The minimum body temperature of a hibernating American black bear is approximately 30°C. This is far warmer than the near-freezing temperatures reached by small hibernators such as ground squirrels, yet it is still several degrees below a black bear’s normal active temperature.
Research has shown that black bears regulate their core temperature within roughly 30–36°C during hibernation, often in multi-day cycles. Their metabolism can fall to about one-quarter of its normal basal rate, while the heart rate may slow from around 55 beats per minute to as few as 9.
This unusual physiology explains why bears can remain in dens for months without eating, drinking, urinating, or defecating. They preserve muscle and bone unusually well and can become active relatively quickly when disturbed. Females may even give birth and care for cubs inside the den.
A common misconception is that bears hibernate exactly like mice or bats. Bear hibernation is shallower in terms of body-temperature decline, but the metabolic suppression is profound. Scientists once debated whether this state deserved the name hibernation; modern definitions generally include black bears as highly efficient hibernators.