Overview

Polar bears (Ursus maritimus) are of special interest because of their large size, white color and position as the top-level carnivore in the remote arctic environment. They occur only in the northern hemisphere nearly always in association with sea ice. They have only two colors of fur: tan and white. Polar bears were created to withstand cold temperatures and are quite adaptable. Polar bears are ferocious and the most dangerous of bears. They can lop a person's head off with one swoop of their paw.

 

Size

Polar bears and brown bears evolved from a common ancestor and are still closely related, as demonstrated by matings and production of fertile offspring in zoos. Polar bears are similar in size to large brown bears. Cubs weigh between 1 and 2 pounds (0.5-0.9 kg) at birth. Male polar bears (boars) grow two to three times the size of female polar bears (sows). Boars weigh about 350 to more than 650 kg (772-1,433 lb.) and are about 2.5 to 3 m (8.2-9.8 ft.) long. Sows weigh about 150 to 250 kg (331-551 lb.) and are about 2 to 2.5 m (6.6-8.2 ft.) long. Pregnant females can weigh as much as 500 kg (1,102 lb.). The largest polar bear ever recorded was a male weighing 1,002 kg (2,209 lb.) and measuring 3.7 m (12 ft.) long. Boars stand between eight and nine feet tall. Polar Bears, when they stand on their hind legs, can be as tall as eleven feet. Their shoulder width can be as much as five feet across. Polar bears are the biggest carnivores on land and are twice as big as a tiger.

 

Body Type

Polar bears have a heavy stout body with strong muscular legs and well-developed neck muscles. Compared to other bears, the head of a polar bear is proportionally smaller. The necks of polar bears are longer than their nearest kin, the brown bear. This adaptation makes it easier for them to keep their heads above water when swimming. They have short, fur covered ears and a very short tail. Polar bears have large paws compared to body size, reaching 30 cm (12 in.) in diameter. The large paws of a polar bear act like snowshoes, spreading out the bear's weight as it moves over ice and snow. The forepaws are round, and the hind paws are elongated. The partial webbing between their toes, polar bears are able to use their front feet much like paddles to propel them rapidly through the water. The hind feet are slightly smaller. On both the front and hind feet, the bottoms are covered with dense fur, which affords better traction when moving on ice. Each toe has a thick, curved, nonretractable claw. The claws are used for grasping prey and for traction when running or climbing on ice. The sole of a polar bear's foot has thick, black pads covered with small, soft papillae (dermal bumps). The papillae create friction between the foot and ice to prevent slippage. Long hairs growing between pads and toes also help prevent slippage. Polar bears walk in a plantigrade manner (i.e., in a manner similar to humans with both heel and toe make contact with the ground when walking). On land, they are not as quick as brown bears and appear to have traded off speed for their extremely massive forelegs which they use to break through seal dens or flip a large seal out of the water. They are able to attain speeds of 40 kilometers per hour (25 miles per hour) for short distances.

 

Lifespan

Polar bears, other than family groups of females and young, are solitary most of the year. Bears in the wild have been recorded as old as 32 years but most probably do not live beyond 25 years. The oldest known polar bear in the Arctic lived 32 years. The oldest known polar bear in a zoological park lived 41 years. A denning female excavates a depression in the snow under a bank, on a slope, or near rough ice. The number of cubs born normally ranges from one to four with two cubs being average. At birth, the cubs are blind, toothless, hairless and very tiny. They weigh approximately 600 to 700 grams (21 to 25 ounces). This body size is roughly that of a chipmunk. Virtually helpless, they are, however, able to move sufficiently to suckle on their mother who remains asleep. Her milk is calorically very rich containing over 40% fat. In contrast, human milk only contains about 4% fat. Pregnant polar bear females are the only polar bears who will enter a winter den and hibernate for any length of time. Other bears will occasionally build a temporary shelter to overcome an extremely severe winter storm or to avoid summer heat and insects. As with other bears, the pregnant female will try to put on as much reserve fat as possible in order to have the resources to hibernate, bring her embryos to term and then nurse the newborn cubs until she leaves the den and is once more able to eat. The den is usually an oval chamber 2 to 3 metres (6.5 to 10 feet) connected to the surface by an long entrance tunnel. Within the denning chamber, the temperature may rise above freezing due to the heat given off by the mother and the excellent insulating qualities of snow. Inside the den the temperature can be as much as forty degrees warmer than the outside temperature.

 

Habitat

Degradation to polar bear habitat is currently of concern. Oil exploration and drilling activities in denning areas could cause bears to den in less suitable areas. Oil spills from drilling platforms or tankers potentially threaten polar bears. A polar bear's fur loses its insulating properties when covered with oil. Oil spills could diminish or contaminate polar bear food sources. The presence of toxic chemicals in polar bears may have long-term effects on their health and longevity. Arctic animals in higher food chain levels concentrate greater amounts of toxic chemicals in their tissues than those below them. Polar bears, at the top of the food chain, develop the highest concentrations of all. Human-made toxic chemicals such as polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), and chlordanes are present in the Arctic. These chemicals have been found in significantly high levels in the tissues of polar bears. Radionuclides, from nuclear waste dumping, may have detrimental effects on polar bears, and the Arctic ecosystem as a whole. 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.

Polar bears have enormous home ranges but are most abundant near coastlines and the southern edge of the ice. They make extensive movements related to the seasonal position of the ice edge. These bears are great roamers as they cover very long distances in their constant search for seals. Leads are water channels or cracks through ice which may remain open (ice free) for only a few minutes to several months, depending upon weather conditions and water currents. Polynyas are areas of water, surrounded by ice, that remain open throughout the year due to winds, upwellings, and tidal currents. Polynyas are important breathing and feeding areas for wintering or migrating marine mammals and birds. A small home range may be 50,000 to 60,000 square km (19,305/23,166 square mi.). A large home range may be in excess of 350,000 square km (135,135 square mi.). Polar bears do not mark or defend their home ranges. Polar bears are capable of traveling 30 km (19 mi.) or more per day for several days. One polar bear was tracked traveling 80 km (50 mi.) in 24 hours. Another polar bear traveled 1,119 km (695 mi.) in one year. It has been estimated that an individual polar bear may potentially cover an area equal to 259,000 square kilometers (100,000 square miles) during its lifetime.

 

Diet

Polar bears are almost exclusively meat eaters and 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. When other food is unavailable, polar bears eat reindeer, small rodents, seabirds, ducks, fish, eggs, vegetation (including kelp), berries, and human garbage. A polar bear's stomach can hold an estimated 15% to 20% of its body weight. It can assimilate 84% of the protein and 97% of the fat it eats. Their large stomachs have a capacity of more than 70 kilograms (150 pounds) of food. Their digestive system, like true carnivores, is also more adapted for processing meat than plant material. They can, however, go weeks between meals. Polar bears need an average of 2 kg (4.4 lb.) of fat per day to survive. A ringed seal weighing 55 kg (121 lb.) could provide up to eight days of energy for a polar bear. While out on the ice, their diet consists mostly of marine mammals. If available, they will kill a seal every few days. When the ice has melted during the summer and early fall, the bears are stranded on land. During this period of limited food sources, the bears will try to minimize their weight loss by scavenging the shoreline and a short distance inland for washed up carrion, bird eggs, rodents, berries and anything else that is edible. A keen sense of smell, extremely sharp claws, patience, strength, speed, and the camouflaging white coat aid in procuring food. Also, in keeping with their carnivorous nature, the teeth of polar bears have re-evolved changing back from a flatter crushing surface to a sharper-edged surface suitable for shearing off bite sized chunks of meat from their prey. The canine teeth, used for seizing and holding prey, are longer, sharper and spaced wider apart than in brown bears. Polar bears have 42 teeth, which they use for catching food and for aggressive behavior. Polar bears use their incisors to shear off pieces of blubber and flesh. The skin and fat are eaten first, followed by the meat. Jagged premolars and molars tear and chew. Polar bears swallow most food in large chunks rather than chewing. Each year as a polar bear grows, a thin layer of cementum is added to the outside of each tooth. Age can be estimated by examining a thin slice of tooth and counting the layers. The bear's strength enables it to break the back of a seal or kill it with one good wallop. Polar bears often stop to wash during feeding, using water nearby or rubbing in the snow. Polar bears do not always eat the entire kill. Other bears, arctic foxes, and gulls scavenge carcass remains.

 

Temperature Regulation

Air temperatures in the Arctic average -34C (-29F) in winter and 0C (32F) in summer. The ocean temperatures in the Arctic are about -1.5C (29F) in summer. In winter the ocean temperatures can drop to -2C (28F), at which point seawater freezes. Adaptions by the polar bear to life on sea ice include a white coat with water repellent guard hairs and dense underfur, short furred snout, short ears, teeth specialized for a carnivorous rather than an omniverous diet, and hair nearly completely covering the bottom of the feet. Polar bears are covered with a heavy fur. The color varies from pure white to more of a yellow hue. The white fur is important camouflage for the bears as they hunt their prey out on the ice pack. The coat consists of two layers - an undercoat of fine white hair and and outer coat composed of long guard hairs. The individual guard hairs are hollow. The hollow hairs also help to make the polar bear more buoyant when swimming. The actual color of the skin of the polar bear is black. It is thought that this is an adaptation for better heat retention. Polar bears are completely furred except for the nose and footpads, which are black. A polar bear's coat is about 2.5 to 5 cm (1.2 in.) thick. A dense, woolly, insulating layer of underhair is covered by a relatively thin layer of stiff, shiny, clear guard hairs. Polar bear fur is oily and water repellent. The hairs do not mat when wet, allowing the bears to easily shake free of water and any ice that may form after swimming. Ice forms when the wet fur is exposed to air temperatures at or below freezing. The hairs reflect light, giving a polar bear its white coloration. Oxidation from the sun, or staining, can make the hairs look yellow or brown. Polar bears completely molt (shed and replace their fur) annually, in May or June. The molt can last several weeks. To keep warm, much like the seals, they have a thick layer of fat on their body through out the year. Body temperature, which is normally 37C (98.6F), is maintained through a thick layer of fur, a tough hide, and an insulating layer of blubber. This excellent insulation keeps a polar bear warm even when air temperatures drop to -37C (-34F). Polar bears have a 4-inch thick layer of blubbery fat. A polar bear's body heat is so well-contained that heat-sensitive cameras cannot detect a bear sleeping on the ice!

Polar bears are so well insulated they tend to overheat. Polar bears move slowly and rest often to avoid overheating. Excess heat is released from the body through areas where fur is absent or blood vessels are close to the skin. These areas include the muzzle, nose, ears, footpads, inner thighs, and shoulders. Polar bears will also swim to cool down on warm days or after physical activity.

 

Physical Activity

As Ursus maritimus means "sea bear", polar bears are well adapted to a marine life. They are able to swim distances greater than 96 kilometers (60 miles) without a pause to rest. They swim across bays or wide leads without hesitation. They have been tracked swimming continuously for 100 km (62 mi.). Using their forepaws for propulsion and their rear paws as rudders, they can maintain an average swimming speed of approximately 10 kilometers (6 miles) per hour. They are also excellent divers being able to remain submerged for up to two minutes and attain a depth of 4 meters (15 feet). They make shallow dives when stalking prey, navigating ice floes, or searching for kelp. Polar bears usually swim under water at depths of only about 3 to 4.5 m (9.8-14.8 ft.). No one knows how deep a polar bear can dive. One researcher estimates that polar bears dive no deeper than 6 m (20 ft.). While underwater, they are able to close their nostrils and flatten their ears. Their eyes remain open so that they can see possible prey. Polar bears have also been observed leaping out of the water up to 2.25 meters (7 to 8 feet) in the air as they surprise a seal resting on a ice floe. A thick layer of blubber (fat), up to 11 cm (4.3 in.) thick, keeps the polar bear warm while swimming in cold water. The hair easily shakes free of water and any ice that may form after swimming.

Polar bears are most active the first third of the day and least active the final third of the day. 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. 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.

Like humans, polar bears walk on the soles of their feet with their heels touching the ground first. Like other bears, they can also stand on their hind feet and walk upright for short distances. Polar bears generally walk with a steady, lumbering gait. The front paws swing outward with each step, landing slightly pigeon-toed. The head swings gently from side to side. The walk has a four-beat pattern, first the right front foot touches the ground, then the left hind foot, then the left front foot, and lastly, the right hind foot. The 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 (3.4 mph). When being chased or charging prey, polar bears can run as fast as 40 kph (25 mph) for short distances. Polar bears are expert hunters and will relentlessly follow tracks in the snow to find their prey. It is rumored that a polar bear will cover its nose with a paw when it comes within striking distance of its prey. This prevents the prey from seeing the polar bear as its black nose would stand out against the white snow and ice.

1. Still hunting: 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.

2. Stalking on land: 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.

3. Aquatic stalk: 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.

4.Stalking birth lairs: Ringed seal birth lairs are caves built under snow drifts next to a hole in the ice. The snow drifts 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.

Like all bears, polar bears love to play! These intelligent creatures will often use ice ridges as water slides, and take turns sliding down for a big SPLASH!

 

Hibernation

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. Polar bears do not enter deep hibernation. Deep hibernation applies to an animal whose body temperature drops to 5C (41F) 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 are not deep hibernators, but enter a state of carnivore lethargy. Though hibernating females sleep soundly, they are easily and quickly aroused. The female polar bear's heart rate slows to about (8-12 bpm at a low) 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 35C (95F), or it may remain normal at 37C (98.6F). Unlike many other animals who hibernate, its body temperature only undergoes a minor reduction of 3 to 7 degrees Centigrade (5 to 9 degrees Fahrenheit). Its metabolism slows down by half. 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. Researchers have found that nonhibernating polar bears, during times of food scarcity, can efficiently utilize their energy reserves much like hibernating bears. During the period of hibernation, the polar bear will neither pass urea or solid fecal waste. While urea poisoning causing death would occur in all other animals within a week, bears have developed a unique process of recycling the urea into usable proteins. During the hibernation period, all bears lose a great deal of weight. It is not uncommon for a female polar bear with newborn cubs to lose as much as 40% of her weight.

 

Senses

A polar bear's eyes are dark brown, set relatively close together, and look forward. The ears are small and rounded, and lay flat when under water. A polar bear's hearing is probably as sensitive as human hearing. Humans can hear sounds with frequencies as low as 0.02 kHz and as high as 20 kHz. Polar Bears have the best eyesight of all three bears. The eyesight of polar bears appears to be similar to human's. Polar bears have a protective membrane over their eyes, that may help shield the eyes from ultraviolet light. Little is known about a polar bear's sense of touch; however, they have been observed delicately moving or touching objects with the nose, tongue, and claws. Polar bears prefer certain foods, but researchers do not know how acute the sense of taste is or how important it is in food preference. A polar bear's sense of smell is acute, and it is the most important sense for detecting prey on land. A polar bear can smell a seal more than 32 km (20 mi.) away. Adult polar bears vocalize most when they are agitated or threatened. Sounds include hissing, growling, champing of teeth, and soft chuffing. Polar bears also communicate through sight, touch, and smell.

Polar Bear Questions