Bears, known for their impressive ability to gain weight in preparation for winter, enter a period of reduced activity to survive when food becomes scarce. This state, often called hibernation, is a natural adaptation allowing them to conserve energy during colder months. Understanding this process, particularly its timing in a diverse landscape like California, offers insight into these large mammals.
California’s Bear Population
California is home to the American black bear (Ursus americanus), the state’s only bear species. While black bears are not always black and can exhibit brown or cinnamon fur, they are distinct from the grizzly bears that once roamed California, which were extirpated by 1924.
The black bear population in California is robust, with estimates ranging from 49,000 to 71,000 individuals as of 2025. These bears are widely distributed across California’s varied habitats, including forests, grasslands, and the urban-wildland interface. Densely populated areas are found in the North Coast, Cascade, and Sierra Nevada regions.
The Science of Bear Torpor
Bears do not undergo the deep, true hibernation seen in smaller mammals like groundhogs, where body temperature drops drastically close to freezing. Instead, bears enter a state known as torpor, a less extreme form of dormancy. This allows them to be more easily roused from their sleep if disturbed.
During torpor, a bear’s physiological functions slow considerably. Their heart rate can drop to as low as 8-10 beats per minute, and breathing may reduce to just one breath every 15 to 45 seconds. While their metabolic rate decreases significantly, body temperature only drops slightly, typically remaining above 88°F (31°C). Bears sustain themselves by relying on fat reserves accumulated throughout the fall. They do not eat, drink, urinate, or defecate while in their dens, recycling waste products to maintain muscle mass. Pregnant female bears give birth to cubs in the den during January or February, nursing them through this period.
When Bears Den in California
Black bears in California typically begin to enter their dens between October and December. The exact timing can fluctuate, influenced by local environmental conditions. Bears remain in these dens for several weeks to several months, depending on the region.
Emergence generally occurs between March and May. Adult male bears tend to enter later and emerge earlier than other bears. Females, especially those with newborn cubs, typically den earliest and emerge latest, ensuring their vulnerable offspring have more time to grow within the safety of the den.
Dens can be found in various sheltered locations, including under fallen logs, in dense vegetation, tree cavities, rock crevices, or under human structures like decks and patios. In some milder southern California areas, bears may remain active year-round and not den at all.
Factors Influencing Denning Timing
Several factors influence when a black bear in California enters its den. Food availability is a primary determinant; abundant natural food sources like berries and nuts in the fall allow bears to build sufficient fat reserves, which can delay denning. Conversely, a scarcity of food prompts an earlier entry.
Weather conditions also play a significant role, particularly the onset of colder temperatures and substantial snowfall. While food availability largely drives den entry, emergence is more closely linked to warming spring temperatures.
Geographic location and elevation contribute to variations in denning behavior. Bears at higher elevations or in colder northern parts of California may den earlier than those in milder, lower-lying areas.
Human activity, especially accessible food sources, can significantly alter traditional denning cycles. Bears accustomed to human-provided food may delay or forgo denning entirely, leading to increased human-bear interactions. Climate change is increasingly impacting these patterns, with warming winters leading to later den entry and earlier emergence, potentially causing bears to wake before natural food sources are widely available.