Do Polar Bears Hibernate? The Answer May Surprise You

Habitat and History

Polar bears (Ursus maritimus) have a circumpolar distribution limited to four distinct sea ice habitats—or ecoregions—of the northern hemisphere. Sea ice is vast and transient, thus polar bears have aggregated into what scientists within the IUCN Polar Bear Specialist Group (PBSG) have identified as 19 subpopulations across the Arctic.

In central and eastern Canada, much of the polar bear habitat lies within the Seasonal Ice Ecoregion, where the sea ice melts entirely in the summer. The Divergent Ice Ecoregion extends around the Arctic from coastal Alaska to Svalbard and is characterized by ice that diverges from the shoreline as it forms and is pushed by ocean currents. As the weather warms, the gap between land and the polar ice pack expands. In the Convergent Ice Ecoregion, locally formed sea ice, along with ice transported from the Divergent Ice Ecoregion, collects along the shore or moves parallel to shorelines. The Archipelago Ecoregion, in addition to the northernmost portions of the Convergent Ice Ecoregion, is likely to provide a final refuge for polar bears as the ocean channels separating the islands of the far north Canadian Arctic have historically been covered by sea ice year-round.

While the polar bear’s Latin name means ‘sea bear’, this mammal’s venerable place in northern culture is reflected in the many titles it’s been given. In Eastern Greenland, Tornassuk means ‘the master of helping spirits’. In Norway & Denmark, Isbjorn means ‘the ice bear’ and in medieval Scandinavia, Norse poets made the following literary references: White Sea Deer; The Seal’s Dread; The Rider of Icebergs; The Whale’s Bane; The Sailor of the Floe. The Sami (or Lapp) Indigenous peoples from northern Europe say God’s Dog and The Old Man in the Fur Cloak in fear of offending the majestic creature. The Ket (Siberian tribe) uses the term Gyp, or ‘grandfather’, which is considered a sign of respect and awe. Nanuk is used by the Inuit to signify ‘Animal Worthy of Great Respect’. Pihoqahiakis—also used by the Inuit—means ‘The Ever-Wandering One’.

Perhaps the polar bear’s earliest denomination, however, originates with Ursus arctos, more commonly known as the brown bear. Brown bears and polar bears diverged between 150-500,000 years ago. Despite this relatively recent split, the polar bear has rapidly evolved unique morphological, physiological and behavioral traits to adapt to the Arctic’s climate and ecology. In 2006, genetic testing confirmed the existence of polar bear-grizzly bear hybrids, also known as ‘grolar bears’ or ‘pizzly bears’. Wild hybrids retain physical features from both species, but often adopt polar bear behavior as a result of being birthed and raised by polar bear mothers.

This genetic history underpins the polar bear’s unique adaptations, such as lack of fur pigmentation, which aids in camouflage. Perhaps the most fascinating characteristic, however, is the polar bear’s sleep patterns—especially in juxtaposition to the other seven species of Ursus.

Hibernating vs. Denning

Throughout most of the world, bears hibernate during the winter months or perform annual denning behaviors in response to seasonal food shortages and cold weather. Hibernating helps the process of thermoregulation in the American black bear (Ursus americanus) and North American brown—or its subspecies grizzly bear (U. arctos horribilis)—in particular. The duration of denning is contingent on latitude and pregnancy status, in addition to more unpredictable environmental factors like climate change. For instance, denning varies from a few days or weeks in Mexico to six months or more in Alaska. Pregnant females generally den earlier and longer than other bears, and in Greater Yellowstone, grizzly bear females without cubs den on average for about five months.

The body temperature of a hibernating bear remains within 12°F of its natural body temperature, allowing it to quickly react to impending threats. Bears sometimes awaken and leave their dens during the winter, but they generally do not eat, drink, defecate or urinate during hibernation. Respiration, normally 6-10 breaths per minute, decreases to 1 breath every 45 seconds and their heart rate drops from 40-50 beats per minute to 8-19 beats per minute. Their well-insulated pelts and lower surface area-to-mass ratio enable bears to reduce their metabolic rate by 50–60%. The urea produced from fat metabolism is broken down and the resulting nitrogen builds protein that maintains muscle mass and tissue reserves.

Unlike brown and black bears, polar bears do not hibernate…not in the traditional sense, anyway. Polar bears contend with extreme environmental conditions and face laborious tasks that result in high energetic demands. In the winter, males and non-pregnant females must maintain a constant body temperature to combat external temperatures that regularly fall as low as −50 °F. These regulation challenges are exacerbated by the Arctic’s biting winds, which can contribute to losses of greater than 75% of the metabolic heat the bears produce.

A polar bear study led by evolutionary biologist, Charlotte Lindqvist, revealed:

“Nitric oxide (NO) can directly regulate cellular respiration, oxygen consumption, and energy production. Also, by decreasing energy production while resting, polar bears may minimize oxygen consumption and therefore decrease heat lost through respiration in subfreezing temperatures. Adaptive, or nonshivering, thermogenesis is a regulated production of heat in response to environmental temperature or feeding and can protect against cold exposure or regulate energy balance after changes in diet.”

Recent work suggests that hibernating brown and black bears have similar energy expenditures to hibernating pregnant female polar bears; however, fasting polar bears that remain active have much higher energy demands. A pregnant female digs a maternity den—usually into a snowbank—in October or November and gives birth to one to three cubs in December or January. Inside the den, temperatures are approximately 40 degrees warmer. In addition to 2 layers of fur, a polar bear’s body fat can measure up to 4.49 inches. The mother relies on her fat storage to go without food, water, defecation and urination for as long as eight months. During this time, her body recycles wastes biochemically (without kidneys) to prevent dehydration and she loses as much as 30% of body mass, which translates to 43% of body weight.

Pregnant or lactating females and their dependent offspring are most vulnerable to climate change. The melting of arctic sea ice increases the length of summer-fall fasting—a period when polar bears lose access to their primary source of food—seals. Usually, a female gives birth to a litter every two to four years, but due to decreasing ice floe quantity and quality, some mothers may only give birth to one or two litters during their lifetime. Though the loss of habitat is the biggest threat to the survival of polar bears, other concerns include lethal response to human-polar bear conflict, toxic pollution in the environment and direct impacts from industrial development, such as disturbance of maternal dens or contact with an oil spill.

Wonder what it would be like to see the ‘The Ever-Wandering-One’?

The Arctic and subarctic Canada are home to two-thirds of the world’s polar bears and Churchill is considered to be “The Polar Bear Capital of the World.” By mid-October, between 600 and 1,000 bears mass along a 100-mile stretch of coast between the Churchill and Nelson rivers. This spectacular gathering is the largest concentration of polar bears on the planet!

Join World Wildlife Fund and Natural Habitat Adventures on our next trip to Churchill to see these majestic terrestrial-marine mammals. We offer special departures and itineraries, including Women-Only, Classic Photo and a trip focused on Climate Change.

Our Polar Bear Expedition Leaders receive additional training and resources from WWF’s top scientists, ensuring the best interpretive experience available. Before you go, check out our wildlife guide to inspire your viewing experience while on the tundra. Keep an eye out for defining characteristics like the polar bear’s black skin, blue tongue and three eyelids, which protect the bear’s eyes from the elements. You’ll have ample opportunity to take up-close photos from our specialized Polar Rovers.