Bears in Yellowstone National Park, including Grizzly and Black Bears, enter a state of winter dormancy each year to survive the cold and scarcity. This period, often called hibernation, allows them to sustain themselves on fat reserves accumulated during the autumn. The denning period is not fixed, but is a flexible schedule governed by biological need and environmental conditions.
Defining Hibernation Timing by Species
Black Bears generally enter their dens earlier than Grizzly Bears, often beginning their denning process in October or early November. They typically favor lower elevation habitats, which can influence their schedule slightly.
Grizzly Bears tend to den later, following a sequence: pregnant females, single females, and finally, adult males. Pregnant females often seek out den sites by late November. Adult male Grizzlies are the last to den, sometimes remaining active until late December if weather conditions allow them to find food sources.
This staggered timeline reflects differing energy needs and survival strategies for each group. The average denning period lasts approximately five months, though this duration varies based on the severity of the winter. During this time, bears do not eat, drink, or pass waste, living entirely off accumulated fat stores.
Environmental Factors Influencing Denning
Den entry is a response to powerful biological and ecological signals, not a conscious choice based on the date. The primary biological driver is the successful completion of hyperphagia, a period of intensive feeding in late summer and fall. During hyperphagia, bears consume massive calories to build necessary fat reserves, preparing them for dormancy.
The physical environment provides the final triggers for den entry, with food scarcity being the most influential factor. As autumn progresses and vegetation dies back, the availability of berries, roots, and other food sources rapidly diminishes. Persistent snow cover and a sustained drop in temperatures signal that foraging is no longer energetically efficient.
Bears often select den sites on north-facing slopes at elevations between 6,500 and 10,000 feet, where prevailing winds accumulate deeper snow. The deep snow acts as an insulating layer, helping the bear maintain its slightly lowered body temperature. The shift in weather, combined with the lack of available sustenance, overrides the impulse to remain active.
Spring Emergence from Dens
Bears begin to emerge from their winter dens in the spring, following a sequence determined by species and sex. Male Grizzlies are typically the first, often leaving their dens in mid-to-late March to begin seeking food. Black Bears, which den at lower elevations where snow melts sooner, may be sighted as early as late February or early March.
Females with cubs are the last to leave the den, usually waiting until late April or early May. This delay ensures the newborn cubs grow and gain strength, and that early spring food sources are more accessible outside the den. The timing of emergence is heavily influenced by rising temperatures and the melting of snowpack.
As the snow recedes, bears are drawn toward areas where food is first exposed, such as the park’s thermal areas. Their initial diet often consists of winter-killed ungulates like elk and bison, providing a calorie-dense meal after months of fasting. The appearance of early spring plant growth helps transition the bears back to their active diet.