Polar bears do not engage in the deep, prolonged dormancy commonly known as true hibernation. The species is active year-round, primarily hunting seals on the Arctic sea ice throughout the winter. Instead of hibernating, polar bears have evolved metabolic and behavioral adaptations to manage long periods of fasting when food access is limited, particularly during the summer when the sea ice melts. The only exception to this active lifestyle is the pregnant female, who enters a temporary state of dormancy known as maternity denning to give birth and nurse her cubs. This adaptation is distinct from the profound physiological changes seen in true hibernators.
Defining True Hibernation
True hibernation is a complex physiological state characterized by a profound reduction in core body temperature, metabolic rate, and heart rate. Animals such as ground squirrels and bats enter this deep torpor, where their body temperature can drop dramatically, sometimes nearing the ambient temperature of their environment, like 35 to 41 degrees Fahrenheit (2 to 5 degrees Celsius). This extreme metabolic suppression, which can reduce metabolism by up to 95%, allows the animal to conserve vast amounts of energy and survive periods of food scarcity. True hibernators are also difficult to rouse, as their brain activity slows significantly.
Polar bears do not meet these scientific criteria for true hibernation. Their normal core body temperature remains relatively stable at about 98.4 degrees Fahrenheit (36.9 degrees Celsius). While they can slow their heart rate and metabolism during periods of fasting, the drop is not nearly as profound as in true hibernators. This less dramatic physiological change means polar bears can wake up and become active relatively quickly if disturbed. A true hibernator would require hours of metabolic warming to fully rouse, making quick activity impossible.
The Exception: Maternity Denning
The closest the polar bear comes to dormancy is the specialized state entered by pregnant females for maternity denning. A pregnant female excavates a den, usually in a snowdrift on land or sea ice, between late fall and early winter and remains there until spring. During this time, the female neither eats, drinks, nor defecates, sustaining herself and her developing offspring entirely on accumulated fat reserves.
Inside the den, the female gives birth, usually to one or two cubs in December or January, who are born tiny and helpless, weighing only around 1 pound (500 grams). The mother’s body enters a state of mild metabolic suppression. This allows her to minimize energy expenditure while maintaining a near-normal body temperature necessary to keep the den warm and nurse her young. The female’s milk is extremely rich, containing over 30% fat, which enables the cubs to grow rapidly. The female emerges from the den with her cubs in the spring, having endured a fast that can last up to eight months and resulting in a body mass loss of up to 43%.
Survival Strategies for Non-Breeding Polar Bears
Male polar bears and non-pregnant females remain active throughout the Arctic winter, primarily hunting seals on the sea ice, their most reliable food source. These bears rely on massive fat reserves to survive long periods of fasting, especially during the summer and early fall when the sea ice retreats and hunting becomes impossible. During these lean months on land, these bears do not possess a specialized, energy-saving metabolic state, which was once speculated to be “walking hibernation.”
Instead, non-breeding polar bears decrease their activity and live off the fat accumulated during the spring hunting season, losing body condition like any other food-deprived mammal. While some may enter short-term snow shelters, this is generally a strategy to reduce thermal stress or conserve energy, not true hibernation. Their ability to endure extended fasting periods is attributed to their capacity to live off immense fat stores, allowing them to conserve energy while remaining active and ready to hunt when sea ice conditions improve.