Many people assume all bears truly hibernate, entering a deep, unresponsive sleep. However, their winter dormancy, particularly for bear cubs, is a more nuanced adaptation. Understanding these distinctions clarifies how they endure periods of scarcity.
Do Bear Cubs Hibernate?
Bear cubs do not hibernate independently. They are born during their mother’s winter lethargy, typically between mid-January and early February, while she is in her den. Newborns are extremely small, weighing less than a pound, and are blind and nearly hairless. They are entirely dependent on their mother for warmth, protection, and nourishment.
Cubs spend their first months nestled closely, nursing frequently. They grow rapidly on exceptionally rich milk, which contains a high fat content, sometimes around 33%. This allows them to gain significant weight, reaching about 5 pounds by spring. The den serves as a secure nursery where cubs develop before emerging.
Understanding Bear Winter Lethargy
Unlike true hibernators like chipmunks or ground squirrels, bears enter winter lethargy or torpor. Their heart rate, breathing, and metabolic rate decrease, but body temperature only drops slightly, typically by 5-7°C (10-12°F) from their normal 37-38°C (99-101°F). This contrasts with true hibernators whose temperatures can drop to near-freezing levels.
Bears can easily be aroused from this state, a significant difference from true hibernation. This allows them to respond to potential threats. The primary purpose of this winter dormancy is to conserve energy and survive periods when food and water are scarce. Bears can remain in this state for several months without eating, drinking, or passing waste, relying on stored fat reserves.
Life in the Den for Cubs
The den provides a sheltered environment where bear cubs grow and develop during winter. Mother bears prepare these dens, sometimes lining them with vegetation for insulation. While in torpor, the mother remains attentive to her cubs, shifting to accommodate and keep them warm.
Newborn cubs nurse often. The mother’s body undergoes a metabolic transformation, breaking down stored fat reserves to produce milk without eating or drinking for months. This demanding process can lead to a mother bear losing a third or more of her body weight over winter. By spring, the cubs have gained enough size and strength to emerge from the den with their mother, ready to begin learning survival skills.