Daily torpor in mice: high foraging costs trigger energy-saving hypothermia.

Many animal species employ natural hypothermia in seasonal (hibernation) and daily (torpor) strategies to save energy. Facultative daily torpor is a typical response to fluctuations in food availability, but the relationship between environmental quality, foraging behaviour and torpor responses is poorly understood. We studied body temperature responses of outbred ICR (CD-1) mice exposed to different food reward schedules, simulating variation in habitat quality. Our main comparison was between female mice exposed to low foraging-cost environments and high-cost environments. As controls, we pair-fed a group of inactive animals (no-cost treatment) the same amount of pellets as high-cost animals. Mice faced with high foraging costs were more likely to employ torpor than mice exposed to low foraging costs, or no-cost controls (100% versus 40% and 33% of animals, respectively). While resting-phase temperature showed a non-significant decrease in high-cost animals, torpor was not associated with depressions in active-phase body temperature. These results demonstrate (i) that mice show daily torpor in response to poor foraging conditions; (ii) that torpor incidence is not attributable to food restriction alone; and (iii) that high levels of nocturnal activity do not preclude the use of daily torpor as an energy-saving strategy. The finding that daily torpor is not restricted to conditions of severe starvation puts torpor in mice in a more fundamental ecological context.