The Ice Bear: Life at the Top of the World

Polar bears are not subtle animals. The largest land carnivores on Earth, adult males can weigh over 1,500 pounds and stretch more than ten feet from nose to tail. They prowl the sea ice of the Arctic, scanning the horizon with small, dark eyes set into a head that seems almost too narrow for a creature so massive. Everything about them is built for a world most other animals could not survive for a day.

Their fur is a good place to start, because it is not actually white. Each individual hair shaft is transparent and hollow, which scatters light in a way that makes the coat appear white or pale yellow against snow and ice. Underneath all that fur, the skin is jet black, which helps absorb heat from the sun. A thick layer of fat, sometimes four inches deep, sits beneath the skin. In temperatures that regularly drop to minus 30 degrees Fahrenheit, none of this is excessive.

Polar bears are classified as marine mammals, which surprises a lot of people. The designation makes sense when you consider how much of their lives are spent at sea. They are powerful swimmers, capable of covering dozens of miles in open water using their large, slightly webbed front paws as paddles. Their scientific name, Ursus maritimus, translates to "maritime bear," and it fits. Researchers have tracked individual bears swimming continuously for more than 400 miles, though this kind of extreme travel comes at a real cost to their fat reserves.

The hunting strategy polar bears are most famous for is called still hunting. A bear will locate a seal's breathing hole in the ice, then stand or crouch motionlessly beside it for hours, sometimes with one paw raised, waiting. When the seal surfaces, the bear strikes with its forepaw and hauls the animal out of the water with remarkable speed. Ringed seals are their primary prey, though they will also take bearded seals, walruses, beluga whales, and the occasional bird or egg when larger prey is unavailable. During the summer months, when sea ice retreats, bears on land will eat berries, kelp, and whatever else they can find, but these foods provide poor nutrition compared to the calorie-dense blubber of a seal.

Female polar bears are dedicated mothers. After mating in spring, a female spends the summer building up enough fat to sustain herself through a pregnancy and the first months of nursing. She digs a maternity den in a snowdrift, usually in November, and gives birth to one to three cubs in December or January. The cubs are born blind, covered in thin fur, and weigh less than two pounds. By the time the family emerges in spring, the cubs have grown substantially on their mother's rich milk, and she has gone without food for up to eight months.

Today, polar bears are listed as vulnerable on the IUCN Red List. Scientists estimate the global population at somewhere between 20,000 and 31,000 individuals. Sea ice is forming later in the year and breaking up earlier, which shortens the hunting season and forces bears to go longer between meals. It is a slow pressure, and researchers are watching closely to understand what it means for a species so precisely adapted to conditions that are quietly shifting beneath its feet.