During hibernation, an American black bear’s metabolic rate can drop to one quarter of its basal rate.
American black bears enter winter dens after building substantial fat reserves, often in October or November. Their hibernation generally lasts three to eight months, depending on climate and location. During this period, the heart rate can fall from about 40–50 beats per minute to around 8, while metabolism drops to roughly 25% of the non-hibernating basal rate.
Bears are unusual among large mammals because their body temperature does not plunge as dramatically as that of small hibernators. It stays near 35°C, allowing the bear to remain responsive and to recover more easily when it briefly awakens. Females can even give birth and nurse cubs inside the den.
The phrase “basal rate” matters: the comparison is with the bear’s normal resting metabolism, not with its energy use while actively walking or foraging. Another common misconception is that bears are simply sleeping. Their hibernation involves coordinated metabolic, cardiovascular, hormonal, and waste-management changes that let them fast for months while preserving much of their muscle and bone.