Energy partitioning in torpor-sensitive and torpor-resistant deer mice (Peromyscus maniculatus)

Abstract

We examined metabolic rates in 25 deer mice, Peromyscus maniculatus nebrascensis, held at an ambient temperature of 10 °C and found that all of them had the ability to enter torpor. However, we found a gradation between the most torpor-sensitive and the most torpor-resistant animals, suggesting that the expression of torpor is more than a simple all-or-nothing phenomenon. On a daily basis, the most torpor-sensitive and torpor-resistant mice partition energy similarly when food is not restricted. On restricted food, both groups of mice decrease the amount of time spent active and increase the amount of time spent at rest and in torpor. In addition, both groups of mice decrease oxygen-consumption rates and body temperatures during rest and the most torpor-sensitive mice further decrease energy costs by entering longer and deeper bouts of torpor. However, the overall energy saving due to torpor is only about 6%. Thus, the widely held view that the adaptive advantage of daily torpor is a large energy saving may be open to question. Alternatively, daily torpor may be important for reducing the rate of energy turnover when these small rodents must rely on body fat reserves.