Polar Bear SP-3666 - History

Polar Bear SP-3666 - History

Polar Bear

(SP-3666: dp. 8,835; 1. 353'3"; b. 49', dr. 23'1", s. 12 k.
cpl. 86)

Polar Bear (SP-3666) was built in 1918 by the Baltimore Dry Dock and Shipbuilding Co., Baltimore, Md., requisitioned by USSB for use in NOTS as a refrigerated cargo ship 28 September 1918; and commissioned at Baltimore 3 December 1918, Lt. Comdr. Richard Russell Lukens, USNRF, in command.

Assigned to NOTS, Polar Bear sailed 19 December 1918 with a full earzo of general Army supplies for the American Expeditionary Force in France. She dlseharged her cargo at La Palliee, Verdun, Bordeaux and Paulliae before returning to the U.S. 3 February 1919.

Polar Bear was decommissioned at New York 10 March 1919, and was returned to the USSB.


The polar bear's life cycle is closely tied to sea ice. Polar bears rely on the ice to travel, hunt seals, breed, and in some cases, den. Scientists believe polar bears are unlikely to survive if ice-free periods exceed their fasting ability (220 days), especially in areas that lack alternate marine mammal prey.

Polar bears are strong swimmers and divers, a characteristic that allows them to swim from one ice floe to the next. But there’s a limit to how far they can swim. Long swims are especially dangerous to young cubs.

Polar Bear SP-3666 - History

One small fossil, one giant step for polar bear evolution
April 2010

As the fuzzy and ferocious poster child for climate change issues, polar bears get plenty of press, whether it's coverage of something as simple as the birth of a cub at a zoo or as political as a rejected ban on trading polar bear parts. Last month, however, saw a polar bear story of a different ilk — a story about the bears' evolutionary past that has implications for their evolutionary future. Polar bears, it turns out, may have evolved surprisingly quickly in response to past climactic changes. Here, we'll examine the different lines of evidence that led scientists to this conclusion.

Where's the evolution?
In 2004, researchers discovered a polar bear fossil preserved in Norwegian coastal cliffs. It was the lower left portion of the jaw, still containing a tooth. And though this might not sound like much information to go on, the single fossil would turn out to have a lot to say about polar bear evolution.

The polar bear jawbone fossil that revealed so much about the history of polar bears.

Based on the jawbone's shape, scientists were confident that it belonged to an adult male polar bear. The rock layers in which it was embedded, along with other dating techniques, suggest that it is 110,000 to 130,000 years old — older than any other known polar bear fossils. This date alone is interesting because previous estimates for the origin of polar bears have ranged from 70,000 to more than a million years ago. But, based on the new fossil's age, we can infer that the polar bear lineage must be more than 110,000 years old. The anatomy and rocks in which the jawbone was preserved provided useful information — but the fossil had an even more powerful line of evidence: DNA.

DNA breaks down over time, but new techniques allow scientists to pull these fragmented bits of DNA out of some well-preserved fossils, copy the pieces, and reassemble them into a very good estimate of the original genetic sequence. An international group of scientists used these techniques on DNA from the polar bear jawbone and announced their findings last month. They had reconstructed the sequence of the bear's mitochondrial DNA — a short loop of DNA that is housed in the cell's "powerhouse" organelles, mitochondria. The ancient mitochondrial DNA was easier for the scientists to work with than the bear's main genome (housed in the nucleus), because each cell contains many copies of its mitochondrial DNA and only one copy of its nuclear genome.

The researchers compared the fossil's DNA sequence to those from different species of modern bears and from extinct cave bears. They used these sequences to reconstruct the animals' family tree. As expected, the modern and ancient polar bear DNA formed a tight group, a clade — and that clade was most closely related to the brown bears of Southeast Alaska.

Having already investigated the fossil's anatomy, stratigraphy, and mitochondrial DNA (as well as the molecular clock in that DNA), the team of researchers studied one more line of evidence unearthed with the fossil: the atoms that compose the tooth embedded in the jawbone. An organisms' diet strongly influences the sort of atoms that are deposited in its body — specifically the ratios of atoms with different numbers of neutrons. By studying the carbon and nitrogen in the bear's tooth, the researchers discovered ratios that were exactly what we'd expect to observe of a bear that gets its nutrition from seafood! Just 20,000 years or so after they diverged from forest-dwelling brown bears, polar bears had already evolved their distinctive marine lifestyle. For a large mammal, that's evolution at breakneck speed!

This rapid pace of polar bear evolution may have been related to changes in the climate going on at the time. The polar bear and brown bear lineage split during a glacial age. This may have provided the ecological setting for the evolution of the seafood eating, sea ice-loving polar bear lifestyle. The newly formed polar bear lineage survived one warm interglacial period before being plunged back into another glacial age.

Now of course, the planet is warming again — this time due to human actions. Will polar bears survive? The answer is not clear, but the new research does shed some light on the issue. We now know that polar bears have evolved surprisingly quickly in the past. However, today, the Earth is heating up much faster than it ever has before — and the unprecedented pace of that change makes it difficult for slow reproducing organisms like polar bears to evolve to keep up. One thing is for certain: knowing more about how polar bears have responded to climate change in their evolutionary past can help us figure out how to help them survive into the future.

    Lindqvist, C., Schuster, S. C., Sun, Y., Talbot, S. L., Qi, J., Ratan, A., . . . Wiig, Ø. (2010). Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceedings of the National Academy of Sciences, USA. 107 (11): 5053-5057.

from Scientific American

Understanding Evolution resources:

Discussion and extension questions

    List at least four different lines of evidence that the researchers were able to glean from the polar bear fossil. For each, briefly describe what that line of evidence suggested about the fossil or polar bear evolution.

. Does the conclusion of that article — regarding polar bears and climate change — conflict with the conclusion of the article above? Why or why not?

. The researchers used the principle of parsimony, along with mitochondrial DNA sequences, to construct the tree shown in the article above. Based on the principle of parsimony, what would make the researchers prefer this tree to other possible trees? Make sure that your answer references the DNA sequences.

    Which polar bear sequence is most different from the polar bear reference sequence? What sort of evolutionary explanation could you give for why this sequence is most different?

Related lessons and teaching resources

    : In this web-based module for grades 6-12, students are introduced to cladistics, which organizes living things by common ancestry and evolutionary relationships.

: This interactive and engaging web activity, for grades 9-12, compares the number of mutations in the mitochondrial genomes of primates to determine ancestry and relatedness.

    Eilperin, J. (2010, March 19). Global conference rejects bans on trade in bluefin tuna, polar bear. The Washington Post.
    Retrieved April 2, 2010 from The Washington Post.

oi bear clade adapted from Lindqvist, C., Schuster, S. C., Sun, Y., Talbot, S. L., Qi, J., Ratan, A., . . . Wiig, Ø. (2010). Complete mitochondrial genome of a Pleistocene jawbone unveils the origin of polar bear. Proceedings of the National Academy of Sciences, USA. 107 (11): 5053-5057.

Learn more about Earth's changing temperature on the Understanding Global Change site.

The perils of eating polar bear

/>Would-be connoisseurs of polar bear meat should keep in mind the possibility of negative side effects, particularly hypervitaminosis A, an excess of the vitamin that can be contracted from eating its liver. (Josh Haner / The New York Times)

Would-be connoisseurs of polar bear meat should keep in mind the possibility of negative side effects, particularly hypervitaminosis A, an excess of the vitamin that can be contracted from eating its liver. (Josh Haner / The New York Times)

Throughout 8,000 years of shared history, humans have regarded the polar bear with wonder, terror and fascination. It has been spirit guide and fanged enemy, trade good and moral metaphor, symbol of ecological crisis and food source. The bear's meat itself is rich with associations that speak of the fraught relationships between our two species.

Paraphrasing the French analyst of totemism, Claude Lévi-Strauss, one could claim that the North's Native peoples are taken with polar bears not only because they are spiritually potent — "good to think" — but also because they are physically potent — "good to eat."

Throughout Arctic history the bear has served as food, though in most indigenous societies, whales, walrus, seals, caribou or reindeer provided the bulk of the diet. Unfamiliar dishes or ingredients like bear meat strike Western palates as surreal or exotic and, in the case of endangered species, might also be seen as politically incorrect — but from our births onward, the culture that surrounds us shapes our food preferences and what we consider normal or acceptable.

Food can be a marker of belonging, contributing to a group's self-image and coherence. Food taken directly from one's surroundings is symbolic of place, forming a link with a people's history. This is why even in countries that banned polar bear hunting, such as the United States, Native groups with a tradition of hunting polar bears are permitted to keep hunting them — and other animals covered by the Marine Mammal Protection Act.

/>Inupiaq hunter and polar bear, circa 1924. (This photo actually shows the re-enactment of a traditional hunt for a silent movie like Robert J. Flaherty’s “Nanook of the North” but was filmed in Nome, Alaska.) Courtesy of Library of Congress.

Together with the bear's humanlike appearance, the richness of bear meat and its rarity in modern diets seem to account for non-Native people's rejection of it. But our culinary preferences have changed. In 19th-century North America, bear meat (though not that of polar bears) was standard fare. Settlers also used bear fat to fry other foods, preferring it to butter.

Unlike medieval royalty who kept polar bears in menageries — or later, zoos — which pampered rare collectibles, explorers and whalers, always near starvation, treated the white bears as survival rations.

For months, "bear-beef" was often the only course on these men's menu. The meat is much greasier, however, than beef. Norwegian explorer Fridtjof Nansen's captain, Otto Sverdrup, called it a "royal dish" and the explorer himself judged breast of polar bear cub to be delicious. Of course, hunger always has been the best sauce and could have swayed culinary opinions.

"Heaven had sent us succor at a time of utter distress," one castaway recalled of a polar bear windfall, "and our gratitude for this miraculous gift was apparent in our overflowing happiness."

Having run out of provisions on one of the numerous searches the British launched after Sir John Franklin went missing in the Arctic, Dr. Elisha Kent Kane ate raw, frozen meat from a polar bear head that he had saved as a specimen and called it a godsend. He described the meat of lean bears as "the most palatable food" and "rather sweet and tender," but he warned against well-fed bears, which were made nearly inedible by "the impregnation of fatty oil throughout the cellular tissue."

Would-be connoisseurs should keep in mind the possibility of negative side effects.

"I did not care to try how it tasted," the English explorer and scientist William Scoresby wrote, "for I was afraid that my hair would turn grey before its time, for the seamen are of opinion that if they eat of it, it makes their hair grey."

More serious is hypervitaminosis A, an excess of the vitamin that can be contracted from eating the liver of polar bears, seals and walrus. Affecting the central nervous system, it can cause hair loss, extreme peeling of the skin, birth defects, liver problems, vomiting, blurred vision and even death. One officer swore never again to eat bear liver, no matter how much it might tempt him, after his crew showed symptoms akin to carbon monoxide poisoning. Native peoples have long been aware of this danger, as have explorers, though some felt no worse after eating the liver.

/>Warning against eating polar bear liver, from a U.S. Navy survival manual, “The Naval Arctic Operations Handbook,” 1949. The organ has concentrations of vitamin A that can be toxic for humans. (Courtesy Woods Hole Oceanographic Institution.)

Research has shown that a healthy adult person can tolerate 10,000 units of vitamin A. Trouble, if it comes, comes between 25,000 and 33,000 units. One pound of polar bear liver — a fist-sized chunk and barely a meal — can contain 9 million units of vitamin A. The occasional lack of liver toxicity that some explorers reported can be explained by differences in the age, hibernation and feeding habits of the bear.

Equally bad is trichinosis, a parasitic disease contracted by eating the raw or undercooked flesh of pigs or wild game, including bear. Symptoms can include fever, muscle pain and fatigue, as well as inflammation of the heart muscle, lungs or brain, which have led to a few deaths.

Native peoples avoided polar bear liver because of its vitamin A concentration, and, like explorers and whalers, fed it only to their dogs. Modern Inuit and Inupiat value the flavor nuances of different bears or parts of a bear. Some prefer den polar bears, instead of bears caught in the open, because they taste better. The Cree consider the front and back paws (tukiq) the best eating.

For many Inupiat, polar bear meat remains a favorite meal and a prestigious gift. Nowadays, when a polar bear has been killed, a call goes out on a village radio channel, asking people to get some. The hunter normally keeps the skin, a trophy and commodity. The rest of a bear still is widely shared, a token of group identity and solidarity, a kind of Arctic communion. Unlike the whalers and explorers, who saw it as staple or last resort, indigenous peoples have always considered eating polar bear a reaffirmation of community as much as an act of physical nourishment.

Like the widespread idea that animal parts such as the blood, heart or testicles give power to those who ingest them, the human craving for novelty and the desire to understand the unknown by tasting it have shaped human culinary exploration from the beginning. It is not surprising that, in a world of potentially lethal pufferfish entrées and coffee ennobled in civet intestines, polar bear meat has found a place in fine dining.

The Norwegian restaurateur André Grytbakk, manager of the upscale Huset in Longyearbyen, Svalbard, occasionally dishes out polar bear steaks with potatoes or a slice of roast in red wine sauce. He also offers a bear meat snack with lingonberry pickle. As it's "a rough kind of meat," the chef recommends a heavy wine with it, such as full-bodied Bordeaux, from the Huset's 1,200-bottle cave.

The Radisson in Longyearbyen, which bills itself as the northernmost hotel in the world, even issues certificates to diners who have "eaten a (sic) polar bear entirely at their own risk." These certificates also serve as liability releases for the hotel. According to one guest, the bear meat there is boiled for six hours and fried another two, to kill parasites.

Arctic gourmet cooking remains an exception, but holidays matter up north. On Alaska's Little Diomede Island, a stormy Bering Strait outcrop near the international date line, turkeys are hard to find. Undaunted by this, the islanders celebrate Thanksgiving by serving common local fare in the village school. Like many in Alaska, these Inupiat still largely depend on the sea's bounty — blue crab and bowhead whale, seal, walrus and polar bear, which they can legally hunt. Butchered properly, a polar bear yields up to 500 pounds of meat, enough food for dozens of guests.

It is hard to anticipate how food preferences will change. In some future day, as a Montreal Gazette column from the 1950s surmised, southern Canadian cooks might be appraising polar bear cuts for steaks or bearburgers.

In that case, or if you ever find yourself at Grytbakk's Huset, don't hesitate. Bon appetit! Nigiñaqsiruq! Dig in!

Michael Engelhard is the author of the essay collection, " American Wild : Explorations from the Grand Canyon to the Arctic Ocean," and of " Ice Bear: The Cultural History of an Arctic Icon," from which this article has been excerpted. He lives in Fairbanks, Alaska, and works as a wilderness guide in the Arctic.

Where did polar bears come from?

Polar bears have a murky origin, and uncertain future. Will they end up merging with brown bears?

Polar bears are such massive, popular and iconic animals that you'd think we'd have long understood whence they came.

However, establishing the origin of polar bears has proved difficult.

We've struggled to reveal when they became the instantly recognisable white bear we know today, and still know relatively little about what happens when polar bears and their darker-coloured cousins come together, and perhaps even mate.

It has been long known that polar bears are indeed bears, belonging to the Ursids, the family of mammals that include brown and black bears, as well as others such as sloth and spectacled bears.

That may seem obvious. But there was, until quite recently, a long-standing confusion around the origin and relationship of one the polar bear's closest cousins the panda, with scientists debating whether that species was a true bear at all, until genetic studies confirmed it was.

It has been difficult to determine the origins of polar bears in part because few preserved ancient polar bear remains have been discovered.

So scientists have turned to studying the genetics of bears to establish when they diverged from each other.

One study published in 2013 suggests that pandas split from the bears anything from 8 to 38 million years ago.

Black and brown bears then split into unique lineages between 1.5 and 6.5 million years ago.

And polar bears diverged from brown bears between 130,000 and 650,000 years ago, with the general consensus that they first appeared in the Pleistocene, and must be at least 115,000 years, the date of the oldest known polar bear fossil.

Now-extinct brown bears that once lived in Ireland, for example, had polar bear ancestry, perhaps because past changes in the distribution of polar ice, for example, stranded polar bears or hybrids on the island.

Brown bears living on islands off the coast of Alaska also appear to have polar bear ancestry.

A study published in 2014 also found tantalising genetic evidence that bears living in the Himalaya mountains, a vast distance from the Arctic, may have derived from polar bears. This unique heritage could have produced bears that look and behave slightly different from the brown bears that usually live in the region, albeit at lower altitudes. And these odd, high-altitude bears may be the origin of the Yeti legend, speculate the scientists who conducted the study.

A meeting of bears

What happens when white and brown bears come together is difficult to answer, as it occurs rarely.

Polar bears do come ashore in summer, and in some regions of the Canadian Arctic brown bears have been observed wandering around on the pack ice. But generally, they live in separate habitats, and there are few recorded instances of polar bears and brown bears mating.

However, that may happen more often if the sea ice in the polar bear's natural habitat melts, forcing more bears ashore and for longer periods.

So far, there is just a single hybrid polar and brown bear known from the wild. In April 2006 a strange-looking bear was shot by a hunter in Nelson Head, on the southern part of Banks Island in the southern Canadian archipelago. A study of its genes revealed it to be a brown-polar bear-hybrid.

Precisely 17 more are known from zoos, the result of bears born to polar bears and brown bears kept together in enclosures, which subsequently mated. And scientists have studied these hybrid bears' features.

They found that polar-brown bear hybrids inherited traits from both parents. Hybrids have visible tails, like polar bears, whereas those of brown bears are barely apparent. They have longer necks more typical of polar bears, but also display small shoulder humps reminiscent of brown bears.

They also inherited blended traits. For example, in terms of overall size, they fall between the larger polar bear and smaller brown bear. The size and shape of their heads is intermediate between the thicker-set brown bear and more slender-headed polar bear.

The bears' feet are also an intriguing blend. The soles of the hybrids' feet are partially covered in hair. Polar bear feet are covered in hair to insulate them from the ice, whereas brown bears have hairless soles and clearly visible toes.

But most intriguing is the bears' hair.

When viewed as a cross section, the shaft of a brown bear's hair is either solid or full of small hollow regions, depending on where the hair is on the bear's body.

The hair of a polar bear is almost completely hollow, with large empty regions within its core. The hybrids' hair was partially hollow.

Behaviourally, the two hybrids have much in common with polar bears.

The existence of these bears proves that polar bears and brown bears in close proximity can and do mate.

What is more, as well as having a combination of features, they are also fertile.

The odds of it occurring may be low, but it raises the possibility that, in a future, warmer world with less sea ice, polar and brown bear may yet consistently breed.

That may either create a new hybrid species, or polar bears and brown bears may merge once more, repairing the split that led to the origin of polar bears in the first place.

KidZone Animal Facts The Polar Bear

Approximate worldwide winter
distribution of polar bears (light gray).
Polar bears are distributed throughout most
ice-covered seas of the Northern Hemisphere.

The polar bear or the sea/ice bear are the world's largest land predators. They can be found in the Artic, the U.S. (Alaska), Canada, Russia, Denmark (Greenland), and Norway. Each of these countries either banned hunting or established rules for how many polar bears could be hunted within its own boundaries. These rules help keep polar bear populations stable. Today, 25,000 to 40,000 polar bears roam the Arctic.

Please note: The photos on this page have come from clipart CD's which allow use on educational internet sites and in school projects or they have been contributed by viewers.
You are free to use all of it in book reports or for your personal website.

Around the age of four or five the female polar bear can start having babies. They usually only have two cubs and they have these babies in a cave they've dug in a large snow drift. They stay there over winter and come out in spring with the babies.

The babies are much smaller than human babies when they're born. They are the size of a rat and weigh little more than a pound. They can grow to full man size in a year if they have lots of food.

Pair of polar bear cubs.
photo by: US Fish and Wildlife

Male polar bears may grow 10 feet tall and weigh over 1400 pounds. Females reach seven feet and weigh 650 pounds. In the wild polar bears live up to age 25.

Despite what we think, a polar bear's fur is not white. Each hair is a clear hollow tube. Polar bears look white because each hollow hair reflects the light. On sunny days, it traps the sun's infrared heat and keeps the bear warm at 98 degrees F (when they're resting).

Polar bear fur is oily and water repellent. The hairs don't mat when wet, allowing the polar bears to easily shake free of water and any ice that may form after swimming.

Underneath the fur, a polar bear's skin is actually black -- the black skin soaks up the sun's heat and helps them stay warm.

Polar bears also have a 4 inch layer of fat underneath their skin. This prevents them from losing any of their heat. In fact, if you look at a polar bear with an infared camera, they are pretty close to invisible (in other words, they don't give off any heat!)

The smallest foot pad is the front track
and the larger is the hind track.

Polar bears have wide front paws with slightly webbed toes that help them swim. They paddle with their front feet and steer with their hind feet. Paw pads with rough surfaces help prevent polar bears from slipping up on the ice.

Polar bears have been known to swim 100 miles (161 kilometers) at a stretch.

Polar bears primarily eat seals. They often rest silently at a seal’s breathing hole in the ice, waiting for a seal in the water to surface. Once the seal comes up, the bear will spring and sink its jagged teeth into the seal’s head.

They have a special liver that allows them to process all of the seal fat they eat - seals store a lot of vitamin A in their blubber which allows them to survive and grow quickly. A polar bear's liver contains 10 times more vitamin A than any other animal on earth - their liver has evolved to allow them to process and eat all of the seal blubber they need to stay alive.

Sometimes the polar bear stalks its prey. It may see a seal lying near its breathing hole and slowly move toward it, then charge it, biting its head or grabbing it with its massive claws. A polar bear may also hunt by swimming beneath the ice.

Humans are the polar bears only predator. Baby polar bears often starve. In fact, 70 percent do not live to their third birthday. Sometimes seals are hard to find, especially in the summer when the ice has melted. All across the Arctic, man is moving in to mine oil and coal and there is less space for the polar bear to live. Oil spills can be very dangerous. A bear with oil on its coat cannot regulate its body temperature properly. If the bear eats the oil while grooming it could die.

Man made pollution is also a cause of death. At each stage of the food chain, pollutants get more concentrated. By the end when the polar bear eats the seal and it could be lethal.

Polar Bears are considered marine mammals -- just like seals, whales and dolphins. Although polar bears are related to the brown bear, they have evolved over time to live in cold northern arctic climates. Their population is on the decline and they are considered a "vulnerable" species.

photo by: US Fish and Wildlife

Protecting polar bears isn't just a matter of being nice. Their body is very different than most mammals -- they are able to process large amounts of fat (seal blubber) and are able to lose and gain large amounts of weight through the year without causing stress health problems. Scientists are studying them to see if they can provide a key to human diseases like diabetes and heart disease!

Recommended reading

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Skin, Ears, & Tail: Staying Warm

To keep them warm, polar bears have black skin over a thick layer of fat that can measure up to 11.4 centimeters (4.49 inches).

In the water, they rely more on their fat layer to keep warm wet fur is a poor insulator. This is why mother bears are reluctant to swim with young cubs in the spring — the cubs just don't have enough fat.

Their skin isn’t the only thing working to keep them warm — their ears are small and round, and their tails short and compact, to conserve the most heat possible.

Natural history

Ursids are mainly animals of northern temperate regions and are found farther north than any other mammal. The Arctic fox is found as far north on land, but the polar bear regularly roams on sea ice hundreds of kilometres from shore. Africa and Australia lack bears entirely. The spectacled bear of the South American Andes Mountains is the only species that lives south of the Equator.

Although clumsy in appearance, bears can move surprisingly fast, even through dense cover that would seriously impede a human or a horse. Their senses of sight and hearing, however, are poorly developed, and most hunting is done by smell. Some, such as black and spectacled bears, are strong climbers, and all are strong swimmers, most notably the polar bear. Bears do not generally communicate by sound and usually are quiet, but they do growl at times when feeding, when being challenged by another bear or by humans, and when competing for mates.

Except for the carnivorous polar bear and the vegetarian giant panda, ursids are omnivorous, consuming many items that seem small for an animal of such large size. Ants, bees, seeds of trees, roots, nuts, berries, insect larvae such as grubs, and even the dainty dogtooth violet are eaten. Many bears relish honey, and the sun bear is sometimes called the “honey bear” because of this. Prey taken by bears include rodents, fish, deer, pigs, and seals. Grizzlies (North American subspecies of the brown bear, Ursus arctos) are known for their skillful fishing during the spawning runs of salmon. The polar bear’s diet is dictated by the Arctic environment, as little vegetation grows within its range. The Asian sloth bear (Melursus ursinus) delights especially in raiding and destroying termite nests, sucking up termites and larvae with its funnel-like lips. The giant panda has a special bone formation of the forefoot that functions as a sixth digit it is opposable to the other five and thus is useful in handling bamboo.

Most bears, including the American and Asiatic black bears (Ursus americanus and U. thibetanus), eat large amounts of food before entering a den for a period of deep sleep during the winter. The polar bear digs a den in the snow, whereas grizzlies build large mounds of dirt in front of their dens. Bears, however, lack the physiological characteristics (lower heart rate, body temperature, breathing rate, and blood pressure) exhibited by animals that truly hibernate.

Male polar bears sometimes aggregate otherwise bears are solitary, except during the mating season. Then they tend to congregate, pair off, and mate in seclusion. The male leaves the female soon after mating and plays no role in raising the young. Gestation periods vary, the fertilized egg remaining dormant in the uterus ( delayed implantation), which ensures the birth of young while the female is in the winter den and guarantees that the cubs will emerge from the den in the spring, when food is abundant. Ursids breed once per year at most, and many bears breed only every two to four years. The breeding season is usually in late spring or early summer. Delayed implantation results in most births occurring in January or February. Newborn bears weigh about half a kilogram (one pound) and are about 23 cm (9 inches) long from the nose to the tip of the short tail. Twins are most common in bears, but up to five young may be produced. The cubs nurse for a few months and stay with the female until the next breeding (about a year and a half or more after birth). Most young, however, can get along on their own by about six months of age. Bears reach breeding condition at three and a half to six years of age, males usually maturing later than females. Longevity of bears in the wild ranges from 15 to 30 years, but in captivity they can live considerably longer.

Because of their large size, bears have few natural enemies in the wild. Most mortality occurs because of hunting by humans. On occasion, bears that fail to accumulate enough fat to last throughout the winter may die of starvation. Young bears are more vulnerable to predation because of their smaller size and thus may be killed by other carnivores such as wolves or cougars but most importantly by other bears, especially males. For this reason, females with cubs are highly protective of their young in the vicinity of males.

Home ranges occupied by individual bears vary in size depending on the abundance of food, and larger areas are used when food is in short supply. Although highly variable among geographic areas and even among seasons, American black bears roam areas of 40 to 200 square km (15 to 77 square miles), grizzlies about 300–700 square km. Some polar bears trek across ranges of more than 125,000 square km (48,000 square miles).

Animal Facts: Polar bear

The polar bear's Latin name, ursus maritimus, means “sea bear.” It is the only bear that is considered a marine mammal because it depends upon the marine environment for survival. It is the largest land carnivore in North America. Its long body, neck and skull distinguish it from other types of bears.

The polar bear is well adapted to life in the extremes of the Arctic. Its distinctive white coat acts as camouflage in the snow and ice. That’s important for polar bears as it makes them hard to see when they are stalking seals or trying to hide from hunters. Meanwhile, the soles of the polar bear's feet have small bumps and cavities. These provide suction to prevent the bear from slipping on the icy terrain.

Polar bears spend most of their lives on sea ice, which they use as a platform to hunt their favorite food: ringed seals. A keen sense of smell is key to the polar bear’s success as a hunter, and these bears can detect a seal's breathing hole in the ice from up to a kilometre away.

In Canada, polar bears can be found from James Bay to northern Ellesmere Island, and from Labrador to the Alaskan border. Churchill, Manitoba, on the western coast of Hudson Bay, is one of the three largest polar bear maternity denning areas in the world. Canada is one of five "polar bear nations," along with the United States (Alaska), Russia, Denmark (Greenland) and Norway.

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