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

Abstract

SUMMARYStripe-faced dunnarts (Sminthopsis macroura) and Ooldea dunnarts (S. ooldea) were acclimated for 2weeks to ambient temperature (T a ) regimes of 12-22°C, 18-28°C and 25-35°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°C under the 18-28°C and 12-22°C acclimation regimes to 35°C under the 25-35°C acclimation regime. MMR increased significantly at the cooler acclimation regimes. EWL increased at T a 35°C, compared with lower T a , in all acclimation regimes, but an increase in evaporative water loss (EWL) at T a 10°C observed in cool acclimations did not occur at the 25-35°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°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.

From doubly labelled water to half‐life; validating radio‐isotopic rubidium turnover to measure metabolism in small vertebrates

“…When we compared our measures of metabolism () made by DLW and FTR , we found that DLW overestimated FTR by about 12% for S. macroura and 24% for S. ooldea . The larger error for S. ooldea likely reflects the higher variability of metabolic rates reported for this species compared with S. macroura due to more labile thermoregulation (Tomlinson, Withers & Maloney ,b), and the large discrepancies may not reflect uncontrolled methodological errors, but represent genuine variability in the metabolic rates of S. ooldea . Some previous studies have also reported that DLW overestimates , generally by about 8% (Speakman & Racey ; Nagy et al .…”

“…Our rationale was that these temperatures would alter metabolic rate and therefore , allowing comparison of respirometry, DLW and 86 Rb k b MRs in situations of altered metabolic rate. It was expected from previous experiments (Geiser & Baudinette ; Song, Kortner & Geiser ; Tomlinson, Withers & Maloney ,b) that 30 °C would be thermoneutral for the dunnarts, while 20 °C would be below the thermoneutral zone and would result in approximately a doubling of the metabolic rate (Tomlinson, Withers & Maloney ,b). Following the second period of respirometry, the dunnarts were maintained at 30 °C and 86 Rb counts were made daily until complete washout had occurred, to determine the ideal time span over which to measure 86 Rb k b .…”