Circadian Rhythms of Body Temperature in Laboratory and Field Marmots (Marmota flaviventris)

For many years, investigators have questioned whether circadian rhythms continue to be expressed in mammals that hibernate when body temperature (Tb) is low. We have examined the circadian rhythm of Tb in marmots (Marmota flaviventris) under laboratory and field conditions during the winter hibernation period. Using temperature data loggers that were implanted in animals, Tb measurements were continuously recorded for up 2 years. We found that animals in the laboratory continued to manifest a circadian rhythm of Tb prior to hibernating, during deep hibernation, and after arousal from hibernation. 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 hibernation. 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 animals must have been exposed to light, because they 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-runs with a period close to 24hr. The fact that we found no circadian rhythm of Tb in our field animals that were in constant dark and very quiet conditions suggests that the circadian pacemaker is uncoupled from the Tb rhythm or is so damped that the output cannot be detected. We believe that circadian system is nonfunctional during deep hibernation under natural conditions.