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Polar Bears
(Ursus maritimus)
It is currently believed that polar bears are direct descendants of brown bears. They still remain genetically very similar.
Somewhere during the mid-Pleistocene period (roughly 100,000 to 250,000 years ago), a number of brown bears became isolated by glaciers. Instead of perishing on the ice, they began a rapid series of evolutionary changes in order to survive. Today, polar bears are totally adapted to their harsh northern environment.
The polar bear is found in all of the polar regions of the entire northern hemisphere (see map). This includes Russia, Norway, Greenland, the United States and Canada.
Their preferred habitat is in the area where the northern seas meet the shoreline. In this area, there is a constant freezing and thawing of the ice. This zone is continually crisscrossed with fingerlets of open water called leads. It is also the preferred habitat of their favourite prey, the seal.
It is now known that there are approximately seven distinct polar bear populations as follows: central Siberia; Svalbard-Franz Josef Land; Greenland; the Canadian arctic archipelago; northern Alaska; western Alaska and Wrangel Island; and the Hudson Bay-James Bay populations. These groupings have developed as a result of separate ice movement patterns.
It is estimated that there are currently somewhere between 20,000 and 40,000 polar bears. Canada has the largest population with approximately 15,000 bears living in the Canadian arctic archipelago and the Hudson Bay-James Bay regions.
The number of bears had been in a sharp decline for many years due to severe overhunting by all the northern nations. Then, in 1967, the five "polar bear" nations, Norway, Russia, Greenland, the United States and Canada, ratified a conservation treaty in order to save the bears. Today, the population is relatively stable.
The polar bear is a large mammal and, along with their close relatives, the brown bear,
are among the largest members of the bear family. There is considerable variability in the
size of individual polar bears.
In general, adult polar bears stand approximately 1 metre (3 1/2 feet) tall when on all
fours and have an approximate body length from nose to tail of 250 to 350 centimeters (8
1/4 to 11 1/2 feet).
Polar bears grow very large. Females typically weigh in the area of 300 kilograms (660 pounds). Males are generally larger and usually weigh 500 to 600 kilograms (1100 to 1320 pounds). They can grow much larger, however. In fact, there is a record of an adult male polar bear weighing over 1000 kilograms (2200 pounds).
The lifespan of polar bears in the wild can be twenty-five years or more.
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. They have short, fur covered ears and a very short tail.
The necks of polar bears are longer than their nearst kin, the brown bear. This adaptation makes it easier for them to keep their heads above water when swimming.
The forepaws on a polar bear are very large. With a diameter approaching 30 centimeters (12 inches) and 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. 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.
Polar bears have also developed very large stomachs with 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.
Also, in keeping with their carnivorous nature, the carnassial 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. Also, the canine teeth, used for seizing and holding prey, are longer, sharper and spaced wider apart than in brown bears.
Polar bears are covered with a heavy fur. The colour 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.
As Ursus maritimus means "sea bear", it is important to understand how well adapted polar bears are to a marine life. They are most at home out on the ice following the leads of temporary open water or hunting near the polynyas, which are larger bodies of water which stay ice free throughout the winter.
Polar bears are able to swim distances greater than 96 kilometers (60 miles) without a pause to rest. 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 1/2 metres (15 feet). 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 metres (7 to 8 feet) in the air as they surprise a seal resting on a ice floe.
Polar bears are almost exclusively meat eaters. While out on the ice, their diet consists mostly of marine mammals such as the ringed seals, bearded seals and occasionally a walrus pup. If available, they will kill a seal every few days. They can, however, go weeks between meals. Their large stomach capacity is designed to allow them to take advantage of unexpected large meals which will serve to tide them over during leaner times.
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 home range is the area an animal travels in its normal activities of gathering food, mating and caring for its young.
Polar bears have enormous home ranges. These bears are great roamers as they cover very long distances in their constant search for seals. 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.
The size of the home range is extremely dynamic and varies from year to year depending on ice conditions and the location of the seal populations.
Polar bears do not defend their home ranges from other bears and thus it is normal for the home ranges of individual bears to overlap each other.
Female polar bears normally become sexually mature in their fifth or sixth year, males in their eighth year. Copulation takes place in a manner similar to members of the canine (dog) family.
Mating normally occurs out on the pack ice between late March and mid-July. Females will mate with a number of males over the approximate three weeks of the breeding season. Females do not come into estrus when they are nursing cubs.
Through a remarkable process, called delayed implantation, the fertilized ovum divides a few times and then floats free within the uterus for about six months with its development arrested. Sometime around September, the embryo will attach itself to the uterine wall and resume its development.
The mother will enter the den in October or November with the cubs being born sometime in December or January while the mother is still hibernating.
Delayed implantation clearly serves an important survival need for the mother. Should the female not have put on enough fat reserves before the time to den arrives, the embryo will not implant and it is simply reabsorbed by her body. She will then continue her winter hunting out on the pack 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 is roughly the size 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.
Within the next several weeks, the cubs will develop rapidly on this rich diet such that they will be able to follow their mother when she leaves the den in late March or early April. By this time, they will weigh between 10 and 15 kilograms ( 22 and 33 pounds).
The survival of bear cubs is extremely tenuous. Many polar bears cubs will die during their first few year of life. Starvation, disease and injury will claim a number of cubs.
The cubs will normally stay with the mother for the first two and a half years. Occasionally, they will remain for an extra year. The cubs are generally weaned between July and September of their first year.
The survival of polar bear cubs is totally dependent on the skill of the mother in both protecting them and teaching them the basics of what to eat, where and how to catch it, where to den in the case of females, and how to cope with the many dangers out on the ice.
As adolescents, the young bears are still in extreme danger. Normally they are driven off by their mother as she prepares to breed once more. They must now become rapidly self sufficient if they are to stay alive and find sufficient food to maintain their fat reserves to last over the long winter and through the period when the ice is out.
Occasionally, young sibling bears will spend a period of time together after they have been driven off by their mothers.
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 pregnant polar bear will usually dig a maternity den into a south-facing slope a short distance inland from the coast. 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.
While hibernating, a bear's heart rate drops from between forty to seventy beats per minute to only eight to twelve beats per minute and its metabolism slows down by half. 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).
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.
The female polar bear will be lethargic during her initial emergence from the den. After a short period, she will return to the ice to resume hunting.
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