Daily torpor and energy expenditure in Sminthopsis macroura: interactions between food and water availability and temperature.

Endothermy allows maintenance of a constant internal thermal milieu for optimal physiological functions but results in high energy expenditure and water loss. Since torpor can reduce both expenditure of energy and loss of water and thus reduces food and water requirements, we determined how different food and water regimes affect torpor occurrence and patterns in the dasyurid marsupial Sminthopsis macroura at ambient temperatures of 18 degrees C (well below the thermoneutral zone) and 28 degrees C (close to the thermoneutral zone). At 18 degrees C, torpor was more frequent and more pronounced than at 28 degrees C. Withdrawal of food reduced the average daily metabolic rate by 20%; withdrawal of water alone had little effect. The average daily metabolic rate of individuals displaying torpor was 20% lower than that of individuals remaining normothermic. At 28 degrees C, the average daily metabolic rate under food restriction was 85% of that with food available ad lib. However, this reduction of average daily metabolic rate at 28 degrees C seems due not to the use of torpor but mainly to a reduction of the resting metabolic rate. The results suggest that at low temperature, torpor is used to adjust energy expenditure to availability of food, whereas at high temperature, reductions of resting metabolic rate are employed.