Why Don’t Polar Bears Hibernate?

Hibernation is a biological process in which animals enter a long-term state of minimal movement and reduced metabolism. It’s much more than a deep sleep; their body temperature plummets while their heart rate and breathing slow down to a barely-there beat. 

Many small mammals – including chipmunks, dormice, hamsters, hedgehogs, and bats – enter this state of dormancy every winter as a way to stay safe and conserve energy when food and water are scarce in the harsh, colder months. 

Do other bears hibernate?

You may have been told in school that all bears hibernate, but the science is a little more complicated. Brown bears and black bears do enter a winter-long dormancy, yet their body temperature barely dips compared to true hibernators. For this reason, scientists prefer to call it torpor, a kind of “light hibernation” marked by a slower metabolism but not the deep, body-chilling sleep of smaller mammals.

With polar bears, it’s even hazier. The species is not considered a true hibernator and they often stay active throughout the year, although there are notable examples when they do succumb to periods of inactivity. For instance, pregnant females are known to stay in a den for months on end, generally between October to March.

“It is important to recognize that winter hibernation represents a suite of physiological and behavioral changes, many of which can be defined on a spectrum rather than a yes/no categorization. Polar bears seem to be a bit better at coping with extended food deprivation – weeks at a time, even during the productive spring hunting season – than other bear species,” Dr John Whiteman, Chief Research Scientist at Polar Bears International, told IFLScience.

“And even ‘non-hibernating’ polar bears may curl up in a snow den for weeks, perhaps months, during winter: at high latitudes, some non-pregnant female bears have had radio locations become static for up to several months during winter before they start moving again,” he continued.

One study suggested that polar bears in the summer months may enter a state of “walking hibernation”, whereby their metabolism slows down and they remain somewhat physically active. However, another multi-year research project later dispelled this idea. While they did become less active in the summer months, the researchers found it was “more akin to normal mammalian fasting levels” rather than a hibernation-like state.

Polar bears in the winter

For this Arctic-dwelling species, winter is a challenging time that also offers new opportunities. The presence of sea ice provides polar bears with crucial hunting grounds, serving as a solid platform from which they ambush seals in the water. Things aren’t easy – it’s cold, dark, and windy – but seal hunting is possible.

So, unlike some other species, polar bears are able to find just enough food throughout the winter and keep the engine running. 

There doesn’t seem to be enough natural selection pressure for them to completely ‘opt out’ of either summer or winter.

Dr John Whiteman

“Winter hibernation in bears is primarily a response to a lack of food during challenging environmental conditions. So, the fact that polar bears (other than pregnant females) have evolved to remain active during winter implies that food is not quite lacking enough, and the environment is not quite challenging enough, to create natural selection towards avoiding winter altogether by hibernating,” explained Whiteman.

Spring is prime feeding time for polar bears, with enough sea ice left to hunt and the landscape newly stocked with seal pups. As the weather warms, the ice begins to melt and seals spend more time in the water; hunting becomes more of a chore. By the start of June, polar bears have started to wrap up their biggest feasting period and those fat reserves will start to slowly burn away over the summer months ahead.

With a lack of sea ice, many polar bears are forced further inland where food is scant and less nutritious. Their usual seal-rich diet is shifted towards a meager mix of berries, bird eggs, and the occasional carcass. Increasingly desperate, some even wander into human settlements, overturning trash bins in search of food scraps.

Even so, polar bears manage to scrape by through the summer months, clinging to survival until the ice returns.

“There doesn’t seem to be enough natural selection pressure for them to completely ‘opt out’ of either summer or winter,” Whiteman continued. 

Climate change and the future of polar bears

Climate change is making survival even more precarious for polar bears. Longer summers and extended ice-free periods mean their crucial hunting season is shrinking, while the availability and distribution of prey are shifting. Despite these changes, scientists don’t expect the bears’ underlying biology, which has been fine-tuned over hundreds of thousands of years, to suddenly adapt to the new reality.

“Polar bears have been around for roughly half a million years, and sea ice has waxed and waned during that period – and the distribution of polar bears has waxed and waned with it. So, as the southern margins of sea ice begin to have such long summers that polar bears can’t survive, I expect that some bears will simply continue to behave as they always have, and they will die,” said Whiteman.

“Polar bears can’t know the future – they don’t know that we have changed the atmosphere and that these longer summers are not temporary but rather a directional trend, and the behaviors and strategies that they have evolved no longer apply,” he concluded.

We did say that polar bears are the “undisputed apex predator of the Arctic.” It’s certainly true they are hypercarnivorous apex predators, but they’re not always at the top of the food chain. The bones of polar bears have been found in the gastrointestinal tract of a Greenland shark, a strange and elusive species that can live in the icy waters of the North Atlantic and Arctic Oceans for centuries upon centuries. The polar bear was likely to be young and scavenged by the shark, yet it just goes to show that even the mightiest hunters of the Arctic are not entirely untouchable.