Polar Bears - Natural History


Text by Liz McKenzie      Photos and Sounds by Richard Nelson


The Sea Bear

Imagine a creamy-white mountain moving gracefully across a frozen sea.

Imagine a land mammal morphing over time into a marine mammal—now with a thick insulating layer of fat for protection against frigid arctic cold, and hollow hairs that trap air to keep it warm and buoyant in freezing water.

Imagine wide furry paws that act like paddles in the water and like snow shoes on ice and snow. Imagine an animal so crafty that it waits motionless for hours beside a seal hole, slowly lifting its paws one at a time to warm them.

Imagine an animal that moves and weaves behind ice jumbles, and each time a seal raises its head the animal stands frozen in place until the seal sleeps again. Then it patiently stalks closer, halting again and again, until it's near enough to rush the seal and kill it.

This animal is the polar bear—Ursus maritimus—one of most remarkable creatures that has ever lived.

Master of the Frozen Sea

Polar bears are big—the largest "land" mammal on earth, a distinction they share with their closest relative, the coastal brown bear. Adult male polar bears are 8 to 10 feet long and weigh 600-1200 pounds, with some very large bears weighing more than 1700 pounds. Adult females weigh 400-700 pounds.

But this is where the similarities end. Polar bears are built for life on the sea ice.

While white fur would be a distinct disadvantage on land, the polar bear's fur conceals it from seals and other prey in a perpetually frozen world. And this camouflage also makes polar bears more difficult to spot by their only predator, man.

Their fur consists of long, glossy guard hairs which are hollow and water repellent, providing excellent insulation and buoyancy; and underfur that is dense and warm.

Polar bears have white fur, but the outer hairs lack pigment so they can appear yellowish when the sun is low on the horizon.

A layer of fat—up to 4 inches thick—insulates against bitter cold, blizzards, and freezing water. This fat also helps the bears to survive during hibernation—which only pregnant females of this species do—and helps them survive in times of food scarcity.

A polar bear's long sleek body is shaped for life in and around the water. It also has a very long neck, so the bear can raise its head further out of the water compared to brown bear relatives.

Not surprisingly, polar bears are excellent swimmers. They paddle with their broad, partly webbed front paws, while their hind legs trail behind, acting like rudders.

Their large furry paws help to distribute their weight on the ice, to give better traction, and to allow quiet stalking on snow or ice.

Polar bears are exquisitely skilled at moving over thin ice by spreading their legs to distribute their weight, even splaying out and "swimming" on the rubbery surface. Unlike fresh water ice, which cracks and shatters, sea ice is more flexible—unless it is so thin that the bear breaks through and has to swim.

Polar bears' teeth are another evolutionary change from those of brown bears. No longer do they have teeth suited for chewing grasses and other vegetation, but rather, they have large canines designed for grabbing prey and tearing flesh. And this makes perfect sense because the polar bear's diet is comprised mainly of ringed seals, and to a lesser degree, bearded seals.

A Diet Rich in Fat

To maintain a thick layer of fat, polar bears have a much higher dietary fat requirement than their terrestrial relatives. Ringed seals provide exactly the kind of rich food these bears need to survive.

The highest concentrations of ringed seals are found near the sea ice edge and along broad open cracks called leads. This is where seals can haul out to rest and sometimes to give birth, but they can easily slip into the water to escape danger.

Polar bears also hunt bearded seals which live around the pack ice. These large seals can reach up to 750 pounds, compared with about 150 pounds for ringed seals.

Polar bears occasionally take walrus pups in the summer, but they have to confront the huge and formidable adults that will use their tusks as defense. In recent times, bears have also hunted walrus that haul out onto remote beaches in the Russian arctic. When the panicked walruses stampede, some members of the herd may be trampled or crushed to death.

Other occasional prey includes beluga whales that have become trapped at narrow cracks in the sea ice. Also, polar bears have been seen swimming underneath seabirds, then rising up, and trying to snatch them from below.

On land, polar bears sometimes catch ground squirrels and lemmings. They will also eat bird eggs, plants and berries. However, these small meals are not high enough in fat to sustain them.

Polar bears have a highly developed sense of smell and can detect carrion many miles away. Inupiaq people often say the bears can smell remains of killed animals from distances "up to fifty miles."

Polar bears have also learned to come ashore during and after the fall whaling season in remote coastal villages, to scavenge on the remains of bowhead whales and belugas. This can cause conflicts because polar bears are mostly solitary animals except when they are mating or when females travel wtih their cubs. In recent years, bears have gathered around whale carcasses or come into close contact while waiting on shore for the sea ice to develop. At these times, there are big arguments and scuffles accompanied by outbursts of angry growling and bellowing.

Listen to bears growling at a bone pile

In times of scarcity, active polar bears can slow their metabolism to conserve energy and use less of their fat reserves. Brown bears and black bears can only do this during the long months they spend inside their winter dens.

Hunting on Ice

Hunting on the ice is where polar bears most brilliantly display their intelligence. Seals are fast and agile swimmers, and if startled on the ice, they quickly dive into the water—a huge challenge for a predator.

Over thousands of years, polar bears have mastered the art of hunting arctic seals. In the winter, ringed and bearded seals maintain tunnel-like breathing holes through the ice. A polar bear will wait motionless for many hours beside one of these holes, and when the seal comes up for air, the bear crushes its skull with one powerful swat and pulls the dead animal out onto the surface.

In the spring, pregnant seals come up through their breathing holes and dig caves in the deep snowdrifts, where they give birth and nurse their pups. The polar bear wanders across the ice until it finds their scent, and then crashes through the cave roof in hopes of catching the pup inside.

In spring and summer, seals come out onto the ice to bask and sleep in the warm sunshine. Polar bears carefully stalk the seals, using piles of broken ice for concealment, creeping on their bellies to cross flat stretches, stopping whenever the seal lifts its head, then moving again until they are close enough to sprint for the kill.

Love and Family

From late March to early May, male polar bears roam widely over the frozen pack ice, following the tracks and scents of females. When he finds a female ready to mate, the bear stays with her for a few days, mating multiple times. Then he leaves her to search for other eligible females.

In the early winter, a pregnant female looks for a large, deep, hard-packed snowdrift—either on the ice or on land not far from the coast—and there she digs a cavelike tunnel with an oval-shaped chamber at the end. Snow is a good insulator and temperatures inside the den drop no lower than a few degrees below freezing, even when it's minus 50 degrees outside.

As arctic temperatures have become warmer, increasing numbers of polar bears are giving birth on land, apparently because the ice has become less stable and dependable.

After about two months in the den—usually in early December—one to three cubs (usually two) are born inside this snug shelter. At birth the cubs are furless, blind, and very small—weighing only about one-and-a-half pounds. They have only a minimal fat layer and could not survive outside the den. But they grow rapidly, nursing on their mother's milk which is the richest in the world—nearly 50% fat.

Mother and cubs will leave the den in March or April, when the cubs are about 3 months old and weigh about 20 pounds.

While males and non pregnant females have spent the winter on the ice eating seals, a mother with cubs has been without food all winter—while she has also nursed her cubs.

The female has lost about half of her body weight and she has a tough job ahead: she must find food for herself, while also keeping her little family safe, teaching them to swim and hunt, and continuing to nurse the hungry cubs.

After the first year, if all goes well, the cubs will be good hunters themselves. The mother bear continues to nurse during the second year, although the milk is not as rich. She will wean the cubs when they are a little over two years old.

Polar bears only have a litter every three years at most--a very low growth rate. Growth of the fetus is delayed after mating and in lean times a pregnant female will reabsorb or abort the fetus. In her lifetime, a female will only have about five litters.

Conservation

Polar bears live throughout the arctic circumpolar regions where sea ice is found during most of the year. Biologists estimate there are 22,000 to 25,000 polar bears worldwide, divided into nineteen distinct populations. Two of these groups live on the ice-covered waters off Alaska—the Beaufort Sea and the Bering-Chukchi Sea populations.

The greatest threat to polar bear survival today is diminishing sea ice due to climate change.

Bears spend most of their time living and hunting on the ice. They come ashore to scavenge beached animals and sometimes to give birth in dens. Otherwise, the polar bear is a creature of the sea ice—in fact, there simply is no other place where these animals can live.

There has been a dramatic decline in arctic sea ice over the past 10-15 years caused by rising temperature. An overwhelming majority of experts conclude that these rising temperatures are caused by increasing greenhouse gasses.

If current trends continue, scientists predict the summer sea ice in the arctic could be gone by the year 2020 and with it, two-thirds of all polar bears. These remarkable animals would become extinct in Alaska and throughout most of the arctic. The only place in the world that they are likely to survive is among the Canadian high arctic islands and on the surrounding polar sea.

Learn more about polar bears and climate change.

Other threats to polar bears include loss of denning sites along arctic coasts because the females are sensitive to disturbance by oil production and other industrial activities. Offshore drilling could also drive bears away, and oil spills would have severe impacts on these animals and on the entire arctic marine environment.

Since the passage in the U.S. of the Marine Mammal Protection Act in 1972, only indigenous people are allowed to hunt polar bears.

In 1973, the International Agreement on the Conservation of Polar Bears and Their Habitat—designed to limit hunting of polar bears and protect their habitat—was signed by all countries within the range of polar bears (the United States, Russia Canada, Norway, Denmark and Greenland).

The United States is the only one of these countries that does not have a designated sanctuary for Polar Bears, although polar bears are currently listed as "Threatened" under the United States Endangered Species Act.

In 2010 an agreement between the US and Russia reduced the number of bears taken annually by indigenous people. In early 2011, Russia announced that no polar bears would be taken in that country this year.

The Sea Bear's Extraordinary Evolution

Paleontologists have learned that polar bears evolved from brown bears about 250,000 years ago. Over this prodigious span of time, they have perfected one of the most extraordinary adaptations of any creature that has ever lived—from land mammal to marine mammal. And not only that—they make their living on the vast jumbled "continent" of sea ice.

Polar bears thrive despite months of midwinter darkness, despite unimaginable and relentless cold, despite shuddering gales and relentless storms. They swim without hesitation during subzero blizzards; they hunt with almost preternatural cunning; and they call the perpetually moving ice home.

Few predators on earth can live in such extreme conditions, hunt with such crafty intelligence, or move with such grace over the frozen world.

Learn more about polar bears and traditional knowledge.
Learn more about polar bears and climate change.

For further reading:

The Last Polar Bear: Facing the Truth of a Warming World by Steven Kazlowski
Hunters of the Northern Ice by Richard Nelson

For listening:

Polar Bear Natural History
Polar Bears and Climate Change
Polar Bears: Inupiaq Native Knowledge
Russian Polar Bears
Patch Dynamics
Ice Algae

For Viewing:

PBS Nature Bears of the Last Frontier: Part 3 Arctic Wanderers

Important links:

US Fish and Wildlife Service
North Pacific Research Board
National Science Foundation
Polar Bears International
National Snow and Ice Data Center
Alaska Wildlife Notebook Series
Alaska Department of Fish and Game

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