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Effect of age and torpor on the circadian rhythms of body temperature, activity, and body weight in the mouse (Peromyscus leucopus).

Age-related alterations in homeostatic mechanisms, as measured by changes in circadian rhythms, have been reported in the literature. The purpose of the present study was to determine if changes in the circadian rhythms of motor activity, body temperature, and body weight occur with age in the long-lived mouse, Peromyscus leucopus, and to determine if the incidence of torpor (hypothermia) was age-dependent. Young and old mice were entrained to an LD 12:12 cycle, and then physiological and behavioral performance was monitored by a data-acquisition system. The major differences between the 6-month and 60-72-month age groups were the following. Significant episodes of torpor occurred in a high percentage of old mice at 0600 hr on consecutive days, but young mice showed no evidence of torpor when food was provided ab libitum. Torpor in young mice was found to occur only during starvation conditions. Old animals phase in and out of torpor over an interval of several days; high-amplitude rhythms and good synchronization of activity and temperature occurred when torpor was expressed, and low-amplitude rhythms, poor synchronization between temperature and activity, and a phase shift of body-weight acrophase occurred on days when torpor was not expressed. Significant changes in acrophase and amplitude occurred between individual animals and days in the old group; no significant change in acrophase and amplitude was observed in the young group. Population-mean cosinor analysis revealed a significant age-related decrease in amplitude and mesor, but no significant change in acrophase was found.