An improved method for detecting torpor entrance and arousal in a mammalian hibernator using heart rate data

We used ECG telemeters to measure the heart rate of hibernating Ictidomys tridecemlineatus . An increase in heart rate from 2.2 to 5 bpm accurately identified arousal from torpor before any change in body temperature was detected. Variability in raw heart rate data was significantly reduced by a forward-backward Butterworth low-pass filter, allowing for discrete differential analysis. A decrease in filtered heart rate to 70% of maximum values in interbout euthermia (from approximately 312 to 235 bpm) accurately detected entrance into torpor bouts. At this point body temperature had fallen from 36.1 o C to only 34.7 o C, much higher than the 30 o C typically used to identify entrance. Using these heart rate criteria advanced detection of entrance and arousal by 51.9 and 76 minutes, respectively, compared with traditional body temperature criteria. This method will improve our ability to detect biochemical and molecular markers underlying these transition periods during which many physiological changes occur.