The polar bear vies with the brown bear for the title of the world’s largest living land carnivore. It is classified as a marine mammal, and its preferred hunting ground is Arctic pack ice. Superbly adapted to its environment, the polar bear has non-retractable claws and dimpled, partially furred foot pads that provide extra grip, allowing it to walk and run easily on ice. The polar bear’s body is covered in double-layered fur—the thickest of any bear species. The inner layer is a dense undercoat, while the outer fur consists of clear, hollow tubes that trap air for insulation. Since the tubes reflect all visible light, the outer coat makes polar bears seem white, allowing them to blend easily into snowy environments. Their skin is black and rests on a layer of blubber up to 4 in (10 cm) thick.
Feasting and fasting
Polar bears can live up to 25–30 years in the wild. Their lives alternate between feasting and fasting, and their intestines are adapted to process fat, which is easier to digest than meat and has more calories. They can also slow their metabolic rate when food is scarce. Their main diet is seals, but they occasionally hunt belugas or narwhals. When starving, they will also hunt walruses, but the risk of injury is high. They can smell prey up to 3 /4 mile (1 km) away, or up to 31 /4 ft (1 m) beneath ice. Polar bears are generally solitary, except for breeding pairs or mothers with cubs. However, they will congregate around large food supplies such as whale carcasses. In fall, polar bears also gather together in “transition” areas such as southwestern Hudson Bay and Churchill, Canada to wait for the sea ice to form that allows them access to ringed seals swimming beneath the ice. Polar bear territories are vast. Bears closest to the Canadian Arctic Islands have an average range of 19,000–23,000 sq miles (50,000–60,000 sq km), but those closer to the Bering Sea can cover up to 135,000 sq miles (350,000 sq km).
Winter births
Polar bears give birth to cubs every two to three years —one of the lowest reproductive rates of all mammals. Mating occurs from late March through May, but embryos may not start to develop until fall. Pregnant females must gain about 440 lb (200 kg) extra weight during summer to survive the winter, when they may have to go up to eight months without food. They dig maternity dens mainly in south-facing snowdrifts, where between one and four cubs are born in early winter. Most litters are of twins. The adult female does not hibernate in the truest sense, but maintains a much warmer body temperature to care for her cubs. Even so, she neither eats, urinates, nor defecates during the months she is in the den. Mother and cubs do not emerge from their den until March or early April, when she leads them toward the sea ice in order to hunt.
Narwhal
Narwhals are unique among whales in having a single long tusk, which is grown mostly by males. The tusk is, in fact, an elongated canine tooth that erupts mainly from the left side of the animal’s upper jaw. It grows in a counterclockwise spiral, and is believed to be the reality behind the unicorn legends of medieval Europe. While scientists once believed that the tusk’s function was purely defensive, relating to dominance disputes over mating rights, recent research has revealed millions of nerve endings at the tusk’s surface. These nerve endings allow the narwhal to detect changes in water pressure and temperature, as well as degrees of water salinity (saltiness). This discovery suggests that the characteristic rubbing of tusks by males may be a sensation- or information-seeking exercise—not simply “jousting.” Tusks can grow to over 8 ft (2.5 m) long and are highly flexible, bending up to 1 ft (30 cm) in any direction without breaking. If a tusk is broken, new growth repairs the damage.
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