Age and temperature related changes in behavioral and physiological performance in the Peromyscus leucopus mouse

Age-related and ambient temperature-related changes in motor activity, body temperature, body weight (b.w.), and food consumption were studied in the long-lived Peromyscus leucopus mouse at environmental temperatures of 29 and 21 degrees C. Major changes in physiological performance were observed between the young (6 months) and old (60-72 month) age groups. The number of daily activity episodes, and total activity output was significantly lower in old mice. Maximum, average and minimum daily body temperature was lower in the old mice and a significant ambient temperature-by-age interaction was found. Maximum, minimum, and average daily b.w. was higher in old mice. Motor activity was evenly distributed over the active (night) phase in young mice but in old mice activity was significantly greater in the late night partition of the active cycle than in the early night partition. Both groups were significantly more active at night than during the day. Most of the food consumption in both groups occurred at night, but young mice consumed significantly more during the late night partition than the early night partition, and the consumption rates for old mice were not significantly different between early and late night partitions. The percentage of activity episodes involved with food consumption in both groups was significantly higher during the night partition, but the percentage during the early night partition was significantly higher in old mice than in young mice. Significant episodes of circadian torpor occurred in a high percentage of old mice at 06:00, on consecutive days, at both environmental temperatures, but young mice expressed no evidence of torpor.