Daily energy expenditure in the pouched mouse (Saccostomus campestris peters 1896)

Previous studies of the pouched mouse (Saccostomus campestris) have revealed that during reproduction there is no increase in food consumption. although resting energy demands, measured by respirometry, increase substantially. One explanation for this anomalous situation is that these mice may routinely use torpor to compensate their energy budgets. We measured the daily energy expenditure of eleven individual pouched mice over three consecutive days using the doubly-labelled water technique. The mice were housed in cages where they had free access to food and water, at an average temperature of 26.5 °C, and exposed to a natural photoperiod (February Pretoria). There was a large day-to-day variation in energy expenditure within each individual. The coefficient of variation in daily energy demand averaged 24.5%. By comparing the correlation of estimates for consecutive and nonconsecutive days we established that this variation was not a consequence of errors in the isotopic technique. The scaling exponent of the measures of energy expenditure to mass was 1.196 (sd = 0.37: not significantly different to 1.0). We corrected all the estimates to a mean mass of 61.3 g using a scaling exponent of 1.0. We then compared the daily energy demands with expected energy requirements for endothermic animals at rest in respirometers. All the estimates of daily energy expenditure exceeded those anticipated from the resting costs, and averaged about 2.1x greater. We found no compelling evidence therefore that Saccostomus campestris routinely utilizes long (>8 hours) bouts of torpor to compensate its energy budget.