During hibernation, black bears and brown bears enter a deep sleep called torpor, during which body functions slow dramatically.
Black Bears: Typically hibernate 4 to 7 months, relying on fat reserves and losing up to 30% of body weight. They generally do not defecate or urinate during deep hibernation.
Brown Bears: Hibernate 5 to 8 months, including grizzlies and Kodiak bears. They also rely on fat reserves, with infrequent urination and very infrequent bowel movements, usually only during brief periods of wakefulness.
Hibernation length can vary based on environment, food availability, and individual differences.