The question of whether a polar bear hibernates often leads to a simple but misleading “no.” While polar bears do not enter the deep, true hibernation seen in many other mammals, their winter behavior is complex. The confusion arises because a specific subset of the population does enter a prolonged state of winter dormancy, which is easily mistaken for hibernation. For the majority of adult polar bears, however, winter is a period of intense activity and continuous hunting. This difference in strategy is linked to their unique physiology and the availability of their primary food source.
Defining True Hibernation
True hibernation is a profound physiological adjustment used by many smaller mammals to survive periods of extreme cold and food scarcity. This deep dormancy is characterized by a massive drop in body temperature, often falling close to freezing or near-ambient levels. True hibernators, such as ground squirrels, experience a drastic reduction in metabolic rate and heart rate, sometimes slowing to only a few beats per minute. Arousal from this state is a slow, energy-intensive process that can take hours, requiring the animal to actively shiver to raise its body temperature back to normal.
The Exception of Maternity Denning
The behavior that most closely resembles hibernation is exhibited only by pregnant female polar bears, which enter a maternity den to give birth and nurse their cubs. These females dig dens, usually into snowdrifts on land or sea ice, and remain there for an extended period, typically from October or November to March or April. The denning period can last between four and eight months, during which the female does not eat, drink, or excrete, relying entirely on her extensive fat reserves.
The female’s physiological state during denning is not true hibernation, but rather a form of winter lethargy or specialized torpor. Her body temperature drops only slightly, unlike the massive reduction seen in true hibernators, allowing her to remain relatively alert. Maintaining a near-normal body temperature is necessary to give birth and provide continuous warmth for the cubs. The mother’s ability to be easily roused is essential for the safety and early development of her newborn offspring.
The Active Winter Strategy
For the majority of the polar bear population—males and non-pregnant females—winter is a time of active hunting on the sea ice, not rest. These bears are built for continuous operation in the extreme cold and do not enter any prolonged state of dormancy. Their survival strategy involves seeking out their primary prey, the ringed and bearded seals, which are most accessible when the sea ice is widespread and stable.
A common hunting technique is “still-hunting,” where the bear uses its powerful sense of smell to locate a seal’s breathing hole, or aglu, in the ice. The bear waits, sometimes for many hours, motionless next to the hole until a seal surfaces for air. This strategy, along with stalking seals resting on the ice, requires significant energy expenditure to secure the high-fat diet necessary to replenish reserves.
Physiological Survival Tools
The polar bear’s existence without true hibernation is made possible by physical adaptations that allow them to thrive in the Arctic environment. Their sheer size is a factor, as their large body mass provides a low surface area-to-volume ratio, which is highly effective at retaining heat, a concept known as Bergmann’s Rule. This massive size reduces the rate at which heat is lost to the frigid air.
The bear is also insulated by a thick layer of blubber, up to 10 centimeters (4 inches) thick, serving as both an energy store and a thermal barrier. Complementing this is their remarkable fur, which consists of a dense undercoat and long, transparent guard hairs. This double-layered coat traps air and prevents almost all heat loss, making the bears so efficient at insulation that they are more prone to overheating than to hypothermia when running.