Grizzly bears are often thought to hibernate through the winter, similar to many smaller mammals. However, their prolonged inactivity, while significant, does not align with the precise biological definition of true hibernation. Understanding these distinctions reveals a remarkable survival strategy adapted to their large size and their specific environmental needs.
The Truth About Grizzly Winter Sleep
Grizzly bears enter a state more accurately described as “denning” or “winter lethargy.” This deep sleep allows them to conserve energy during times of food scarcity and harsh weather. Unlike true hibernators, grizzlies do not undergo profound physiological changes and remain capable of relatively quick arousal, a key difference. This period typically lasts five to seven months, depending on the region and individual bear.
During denning, grizzlies significantly reduce their metabolic rate, heart rate, and breathing. Their heart rate can drop to 8-12 beats per minute, and respirations may slow to one breath every 45 seconds. Despite these reductions, their body temperature only decreases by a few degrees, generally remaining between 30-35°C (86-95°F).
Distinguishing Denning from True Hibernation
The physiological differences between a grizzly’s winter lethargy and true hibernation are significant. True hibernators, like ground squirrels, experience a drastic drop in body temperature, sometimes nearing freezing, and their metabolic rate can slow by as much as 98%. In contrast, a grizzly’s body temperature reduction is modest, typically only 5-6 degrees Celsius, allowing them to maintain a temperature close to normal.
Another distinction is arousal. True hibernators are difficult to rouse, as warming their bodies requires significant time and energy. Grizzlies, however, can be easily awakened and respond to disturbances or threats. This ability to quickly become active is a defining characteristic of their winter lethargy. Furthermore, while true hibernators may wake periodically, grizzlies do not eat, drink, urinate, or defecate during their entire denning period.
Preparing for Winter and Denning Behavior
Grizzly bears undertake extensive preparations for winter denning. In the fall, they enter hyperphagia, characterized by excessive eating. They consume vast quantities of food, sometimes eating up to 45 pounds a day and gaining 3-5 pounds daily. This intense feeding allows them to accumulate significant fat reserves, sustaining them through months without food or water.
Den selection is a careful process. Grizzlies often choose sites on moderate to steep slopes with northern exposures, which aid in snow accumulation for insulation. Dens are typically excavated into the ground, consisting of an entrance, a short tunnel, and a chamber lined with bedding material to trap body heat. While bears may use the same general area year after year, they rarely reuse the exact same den site, often digging a new one. The denning period typically begins in late fall, often triggered by snowfall, and lasts until spring.
Survival Advantages of Winter Lethargy
Winter lethargy offers several survival advantages for grizzly bears. This adaptation allows them to conserve energy when food is scarce and environmental conditions are harsh. By reducing metabolic activity, grizzlies survive months without consuming food or water, relying on stored fat reserves. They typically lose 15-30% of their body weight during this time.
Denning also provides refuge from cold temperatures and deep snow. For female grizzlies, the den serves as a secure environment for giving birth and nursing cubs. Cubs are born in the den during late winter and remain there with their mother until spring, benefiting from the den’s protection and warmth. This strategy ensures the survival of both the adult bears and their vulnerable offspring through the most challenging season.