Thousands of miles north of the Svalbard archipelago, a male polar bear lopes across the wind-swept sea ice. From afar he appears to be nothing but an undulating white mass. And that is precisely his purpose: despite the outward appearance, a polar bear is not white. In fact, every individual guard hair is transparent and hollow; it is this scattering of light that makes him appear so brilliantly white. His actual skin, however, is black, to maximize what little radiation from the sun he can possibly intercept.

Biologists recognize that polar bear as Ursus maritimus — the sea bear — which makes it the world's largest land-based carnivore on Earth. Males grow to between 300 and 800 kilograms and measure 200 to 250 centimeters from nose to rump, while the females are about half their size, 150 to 300 kg. Polar bears are distinctly dimorphic: males have broader skulls and heavier shoulders, and compared to brown bears they are long-skulled, slender-necked and sleek-bodied. This body shape is particularly helpful when swimming and when reaching into a seal hole to drag a seal pup onto the surface for consumption.

The fact that the polar bear is truly a marine mammal is readily apparent. It has up to 11 cm of subcutaneous blubber and a thick, dense coat of fur with two types of hairs: guard hairs and underfur. The paws are very large (up to 30 centimeters or one foot in diameter) with thick fur on the underside and claws that are not as sharp and hooked as other species of bear. The claws are better suited to gripping slippery seal skin than digging them into the ground. The polar bear belongs to the superorder Pinnipedia, the order of marine mammals that includes seals, sea lions and walruses.

It lives in the Arctic Ocean and its surrounding seas in areas covered with pack ice, which occurs in all five Arctic nations: Russia, the United States, Canada, Greenland (a territory of Denmark) and Norway. It is known in 19 different subpopulations, which migrate with the seasonal advance or retreat of the ice. Some bears stay year-round close to the shore on the fast ice. Others are associated with the pack ice and travel hundreds or even thousands of kilometers with the drifting pack ice, sometimes as far as the Beaufort Sea to Siberia. During the summer months, when there is no pack ice, many subpopulations are forced to move on land and wait for the winter to return.

Seals are the polar bear's main source of food during most months of the year. Ringed seals and bearded seals are by far the most commonly consumed. Bears catch most of their food by still-hunting. The bear seeks out a breathing hole on the sea ice (either the seal's own hole or an old abandoned hole), crouches next to the hole on the downwind side and waits for the seal to return. If a seal approaches the breathing hole, the bear usually waits about an hour to be certain that it is the seal that made the hole, and that the hole was not made by some other animal. When a seal approaches and raises its head out of the water to breathe the bear strikes, dragging the seal out of the water and onto the ice where it kills it. The bear usually eats the fat (blubber) first, leaving the protein-rich flesh for scavengers like the arctic fox and glaucous gull. Other prey include walrus, and the occasional beluga, bird or bird egg, although these do not provide the same high-fat content. Females rely on the fat-rich blubber for the energy to carry a pregnancy and produce milk.

Breeding occurs from April to May, with fertilization being delayed and not taking place until the autumn. Between October and November, pregnant females dig maternity dens into the snow (on the sea ice or on land, depending on where they are), which they use to shelter the cubs during the winter. From one to usually two blind cubs are born in December or January. Each of them weighs less than 1 kilogram at birth. The mother does not eat or drink during this time, but instead draws upon her fat stores to feed them with her milk. Between March and April, the mother, accompanied by the young, leaves the den, and they live together for approximately two and a half years while the mother teaches the cubs how to hunt.

Because it is so difficult to survey polar bear populations throughout their vast home range, exact numbers are not available. According to the IUCN the polar bear is listed as vulnerable globally, with a current population estimate of about 22,000 to 31,000 individuals. However, populations vary widely. As sea-ice season length decreases in certain locations, the Southern Beaufort Sea population (located off the coasts of Alaska and western Canada) continues to decrease. As the climate warms, sea ice will continue to disappear. This reduces the amount of time that polar bears have to hunt on land, forcing them to come closer to humans. With fewer places to hunt, polar bears may become more malnourished, their young may become more malnourished as a result. The United States lists the polar bear as threatened, under the Endangered Species Act. In addition, there is the pollution and industrial threats.

Wild bears have an average life expectancy of between 15 and 18 years, although females often survive into their late twenties. In captivity they commonly live longer than 30 years. They are excellent swimmers and can swim continuously for days and have been tracked as far as 100 kilometers from land without rest. Polar bears can sense a seal nearly 1 kilometer away due to its excellent sense of smell. They are the nanook to the Inuit — an animal that they respect and approach with caution — and feature frequently in oral tradition, local culture and place names.

This male polar bear stops. He tests the air, and then lowers himself next to a dark hole in the ice. He sits still, waiting. This is something the bear, the seal, and the ice have been doing for at least 120,000 years. The only thing that has changed is the length of time the pack ice has lasted.