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 T_env's) ranging from -3.9°C to 6.6°C. Captive possums in deep torpor were slowly cooled with ambient temperature (T_a), while their MR was measured to determine the minimum torpor metabolic rate (TMR), the T_a at which their MR increased for thermoregulation (min T_a), and the corresponding minimum T_b (min T_b). Partial least squares regression analysis revealed that T_a and T_env were the strongest explanatory variables for the min T_b. The min T_b and T_a were also correlated with latitude but not elevation of the capture sites. However, the best correlations were observed between the min T_env and the min T_b and T_a for individuals experiencing min $T_{\mathrm{env}}>0°\text{C}$; these individuals thermoconformed to min T_a's between -0.8°C and 3.7°C, and their min T_b ranged from 0.5°C to 6.0°C and was 0.5°C-2.6°C below the min T_env at the capture site. In contrast, individuals experiencing a min T_env of -3.9°C regulated T_b at $0.6°C\pm 0.2°\text{C}$ or 4.5°C above the T_env. The minimum TMR of all possums did not differ with T_a 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 T_b is a function of the min T_env but only above 0°C, perhaps because the $T_b-T_a$ differential for torpid possums in the wild, at a min T_env 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 T_b below 0°C.