Selection of High Temperatures for Hibernation by the Pocket Mouse, Perognathus Longimembris: Ecological Advantages and Energetic Consequences

Daily metabolism was calculated from food consumption in pocket mice, Perognathus longimembris, at 8 degrees C, 18 degrees C, and 31 degrees C. At temperatures below thermal neutrality for this species, daily metabolism was related to the amount of time the mice spent in torpor. Ambient temperature has no net effect on the minimum energy expenditure during a typical 5—mo hibernation season. Once an animal has accumulated a food store of 130 g of millet seeds, it has the minimum energy necessary to hibernate at any environmental temperature. Such temperature compensation results from the complex effects of temperature on (1) the ratio of time of euthermy to time of torpor, (2) the energetic cost per hour of torpor, (3) the energetic cost per hour of euthermy, and (4) the energetic cost of arousal from torpor. The amount of time spent in torpor was inversely dependent on the food supply, indicating that euthermia is preferred even during the hibernation season. Mice also maximize the time of euthermia by selecting high environmental temperatures at all times of the year. Torpor probably occurs naturally only during the winter when the highest temperatures available to the mice are below thermal neutrality. The maximization of the time of euthermia reduces the chances of freezing during hibernation and enhances the animal's ability to escape from predators.