Circadian rhythms of body temperature in laboratory and field marmots (Marmota flaviventris)

For many years, investigators have questioned whether circadian rhythms con tinue to be expressed in mammals that hibernate when body temperature (Tb) is low. We have examined the circadian rhythm of Tb in marmots (Marmota fla viventris) under laboratory and field conditions, juring the winter hiberna tion period. Using temperature data loggers that were implanted in animals, Tb measurements were continuously recorded for up 2 years. We found that a nimals in the laboratory continued to manifest a circadian rhythm of Tb pri or to hibernating, during deep hibernation, and after arousal from hibernat ion. Whereas animals in the field had robust circadian rhythms of Tb prior to hibernation, we were not able to detect a Tb rhythm during deep hibernat ion. We did find a diurnal rhythm of Tb once field animals had aroused for the last time in the spring and came above ground. In fact, we were able to determine when field mammals must ha,le been exposed to light, because the y were entrained to the daily light-dark cycle, Our results suggest that in the laboratory, animals may be picking up cues and their Tb rhythm free-ru ns with a period close to 24hr. nle fact that we found no circadian rhythm of Tb in our field animals that were in constant dark; and very quiet condi tions suggests that the circadian pacemaker is uncoupled from the Th rhythm or is so damped that the output cannot be detected. We believe that circad ian system is nonfunctional during deep hibernation under natural condition s.