How Do Polar Bears Spend Their Winters?

Polar bears are creatures of the sea ice, and so it would be reasonable to assume that they are at their happiest and most productive when the ice is at its most extensive: during the long Arctic winter. But that is far from the case: Even polar bears can struggle in the cold and the dark.

It isn’t easy being a polar bear. The Arctic is a harsh environment at the best of times, and its dominant carnivore species has had to adapt to a cycle of plenty and paucity. For some populations such as those in Hudson Bay, summer is the least productive season of all, as sea ice melts completely and the bears are forced to come ashore to wait until it freezes again. Throughout the polar bear’s range, the most bountiful period is spring, when their seal prey spends less time underwater and more time on the surface of the ice to give birth to and nurse pups. Winter lies somewhere in between: there is plenty of ice for them to traverse and search for prey, but not much prey to be found and no easy way to access any they do come across.

Difficult hunting season

Part of the challenge facing polar bears in winter, explains Dr. John Whiteman, our chief research scientist and assistant professor of biology at Old Dominion University, is that their favored environment is ice that is expansive but fractured, with plenty of open water leads through which ringed seals can emerge.

But in winter, Whiteman explains, “The ice is pretty tight. There aren't a lot of leads, there aren't a lot of cracks, the ice is reaching its maximum coverage. And so, there aren't many openings for seals, aside from their breathing holes.” And even if there were, he adds, there are fewer seals to hunt at or near the surface anyway.

“During the winter months, ringed seals are spending less time on the surface of the ice than they do at any other point in the year. In the spring, seals are giving birth on the sea ice. In early summer, they're going through the molt; to go through the molt, they have to bask on the sea ice surface to raise their skin temperature, and they're just not doing either of those things in the winter. The seals aren't bothering to come up to the surface except for air or an occasional rest, so they're hard to hunt.”

Added to those challenges is the very nature of the Arctic winter, a period dominated by cold, dark, and, particularly out on the frozen sea surface, vicious winds.

“Polar bears don't seem to rely all that much on sight,” explains Whiteman. “But they certainly rely a lot on hearing and on scent. And so, if you put all those conditions together it's just not an easy time to hunt. There are bears out there actively hunting, but they're not having a lot of success.”

There are a few places in the polar bear's range where food is more easily accessible year-round. For example, notes Whiteman, there have been observations of bears feeding in winter on the carcasses of bowhead whales that have been killed by Inupiat whalers in communities along Alaska’s North Slope. Similarly, some bears that stay on or near shore rather than follow the sea ice during winter – for example, on Svalbard in the Barents Sea or Wrangel Island in the Chukchi Sea off Russia – may have access to food sources, such as terrestrial mammals or birds, which, while not providing the nutritional benefit of big, fat seals, help sustain them during the long winter months.

Shelter denning

Whiteman emphasizes that, because the Arctic in winter is such a harsh environment, it is the period for which scientists have the least data on polar bears. However, there is enough data from radio-collared bears to show that there are periods in winter in much of the species’ range when bears just curl up and hunker down – a behavior known as “shelter denning.”

"The bears are active, but then the signals from the collars will stay in one spot for a matter of weeks, which indicates that the bears are not moving. There’s nobody on site to observe what they’re doing, because again it’s pitch black, but it would make a lot of sense that that is a period where the weather is particularly cold, or the winds are particularly strong or they’re in a region where there are just zero cracks to find the seals, and so they decide to wait the conditions out.”

Mothers with newborn cubs

While most bears are struggling to eke out a living in the cold, one group spends its winters well and truly sheltered from the elements but not eating at all. Pregnant females form maternity dens out of snow at the end of fall, and during winter they give birth to their tiny cubs — normally two of them, each weighing a little more than a pound. Exactly when in winter cubs are born is difficult to know with precision and almost certainly varies throughout the polar bear range, but data from radio collars worn by pregnant females suggests that January 1 is a rough average – although that may vary by about a month in either direction.

The mothers do not eat at all for the several months that they are in the den — and bears in Hudson Bay, where sea ice disappears entirely during the summer, have not eaten since coming ashore at the end of spring — and the energetic requirements of feeding their cubs with milk are significant. The likelihood of a successful pregnancy depends very much on how much food the mothers are able to consume before entering the den.

“It's a very fine line where the difference of eating an extra seal or one seal less prior to the winter could end up making all the difference,” says Whiteman.

Unfortunately, with climate change causing sea ice to break up earlier in summer and form later in fall, polar bears’ already tenuous existence is becoming even more perilous, as they have less time to gorge themselves and build up fat reserves before the harshness of winter.

"Ringed seals are much smaller than polar bears, so it doesn't necessarily sound like a whole lot, but on average a polar bear may catch and eat a seal per week or a seal every five days, something like that,” explains Whiteman. “So, if you miss a seal or two, you're talking a week or two without food. And that's a long chunk of time to suddenly get through without food.”

Sadly, if unsurprisingly, new research suggests that one consequence of that for pregnant females is that their milk may be of lower quality, which decreases the cubs’ likelihood of surviving the winter, and can put them at a disadvantage when they emerge into the open because they may be smaller and comparatively malnourished.

It all adds up to a challenging existence, one made even more difficult by human activities. For polar bears, like many of us, winter is a season to be endured in the hope that it will be succeeded by a rich and productive spring.