Marvels of Adaptation

Perhaps it was because nature had equipped polar bears so uncannily that their Inuit hunters credited them with supernatural powers. How other than magic could Nanook, as they called the bear, so easily survive the fierce polar elements, including temperatures of minus 50°F (minus 46°C)? One way, scientists now know, is the creature's natural clothing, its fur and skin. The polar bear's fur is thicker than that of any other bear, consisting of guard hairs of varied lengths and a dense undercoat. Bears also are protected from cold by a thick layer of fat. Look at the white bear's nose, and you will see the color of its skin: black, a pigment designed for maximum heat absorption. As for that white coat, well, it's not really white at all. Each of the individual hairs is translucent; the polar bear's fur appears white only because it reflects light. This quality led some to speculate that the hollow hairs absorbed ultraviolet light and funneled it to the polar bear's skin, helping to keep the bear warm. This theory, however, has been disproved.

Nanook is a masterful swimmer, able to paddle for an incredible 60 miles (97 kilometers) without rest, according to some observations. Using its forepaws as flippers and its rear feet as rudders, it can swim more than twice the speed that it normally walks. Eyes wide open, it apparently boasts acute vision underwater, where it easily spots much of its prey—sometimes looking up from the depths at a seal resting on a shelf of ice.

Binging Carnivores

The most carnivorous member of its family, polar bears subsist almost entirely on seals and other sea mammals, such as walruses and the occasional beached whale. As with many other predators—though even more so—polar bears binge on large kills whenever and wherever they can. Smaller prey, like crabs, arctic foxes, and birds, are basically hors d'oeuvres that do little to fill the bear's enormous stomach, which can hold up to 150 pounds (68 kilograms) of seal meat from a single sitting. It would take killing a seal every five or six days for an adult bear to maintain its weight. But polar bears exhibit a remarkable metabolism, which allows them to live off accumulated body fat for amounts of time that would spell death for most species. After a winter of binging on seals and reaching their top weight—typically 1,000 pounds (454 kilograms) for a male and 450 pounds (204 kilograms) for a female—landed bears can go months without a decent meal, only occasionally exercising their gullets on kelp and carrion. During this time, they may lose half of their peak body weight.

Momma Bear Has Landed

Though ice is the life for adult polar bears, land is where the family usually begins. After having reached maturity in five or six years, most pregnant females swim ashore to dig dens in snowdrifts. Some, however, remain on the pack ice and dig dens there. They give birth to litters of one to three cubs in December or January. Weighing little more than one pound (0.5 kilograms) at birth, the diminutive cubs experience explosive growth in the next three months, thanks to a diet of fat-rich mother's milk. By the time they leave their snow den with their mother in March or April, they weigh 25 to 33 pounds (11 to 15 kilograms). Out on the ice the following fall, they are slowly weaned off mother's milk and taught to hunt. Then, following a second summer on land, they will normally remain with their mother for a second season on the ice, after which they begin to fend for themselves.

While lactating, females are never in heat, and male bears ignore them. Otherwise, during the mating season (usually March until May) they are objects of fierce competition among courting males, now massive in size after a season of hunting seals. As their scarred faces frequently testify, blood is often let in these courtship battles. But mating does not guarantee birth. If the fertilized female fails to find sufficient food in the months that follow, her embryos will be reabsorbed into her bloodstream, and she will be back to the beginning in the heat-and-courtship cycle. While female bears in the Churchill region have historically produced a litter every two or three years, those in some of the more inhospitable regions of the Arctic may give birth only a couple of times in their lives. Birth rates in the Churchill area have been dropping, however, possibly due to a shorter hunting season and a corresponding weight loss in the bears.