Seasonal changes in thermogenesis, organ weights, and body composition in the white-footed mouse,Peromyscus leucopus.

1. Seasonal adjustments in wild-caughtPeromyscus leucopus include autumn increases in basal metabolic, nonshivering thermogenesis, and interscapular brown fat and decreases in weights of gonads, liver, adrenal glands, and total lipid. Body weight and nonextractable dry weight do not change. 2. Basal metabolic rate, nonshivering thermogenesis, and interscapular brown fat increase following initial cold exposure in mid-September and are maintained at similar levels through January. 3. There is a positive correlation between nonshivering thermogenesis and interscapular brown fat, and negative correlations for both nonshivering thermogenesis and interscapular brown fat with body weight. 4. These seasonal changes in wild-caughtP. leucopus are compared with laboratory studies on the effects of chronic exposure to differences in temperature and photoperiod on these characters. It is concluded that disparities between the effects of cold acclimatization and cold acclimation could result from the influence of additional environmental cues, such as photoperiod, in cold acclimatized mice.