Influence of Ambient Temperature, Nest Availability, Huddling, and Daily Torpor on Energy Expenditure in the White-Footed Mouse Peromyscus Leucopus

Abstract

Short-day (9 light:15 dark), cold-acclimated Peromyscus leucopus known to enter torpor (no. = 24) and P. leucopus never observed in torpor (no. = 12) were compared with and without a 5-g nest at 13, 7, and 1 C over 9 wk. Oxygen consumption and body temperature were monitored approximately hourly for 2-3 days under each treatment. A second group of short-day, cold-acclimated mice (13 torpid and 14 nontorpid) were monitored for oxygen consumption with and without a 5-g nest at 13 C, first individually and 10 days later as huddles of three. For individual mice, the time required to enter torpor (3.0-4.5 h), duration of torpor (4-5 h), and minimum body temperature during torpor (21-23 C) remained fairly constant at different ambient temperatures. The metabolic rate [cm³O₂/(g · h)] necessary to maintain minimum body temperature during torpor without a nest was 2.1 at 13, 3.0 at 7, and 3.7 at 1 C, while metabolic rates during the normothermic period at these temperatures were 4.8, 5.5, and 6.7, respectively. A similar trend, but with lower values, occurred with a nest. Without a nest, torpid mice had a mean daily metabolic rate which was 20% less at 13, 21% at 7, and 9% at 1 C than the corresponding value of nontorpid animals. Torpid mice with a 5-g nest reduced metabolic rate at those temperatures by 43%, 44%, and 34%, respectively, relative to nontorpid mice without a nest. Daily torpor and huddling together provided a 58% energetic saving at 13 C relative to individually housed, nontorpid mice. The addition of a nest fostered a 74% daily energy savings versus nontorpid, individual mice without a nest.