Most polar bears don’t hibernate. Winter is their main hunting season, and they spend it roaming the sea ice for seals. The exception is the pregnant female, who seals herself into a snow den for months to give birth. Whether even that counts as hibernation was argued for decades, because a bear’s body barely cools compared with a hibernating ground squirrel’s.1Wikipedia. Hibernation So the answer depends on which bear you mean and what you count as hibernating.
Why Winter Is Hunting Season for Most Polar Bears
Polar bears hunt seals from the sea ice, and the ice is most widespread in winter and spring. For a bear that isn’t pregnant, winter is a time to eat, not to sleep. Seals are thinly scattered, so female bears on the pack ice travel long, straight routes to find them.2PubMed Central. Females roam while males patrol: divergence in breeding season movements of pack-ice polar bears (Ursus maritimus) Across their time on the ice, polar bears spend roughly half their time drifting with the ice, about 3 in 10 hours sniffing for prey, and about a quarter searching closely around one area.3PubMed Central. Drivers of polar bear behavior and the possible effects of prey availability on foraging strategy
One classic method is waiting at a breathing hole. Seals keep holes open in the ice all winter so they can come up for air. A bear finds a hole by smell and waits beside it, sometimes for hours. When a seal surfaces, the bear grabs it and hauls it out onto the ice. This only works while the sea is frozen.
Seals are spread across a huge area, and a seal can surface at any of its many holes, so a bear that finds no sign of seals in one spot gains more by moving on than by staying. The ice also drifts, so a bear often has to travel just to stay in good hunting range.
Bears do sometimes hole up. Adult males, non-pregnant females and young bears often dig temporary dens in the snow.4CANADIAN WILDLIFE SERVICE REPORT SERIES. Denning habits of the polar bear (Ursus maritimus Phipps) They use them to ride out storms or to rest after hunting.5CANADIAN WILDLIFE SERVICE REPORT SERIES. Denning habits of the polar bear (Ursus maritimus Phipps) These range from small, shallow hollows to large rooms with long entrance tunnels.6CANADIAN WILDLIFE SERVICE REPORT SERIES. Denning habits of the polar bear (Ursus maritimus Phipps) A bear may stay a day or two, and occasionally up to 3 or 4 months, or up to about 6 months for some non-pregnant females.7CANADIAN WILDLIFE SERVICE REPORT SERIES. Denning habits of the polar bear (Ursus maritimus Phipps) Even so, this isn’t hibernating. The bear’s body keeps running normally, it can leave as soon as the weather clears, and there’s no long fast or slowdown.
Pregnant Females Spend Winter in a Maternity Den
A pregnant female digs a maternity den, a snow cave where she gives birth and raises tiny cubs out of the cold. She chooses a deep snowdrift, on land or on sea ice, and drifts on slopes near the coast are common spots. She digs a tunnel and one or more rooms. Snow seals the den and keeps it much warmer than the air outside.
Before she digs, she needs to be at her fattest, because everything she needs for pregnancy, birth and nursing must come from the fat she carries in. In places where bears wait out the summer on land, she comes ashore thin and has to find her way to a den area once the sea ice is gone. She starts digging in autumn as snow piles up, so timing depends on when enough has fallen. Most go in between mid-October and late November, though some enter as early as late August or as late as January.8PubMed Central. Denning Phenology Mediates Sea‐Ice Loss Impacts on Early Reproductive Success in Polar Bears Most come out between mid-February and early April.9PubMed Central. Denning Phenology Mediates Sea‐Ice Loss Impacts on Early Reproductive Success in Polar Bears
The cubs are born in midwinter, blind and tiny, usually one or two of them. They nurse on fatty milk the whole time they’re in the den, and the mother doesn’t leave to eat or drink. In Hudson Bay, pregnant females have already fasted about 4 months on land in summer before they den, then fast another 4 inside.10Journal of Zoology. Seasonal, sexual and anatomical variability in the adipose tissue of polar bears (Ursus maritimus) A new mother there can go about 8 months without eating.11Journal of Zoology. Seasonal, sexual and anatomical variability in the adipose tissue of polar bears (Ursus maritimus)
Leaving the den is gradual. Families with cubs linger near the den for about 8 days after coming out, more than four times longer than mothers without cubs.12PubMed Central. The post-emergence period for denning Polar Bears: phenology and influence on cub survival That gives the cubs time to adjust to the open air before the long walk to the sea ice. It also pays off: in one study, staying about 4 days longer raised the chance that a litter survived the spring from about half to about 9 in 10.13PubMed Central. The post-emergence period for denning Polar Bears: phenology and influence on cub survival
Does Denning Count as True Hibernation
A hibernating bear’s body cools only about 3 to 5 °C, while many small hibernators drop 32 °C or more.14Wikipedia. Hibernation Because of that, many scientists once called bear dormancy just a deep sleep.15Wikipedia. Hibernation Then studies of black bears in 2011 and brown bears in 2016 showed bears do truly hibernate.16Wikipedia. Hibernation
What changed is the definition. Hibernation is now judged by what the body does, not just by how cold it gets, and a long, deep drop in energy use is the key sign. Under the older, stricter view, the animal had to cool to near freezing, which left bears out. Under the newer view, a bear qualifies because it cuts its metabolism sharply for months, even though its body stays relatively warm. When people argue about whether bears “really” hibernate, they are often disagreeing about the definition, not about what the bears’ bodies do.
The numbers are striking. Hibernating black bears cut their energy use to a quarter of normal while their body stays around 30 to 36 °C.17PubMed. Hibernation in black bears: independence of metabolic suppression from body temperature Their heart slows from about 55 beats a minute to as few as 9.18PubMed. Hibernation in black bears: independence of metabolic suppression from body temperature They go 5 to 7 months without eating, drinking, urinating or defecating.19PubMed. Hibernation in black bears: independence of metabolic suppression from body temperature After leaving the den, their bodies stay slowed down for up to 3 weeks.20PubMed. Hibernation in black bears: independence of metabolic suppression from body temperature Denning polar bears burn energy at the same rate for their size as hibernating brown and black bears.21Journal of Mammalogy. Hibernation and seasonal fasting in bears: the energetic costs and consequences for polar bears So a bear can save a great deal of energy without cooling down much.
Staying warm has a practical side. A ground squirrel has to rewarm for hours before it can act, but a bear can rouse fast. A disturbed denning bear can defend itself and its cubs, which is one reason never to approach a den.
How a Denning Mother Survives Months Without Food
Hibernating bears live almost entirely on fat stored before denning.22PubMed Central. Body Protein Sparing in Hibernators: A Source for Biomedical Innovation They lose about 15 to 25% of their body weight over the winter, but black and brown bears keep nearly all their muscle.23PubMed Central. Body Protein Sparing in Hibernators: A Source for Biomedical Innovation Most animals that fast this long start breaking down muscle for fuel. Polar bears burn mostly fat and conserve protein whether they’re fasting or well fed.24University of Saskatchewan. Biochemical and Physiological Aspects of Obesity, High Fat Diet, and Prolonged Fasting in Free-Ranging Polar Bears A bear also recycles waste from fat breakdown instead of discarding it, which helps her avoid burning muscle. That’s why a mother can come out thin yet strong enough to walk a long way to the sea ice and lead her cubs. Still, mothers leave the den with much less fat than they had in autumn.25Journal of Zoology. Seasonal, sexual and anatomical variability in the adipose tissue of polar bears (Ursus maritimus)
Her bones are protected too. Before denning, pregnant polar bears build up bone, which seems to guard against bone loss during the fast.26PubMed. Polar bears (Ursus maritimus), the most evolutionary advanced hibernators, avoid significant bone loss during hibernation Her milk is the part that gives. As the fast grows longer, it carries less energy and may stop altogether.27USGS. Lactation performance in polar bears is associated with fasting time and energetic state The drop in milk energy is about 2.6 times steeper for mothers nursing twins.28USGS. Lactation performance in polar bears is associated with fasting time and energetic state Twins drain her reserves faster than a single cub, and her summer and autumn condition shapes how the cubs grow. Thinner milk saves the mother’s reserves but slows her cubs’ growth.29USGS. Lactation performance in polar bears is associated with fasting time and energetic state
When the family emerges, the cubs weigh about 10 kg by early April and have thick fur but almost no fat. Fur and rich milk keep them warm down to about −30 °C, but icy water chills them dangerously, so swimming is risky early on.
Walking Hibernation and Polar Bears in Summer
Scientists once suggested that bears stuck on shore enter a hibernation-like state while awake.30PubMed. Summer declines in activity and body temperature offer polar bears limited energy savings Bears waiting on land for months look sluggish and lie still for long hours. Some research on body chemistry even hinted that polar bears might be able to hibernate at any time of year.31PubMed. Is the polar bear (Ursus maritimus) a hibernator? Continued studies on opioids and hibernation.
Later measurements tested the idea directly, and it didn’t hold up. Polar bears can’t switch into a special energy-saving state, and their summer drop in activity and body temperature saves only a little energy.32Polar Bears International. Walking Hibernation Not a Factor for Polar Bears
On land in summer, daily energy use differed up to 5-fold from one bear to another, and activity up to 19-fold.33PubMed Central. Polar bear energetic and behavioral strategies on land with implications for surviving the ice-free period Some bears slowed to the energy use of a hibernating bear by resting a lot.34PubMed Central. Polar bear energetic and behavioral strategies on land with implications for surviving the ice-free period So some bears save energy by lying low, but that’s a behavior, not hibernation.
On the Hudson Bay shore, bears spend about 4 months ashore with little to eat.35Journal of Zoology. Seasonal, sexual and anatomical variability in the adipose tissue of polar bears (Ursus maritimus) Many rest for long stretches in cool spots, while others roam, scavenge or nibble at vegetation. They live mainly off fat built up on the ice. Unlike a denning mother, they keep a normal heart rate and metabolism, keep drinking and moving around, and lose weight according to how active each bear is. The fast ends when the sea refreezes in late autumn.
Spring Feasting After the Den
In spring, mothers lead their cubs onto the ice for their first meal in about 8 months.36Polar Bears International. Polar Bear Tracker Update: Spring 2026 This is the feasting season, when bears eat heavily.37Polar Bears International. Polar Bear Tracker Update: Spring 2026 Young seal pups are easy, fat-rich prey.38Polar Bears International. Polar Bear Tracker Update: Spring 2026 Bears pack on fat in spring to last through the leaner months ahead.39PubMed Central. Age and Sex Composition of Seals Killed by Polar Bears in the Eastern Beaufort Sea
How Climate Change Is Disrupting Denning
In the European Arctic, pregnant bears can reach only about a third of the denning areas they could in the 1980s.40Polar Biology. Shifting polar bear Ursus maritimus denning habitat availability in the European Arctic The sea ice no longer forms in time for them to reach those areas before denning.41Polar Biology. Shifting polar bear Ursus maritimus denning habitat availability in the European Arctic By the 2090s, all of those areas are projected to be out of reach.42Polar Biology. Shifting polar bear Ursus maritimus denning habitat availability in the European Arctic Not every region is affected the same way: on Wrangel Island, denning habitat hasn’t shrunk in 41 years and isn’t yet a limiting factor.43Ecological Modelling. Assessing past and future climatic influences on the availability of polar bear maternal denning habitat on Wrangel Island
For a pregnant female, a late freeze-up can strand her far from a good den site. A longer fast on land leaves her thinner before denning, and thinner mothers make less milk, so fewer cubs may survive to their first summer.
Less sea ice also keeps bears on land longer with less to eat.44Arctic Science. Polar Bear-human conflicts: state of knowledge and research needs Conflicts between bears and people are expected to rise,45Arctic Science. Polar Bear-human conflicts: state of knowledge and research needs since hungry bears are drawn to garbage dumps and stored food.46Arctic Science. Polar Bear-human conflicts: state of knowledge and research needs In polar bear country, store food and garbage where bears can’t reach them, go with a local guide or follow town bear alerts, and never approach a bear or a den.