Polar bears do not truly hibernate in the summer. While they must survive extended periods without food when the sea ice melts, the resulting metabolic slowdown is not true hibernation. They instead enter a state of deep fasting and lethargy. This adaptation allows them to conserve energy during the ice-free season when their primary food source, seals, is inaccessible. Only pregnant females enter a deeper, more prolonged state of denning that shares some characteristics with hibernation.
Polar Bears and the Definition of True Hibernation
The concept of true hibernation involves a dramatic physiological shift, distinct from regular sleep. True hibernators, such as ground squirrels, achieve a state of regulated hypothermia where their body temperature plummets to near ambient levels. Their metabolic rate can drop to as little as two percent of their normal summer activity, and their heart rate slows drastically, making them very difficult to wake. This deep, regulated hypothermia allows them to conserve massive amounts of energy over the winter months.
Polar bears are not classified as true hibernators because they do not undergo this profound physiological change. Their core body temperature remains relatively high, dropping only a few degrees, unlike the deep drop seen in smaller mammals. This slight reduction allows them to be easily aroused, which is a fundamental difference from deep torpor. Other species of bears, for example, typically reduce their body temperature from around 100°F to approximately 88°F during their winter dormancy.
Fasting and Torpor During the Ice-Free Season
The appearance of summer hibernation is actually a forced, prolonged fast induced by environmental changes. As the Arctic sea ice breaks up, polar bears are forced ashore because they cannot effectively hunt seals without the ice platform. This annual retreat triggers a state of fasting-induced torpor or lethargy in male and non-pregnant female bears. This response is a direct survival adaptation to seasonal food scarcity, relying entirely on fat reserves accumulated during the previous hunting season.
During this period, the bears conserve energy by depressing their metabolic rate. The inferred rate of an inactive bear on land is comparable to a basal mammalian rate. They are able to sustain this fast for many months without eating, drinking, or excreting waste. This is achieved through a physiological process where the body recycles nitrogen—a byproduct of fat metabolism—to rebuild protein, preventing muscle atrophy and maintaining organ tissue.
The bear’s primary goal during summer fasting is to minimize energy expenditure while waiting for the ice to return. The length of the ice-free season is increasing, pushing the limits of the bear’s fasting capacity. Prolonged fasts of 180 days are now predicted to cause significant mortality, particularly in subadults, underscoring the severity of this survival strategy.
The Unique State of Maternity Denning
Maternity denning is the one instance where a polar bear’s dormancy closely resembles true hibernation. Pregnant females enter a snow den in the late fall, typically between October and November, and remain there until emerging in the spring. This compulsory fast can last for over eight months, during which the female does not eat, drink, or pass waste while giving birth to and nursing her cubs.
The den serves as a safe environment for the altricial cubs to grow on their mother’s highly concentrated milk, which is about 31 percent fat. This reproductive fast requires a deeper metabolic suppression than summer torpor, allowing the mother to conserve energy for survival and lactation. However, the female is still easily aroused compared to small mammalian hibernators, which is necessary to care for and defend her newborn offspring. After emerging, the mother and cubs may remain near the den site for up to a month before traveling onto the sea ice to hunt.