Roaming the frozen North in search of food, polar bears are the top predators of the Arctic ice. These hunters sometimes go ashore, but their survival depends on the marine ecosystem. In fact, their scientific name, Ursus maritimus, is Latin for "sea bear." Polar bears migrate hundreds of kilometers to stay on pack ice, which extends south in the winter and then retreats north as it melts in summer. Without ice, the bears have no way to stalk and capture seals—their staple food—as depicted in this Bering Sea scene. When food is scarce, polar bears wait out lean times by living off stored body fat.

CONSERVATION: The Big Thaw
Polar bears depend on shifting pack ice for access to seals—their staple food. But global warming trends are melting the ice earlier in the summer and freezing it later in winter. The result: a shorter seal-hunting season threatens the survival of polar bears in some regions, such as Hudson Bay.

Bears that spend the summer on land may not have enough stored fat to keep them alive until the sea refreezes. Although starving bears will hunt whatever is available—walruses, beluga whales and, on rare occasion, caribou—such attempts are seldom successful.

Polar bears can swim up to 100 kilometers (60 miles) across open-water gaps between ice floes, and they dive underwater to ambush prey from below. They have broad feet that act like paddles to propel their massive bodies through water and help distribute their weight on the ice. © Lynne Ledbetter / Visuals Unlimited

Life on the Ice
Adaptations to ice and freezing temperatures enable polar bears to survive conditions too harsh for other bears.

They are closely related to brown bears, but polar bears have evolved specialized features to thrive in the Arctic. Females, for example, produce milk that is up to 50 percent fat to boost their cubs' growth, energy and fat insulation, all of which help them survive Arctic temperatures down to -45 degrees Celsius (-50 Fahrenheit). Their coats have water-repelling guard hairs and dense underfur to ward off a chill after swimming between ice floes. No vegetation grows on the ice, so polar bears eat only what they can hunt; they are the most carnivorous of any bear.

Seal Stalking
Ribbon seals are closely linked to the sea ice. They haul out onto ice floes to breed and give birth—sometimes falling prey to polar bears.

Found along the southern fringes of polar bear habitat, ribbon seals are the only seals that spend their lives entirely at sea. They rely on ice floes to mate, molt and give birth. Within weeks, females wean pups and teach them to swim and hunt fish, squid and crustaceans. When the ice melts in the summer, ribbon seals take to the open ocean.

Seals are welcome feasts for hungry polar bears. Swimming under the ice, seals come up to breathe at holes and gaps, or they haul out onto the ice to rest. A polar bear's keen sense of smell can pinpoint seal breathing holes a kilometer (over a half mile) away, even holes covered by snow.