When food is limited and/or environmental conditions are unfavourable, many mammals reduce activity and use torpor to save energy. Nevertheless, reliable predictors for torpor occurrence, especially in the wild, are currently not available. Interrelations between torpor use and other energy conserving strategies are also poorly understood. We tested the hypothesis that reductions in normothermic body temperature ("T[b]") and the period of activity before torpor events could be used as predictors for torpor occurrence in sugar gliders, 'Petaurus breviceps' (body mass, ~125 g), known to display daily torpor in the wild. Occurrence of torpor was preceded by significant (~10-25%) reductions of the duration of the activity phase. Moreover, the normothermic resting T[b] fell by an average of 1.2°C over 3 days before a torpor event, relative to individuals that did not display torpor. Our new findings suggest that before entering torpor, sugar gliders, which appear to use torpor as an emergency measure rather than a routine energy saving strategy, systematically reduce activity times and normothermic resting T[b]s to lower energy expenditure and perhaps to avoid employing torpor. Thus, reduced activity and normothermic T[b] may provide a predictive tool for the occurrence of daily torpor in the wild.

We report new findings about torpor in marsupials with regard to three energy-demanding processes: (i) development and growth, (ii) reproduction, and (iii) rewarming. Young marsupials use torpor extensively after they develop endothermy, and torpor is generally deeper and longer than in the same individuals when they reach adult size. Adult marsupials also employ torpor during pregnancy and/or lactation to reduce energy expenditure and perhaps to store fat for later use. Moreover, to enhance the energy-conserving potential of torpor, desert marsupials bask during arousal to minimise energy costs of rewarming. We show that the functions of torpor extend beyond merely reducing energy expenditure during food shortages and that torpor can save substantial amounts of energy even during the rewarming process.