Induction of torpor in response to a common chronic food restriction paradigm: implications for behavioural research using mice

Background Many behavioural, pharmacological, and metabolic studies in mice require fasting, yet the possibility of fasting-induced torpor affecting data is rarely considered. Torpor is a state characterised by depressed metabolism and profound alterations in physiology and behaviour. In this study, we aimed to determine whether a chronic food restriction paradigm, common in behavioural studies, was sufficient to induce torpor in mice. Methods Mice were food restricted to ~85-90% of their bodyweight, as is typically done, and monitored using continuous thermal imaging. Results We observed that body temperature significantly decreased over days of food restriction, and it was significantly related to the drop in bodyweight (r2=0.8989, p<0.0001). All mice reliably entered torpor daily from day 8 of food restriction which coincided with bodyweight stabilisation at ~85%. We found a strong positive relationship between the magnitude of the decrease of bodyweight and the proportion of mice entering torpor each day (r2=0.8715, p<0.0001). Conclusions Overall, we found that torpor is readily induced in response to food restriction. Considering that hunger is frequently used as a motivational drive in behavioural tasks, it is likely that torpor occurrence is common in such studies, while remaining undetected and unaccounted for. Due to the profound effect of torpor on physiology, it is possible that torpor induction may be confounding subsequent data and represents an important source of variation. We recommend that body temperature is always monitored noninvasively in studies where food restriction is employed, to determine when torpor is occurring, and that torpor history is appropriately controlled for within and across experimental groups.