To use or not to use torpor? Activity and body temperature as predictors

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, approximately 125 g), known to display daily torpor in the wild. Occurrence of torpor was preceded by significant (approximately 10-25%) reductions of the duration of the activity phase. Moreover, the normothermic resting T(b) fell by an average of 1.2 degrees 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.