Physiological dynamics of body temperature and heart rate during hibernation periods in male Japanese black bears ( Ursus thibetanus japonicus )

Background: ) exhibits a profound seasonal metabolism shift, with a substantial increase in body weight in autumn and metabolic suppression during hibernation in winter. This species hibernates under the strict regulation of endogenous and environmental factors, making it a compelling subject for studies on the interrelationships between vital signs and metabolism. Studies continuously monitoring body temperature (Tb) and heart rate (HR) in Japanese black bears is limited, leaving gaps in understanding their synchronized fluctuations during hibernation. Methods: Here, we conducted long-term monitoring to investigate changes in the Tb and HR of Japanese black bears across the pre-hibernation, hibernation-induction, hibernation, and post-hibernation periods. Subcutaneous Tb and HR loggers of six captive male bears were monitored for Tb and HR from October 2017 to May 2018. Results: The Tb of male bears decreased gradually from the pre-hibernation period to the hibernation-induction period, experienced a predicted mean Tb of 35.2 °C (95% confidence interval (CI) [35.00-35.38]) during the hibernation period, and subsequently increased gradually during the post-hibernation period. The HR decreased rapidly at the end of the pre-hibernation and hibernation-induction periods, stabilized at a predicted mean of 44.76 bpm (95% CI [34.54-54.98]) bpm during the hibernation period, and subsequently increased rapidly at the beginning of the post-hibernation period. Tb and HR followed ~24-h cycles during the pre-and post-hibernation periods. Conversely, Tb and HR fluctuations exhibited synchronization with multi-day cycles during the hibernation periods. These results suggest that Japanese black bears exhibit a daily (24-h) rhythm of Tb and HR during the pre- and post-hibernation periods, while autonomic nervous system regulation predominates during hibernation period. We observed a difference between changes in Tb and HR, Tb decreased only moderately by approximately 4% from the pre-hibernation level, whereas HR dropped dramatically by approximately 38%, with rapid changes in HR and gradual changes in Tb. Our findings enhance the understanding of hibernation physiology and highlight synchronized subcutaneous Tb and HR as key biomarkers in large hibernators.