By late December, ice covers much of Hudson Bay. Bears, young and old, have returned to the ice, but their work has just begun. While seals are available, the ice is thick and hunting is difficult.
In order to survive these extreme climates polar bears have developed complex physical adaptations. By late October, the bears’ undercoat gets much thicker, so thick that it gets very hard to find their skin through it. Researchers estimate that a bear’s coat consists of almost ten thousand hairs per square inch by midwinter.
Obviously, their cold resistance is considerable. This is not to say, however, that polar bears are completely resistant to the arctic winter. Strong winds and cold ambient air temperatures still affect them.
The guard hairs keep a warm layer of air next to the skin and strong winds penetrate these hairs, undermining their insulative value. For this reason, bears often wait out the winter storms, taking shelter on the lee side of snow banks and ice ridges. Sometimes, bears will stay motionless for days, only to rise and shake away a layer of snow as the storm breaks.
As well, water affects their ability to maintain heat. This affect is very noticeable during Churchill’s bear season. Polar bears go to some incredible efforts to avoid getting wet! They will divert their path to avoid the many tundra ponds near Churchill. In fact, the bulk of visiting polar bears do not arrive in the Churchill area until many of the inland ponds have a fresh layer of ice.
However, the anticipation of winter is strong and testing ice is a common pass time during bear season, sometimes ending with the groan and crack of breaking ice and a very wet bear. On a warm day this is welcome, on cold days it is less than comfortable.
Absorbing heat twenty five times faster than air, water can soak to the bear’s skin and cools it quickly. To a large male with four inches of fat, this may not matter but to a younger, thinner bear, it could be fatal. To avoid this, their guard hairs are oily and shed water quickly. Emerging from the ocean or a partially frozen tundra pond, bears quickly shake excess water from their coat.
In a further adaptation to life on the ice, polar bears are almost completely furred, even much of their paws are covered. Their foot pads are also covered with little bumps. These bumps, or papillae, provide extra traction while traversing ice ridges on the rugged sea ice. Finally, their claws, short and strong, are also sharply curved; providing even greater ability to navigate the frozen ocean.
As with all arctic animals, their appendages, mainly their ears and tail, are smaller than other bears. This follows the idea that less surface area results in less heat loss. Polar bears emanate almost no heat, only their black noses and a wisp of breath showing up on infrared or heat sensitive cameras.
Beneath the ice – algae blooms in the cracks and crevices under the ice in late winter – as soon as the sun returns. Amphipods live on this – they are also called sea lice.
Ultimately, the extended period of algal growth is the foundation on which polar bears depend for their survival.
Not only female bears dig dens. Many male bears, particularly the older ones, also burrow out shelters under the snow, in which they sleep for most of the dark period of winter. Dens require a good snow roof over the top, and a layer of snow on top of the earth underneath, so they can only be 1ocated in areas with lots of snow. Most bears place their dens on hillsides in the lee of the wind, or in other places where deep snow accumulates. Because suitable areas are not common, bears return year after year to the same locations for the winter. Some bears enter their dens at the beginning of the dark period and stay hidden under the snow, almost hibernating, until the light returns. Others, particularly younger bears, stay active for most of the winter, hunting seals at the floe edge and only sleeping in a den for a short period of time. In this case, their temporary den, which has an opening through which they can go in and out, is called a tisi. The den of a sleeping bear which remains hidden for several months is called an apitiq, and has no exit hole until the bear wakes up in early spring and digs out through the snow (Randa 1994). Bears with dens are called apitiliit, “those that have snow to cover them”.
Despite their thick layer of fat and woolly undercoat, polar bears are still susceptible to heat loss from strong winds and plummeting temperatures. Many bears will simply ‘hunker down’ and conserve energy through the coldest months.
Polar bears can enter in and out of their ‘walking hibernation’ at any time of the year. After seven to ten days without food their metabolic rate (heart beat, etc) will slow down allowing them to withstand prolonged periods of fasting.
Essentially, it allows polar bears to survive periods of food deprivation that would be fatal to other mammals.