During hibernation, a bear's heart rate drops to about 8 to 10 beats per minute. A commonly cited value is approximately 9 beats per minute, compared with a normal resting rate of roughly 55 beats per minute.
This dramatic slowdown is part of a broader metabolic adjustment that lets bears survive winter without eating, drinking, urinating, or defecating. Their metabolism decreases, and their body temperature falls modestly compared with the much deeper cooling seen in many small hibernating mammals. Bears can therefore remain in their dens for months while relying on fat stored during the autumn feeding season.
Bears are not completely inert during this period. They can wake, shift position, and respond to danger, and their muscles remain in surprisingly good condition. Researchers have linked the low heart rate partly to a super-relaxed state of myosin, a muscle protein that reduces ATP consumption in the heart and other striated muscles. Not every bear species follows exactly the same winter pattern, and pregnant polar bears den rather than undergoing identical dormancy.