Maximal Thermogenic Capacity and Body Temperatures of White-Footed Mice (Peromyscus) in Summer and Winter

Freshly captured white-footed mice (Peromyscus leucopus) from summer and winter populations were returned to the laboratory and metabolism experiments performed immediately. Oxygen consumption (V̇_O₂), maximal oxygen consumption (V̇_O₂max), and body temperatures (before and after metabolism runs) were measured as a function of ambient temperature. In order to elicit V̇_O₂max, metabolic rate was measured in a mixture of He-O₂ as well as in air. Thermoregulatory capabilities were enhanced at low temperatures in winter-acclimatized individuals. (1) The minimum thermal conductance decreased from 0.286 cm³ O₂ (g·h·°C)⁻¹ in summer to 0.231 cm³ O₂ (g·h·°C)⁻¹ in winter. (2) Winter animals were capable of maintaining higher body temperatures at the lower ambient temperatures. (3) The mean V̇_O₂max increased approximately 70% from 11.6 cm³ O₂ (g·h)⁻¹ in summer to 19.5 cm³ O₂ (g·h)⁻¹ in winter. (4) Body temperatures measured at the conclusion of metabolism experiments ranged from 39 to 28 C for winter animals in steady-state oxygen consumption in He-O₂. Such a reduction in body temperature at low ambient temperatures could serve to lower energetic expenditures significantly.