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Polar bears are strong swimmers, they can swim several hours at a time over long distances. A polar bear's front paws propel them through the water dog-paddle style, while the hind feet and legs are held flat and are used as rudders. Polar bears can obtain a swimming speed of 10 kph. The hair of a polar bear easily shakes free of water and any ice that may form after swimming. Polar bears will also swim to cool down on warm days or after physical activity.
| Polar bears make shallow dives when stalking prey, navigating ice floes, or searching for kelp. The polar bear's nostrils close when under water. A thick layer of fat, up to 11 cm thick, keeps the polar bear warm while swimming in cold water. Polar bears usually swim under water at depths of only about 3 to 4.5 m. They can remain submerged for as long as two minutes. No one knows how deep a polar bear can dive. One researcher estimates that polar bears dive no deeper than 6 m. |
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| In the Canadian Arctic, adult female polar bears with cubs hunt about 19% of their time during the spring and about 38% of their time during the summer. Adult male polar bears hunt about 25% of their time during the spring and about 40% of their time during the summer. When not hunting, polar bears are often sleeping or resting. |
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On warm days polar bears sprawl out on the ground or ice, sometimes on their backs with their feet in the air. They may also make temporary snow or earthen pits to lie in. On cold days polar bears curl up and often cover their muzzle area. During the winter, some polar bears excavate temporary dens or find natural shelters to stay warm. They may use these shelters for several months at a time.
Their bulky build and swinging gait cause polar bears to use more than twice as much energy to move at a given speed than most other mammals. The average walking speed of a polar bear is 5.5 kph. When being chased or charging prey, polar bears can run as fast as 40 kph (25 mph) for short distances.
| Polar bears are basically solitary. Usually, only two social units exist: (1) adult females with cubs and (2) breeding pairs. The most constant social interaction occurs between mother and cubs. Polar bear mothers are attentive, frequently touching and grooming their cubs. Polar bear breeding pairs remain together for one week or more, mating several times. |
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Polar bears will aggregate to feed on large whale carcasses and at dumpsites. In some southern regions, like Hudson Bay, bears aggregate on land during the ice-free summer and autumn months. On occasion, adult and sub adult (ages 30 months to five or six years) polar bear males will feed and travel together for short periods of time.
Aggression occurs between males during the breeding season and when males attempt to steal food caught by other polar bears. Play fighting has been observed between aggregating sub adult and adult male polar bears. Young polar bear cubs chase and tackle their siblings.
Hibernating means to pass the winter in a dormant or lethargic state. Animals that hibernate store body fat when food is plentiful. When food is scarce, they hibernate, living off their stored body fat. 2. Polar bears don't enter deep hibernation. Deep hibernation applies to an animal whose body temperature drops to 5oC for a period of days or weeks. Deep hibernators also show a marked drop in heart rate, and are slow to wake up when disturbed.
Only pregnant female polar bears hibernate. Polar bears aren't deep hibernators, but enter a state of carnivore lethargy. Though hibernating females sleep soundly, they're easily and quickly woken. The female polar bear's heart rate slows to about 27 beats per minute from a normal resting heart rate of about 46 beats per minute.
When hibernating, a female's body temperature may drop slightly, perhaps to 35oC, or it may remain normal at 37oC. Unlike most other hibernators, female polar bears give birth while hibernating. High body temperature is needed to meet the demands of pregnancy, birth, and nursing.
Polar bears feed mainly on ringed seals and bearded seals. Depending upon their location, they also eat harp and hooded seals and scavenge on carcasses of beluga whales, walruses, narwhals, and bowhead whales. Polar bears feed mainly on ringed or bearded seals. When seals are unavailable, polar bears eat other marine mammals, reindeer, small rodents, sea birds, ducks, fish, eggs, vegetation (including kelp), berries, and human garbage. Polar bears need an average of 2 kg of fat per day to survive. A ringed seal weighing 55 kg could provide up to eight days of energy for a polar bear.
Still hunting is the most common method of hunting year-round. The polar bear remains motionless beside a breathing hole or lead edge waiting for a seal to surface. When a seal surfaces, the polar bear bites onto the head or upper body, then flips the entire seal onto the ice. Still hunting usually takes less than one hour, but polar bears will wait much longer. Stalking is a hunting method used in summer when seals haul out on sea ice. Once spotted, the seal is slowly and steadily stalked by the polar bear. At 15 to 30 m (49.98 ft.) away, the polar bear suddenly charges the seal. With its claws or teeth, the polar bear grabs the seal before the seal can leave the ice. |
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The aquatic stalk is a method also used in summer when seals haul out on sea ice. The polar bear swims toward a hauled-out seal. Once the polar bear reaches the ice edge, the bear quickly emerges from the water and grabs the seal with its claws or teeth.
Stalking ringed seals at their birth lairs is a hunting method polar bears use in spring, when ringed seals give birth to their pups. Ringed seal birth lairs are caves built under snowdrifts next to a hole in the ice. The snowdrifts are on stable sea ice attached to land.Once a polar bear identifies a birth lair, it slowly and quietly positions itself next to the lair. If a polar bear smells or hears a seal in the lair, it slowly raises up on its hind legs and crashes down with its front paws to break through the lair roof. To break the roof's hard surface, several tries are sometimes needed, which may allow the seal to escape into the water. This method is most commonly used by polar bear females with cubs under one year old.
Polar bears don't always eat the entire kill. Carcass remains are scavenged by other bears, arctic foxes, and gulls.
The total gestation period is about eight months. Gestation includes a period of delayed implantation. The fertilized egg divides into a hollow ball of cells one layer thick (a blastocyst),then stops growing and lies free-floating in the uterus for about four months. The blastula then implants in the uterine wall and continues to develop. Delayed implantation assures that the cub is born during the best time of the year for survival and allows the female to get into good physical condition and use her energy for nursing her newborn cubs. The actual embryonic development is estimated to be four months.
Once mated, females begin depositing fat in preparation for cubbing. Females need to gain at least 200 kg for a successful pregnancy. Some females may seek out maternity dens as early as late August, but most enter dens in mid to late October. Dens protect newborn cubs from winter's temperature extremes. Females usually dig dens in snowdrifts on southerly facing slopes, most dens are on land, within 16 km of the coast. In some areas, dens are more than 100 km from the coast. A few polar bears make dens on the sea ice. Most dens consist of a single chamber slightly elevated from a short entrance tunnel. On average, the chamber is 2 m long, 1.5 m wide, and 1 m high. Polar bears maintain a ventilation hole through the chamber ceiling to provide fresh air. Because of body heat and snow insulation, the den stays warmer than the outside air temperature.
| Polar bear cubs are born November through January in a den. Mother and cubs emerge from their den in late March or April. During their first few weeks of life, polar bear cubs nurse most of the time and stay close to their mother to keep warm. Most adult females give birth once every three years. In some populations, birth occurs once every two years. The most frequent litter size is two, followed by litters of one. Litters of three are less common than twins or singles, and litters of four are rare. |
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Polar bear cubs are born small and helpless, with their eyes closed,; weighing about 454 to 680 g and are about 30 cm long. Males are born slightly larger than females. The fur is very fine at birth, making the cubs look hairless. Mother polar bears nurse their cubs for as long as 30 months. Some cubs stop nursing as young as 18 months of age, but remain with their mothers for survival until they are 30 months old. Cubs open their eyes within the first month. The cubs begin walking while in the den at about two months. By this time, they also have thick, whitish fur and their teeth have erupted. By the time the mother and cubs emerge from the den in late March or April, the cubs weigh 10 to 15 kg. Mother and cubs remain around the den for about 12 more days, sometimes longer. This enables the cubs to acclimate to the colder weather and develop their walking muscles.
| During this time the cubs still spend about 85% of their time in the den, sleeping there at night. When ready, the mother polar bear leads her cubs to sea ice. Travel is slow with frequent rest and nursing stops. A mother will sometimes carry her cubs on her back through areas of deep snow or water. Cubs begin eating solid food as soon as their mother makes her first kill on the sea ice (about three to four months of age). |
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A mother polar bear can comfort, protect, or punish her cubs by using her body, muzzle, or paws. Cubs will try hunting in their first year, but don't seem to be successful until they're over one year old. Even then, they only spend about 4% of their time hunting. By the time they're two years old they spend about 7% of their time hunting and can catch a seal every five or six days. When her cubs are about 30 months old, a female polar bear is ready to breed again.
Longevity and Disease
Polar bears can live 20 to 30 years, but only a small proportion of polar bears live past 15 to 18 years. The oldest known polar bear in the Arctic lived 32 years. The oldest known polar bear in a zoological park lived 41 years. Adult polar bears have no natural predators. Males occasionally kill other males competing for mates. Males periodically kill females protecting cubs. Cubs less than one year old sometimes are prey to adult male polar bears and other carnivores, such as wolves. Newborn cubs may be cannibalized by malnourished mothers. Starvation is the greatest threat to sub-adult polar bears. Sub-adults are inexperienced hunters, and often are chased from kills by larger adults. Older, weaker bears also are susceptible to starvation.
Today, polar bears are hunted by native Arctic populations primarily for food, clothing, handicrafts, and sale of skins. Polar bears are also killed in defence of people or property. Hunting is government-regulated in Canada, Greenland, and the United States. Hunting is currently banned in Norway and Russia.
As in any animal population, a variety of diseases and parasites can be responsible for polar bear illnesses. Polar bears are especially susceptible to the parasitic worm Trichinella, which they contract by feeding on infected seals. Trichinella larvae encyst in various parts of the polar bear's body, usually muscle tissue. If enough larvae encyst in one area, such as the heart, the tissue becomes severely damaged. Death may result.
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