Today's News
Today's News

Fall's Journey South
Fall's Journey South
Report Your Sightings
Report Your Sightings
Teacher's Manual
Teacher's Manual
Search Journey North
Search Journey North
return to:
JNorth Home Page



Alaska's Polar Bears Overdue at Maternity Dens, Scientists Still Expecting Them

Photo credits USFWS

November 24, 1997
"I've just gotten back from nearly a month of field work with the polar bears on Alaska's North Coast," reports Steve Amstrup, polar bear biologist with the U.S. Fish and Wildlife Service. ãWe generally find the females moving into their dens by now. We have five bears outfitted with satellite collars to track their movements. This year, none of our collared bears, or any others that we know about, has moved into their dens. This is already more than two weeks behind schedule."

Steve believes that this has been one of the most unusual field seasons of his career. Unseasonably warm weather has kept near-shore ice from forming. Typically, polar bears which spend the summer and fall on large, off-shore pack ice will make use of the near-shore ice flows to move into the coastal waters to hunt, or onto the land to build dens.

In addition to the absence of near-shore ice, there has been lower than average snow fall and therefore little accumulation of the drifting snow which polar bears use to build their dens. Steve is concerned that these conditions may be altering the polar bear's denning behavior. While he can not be sure, Steve wonders if El Nino may be the cause of these unusual weather patterns.

Migration On Ice

Alaska's North Slope
(Click map to enlarge)

Two U.S. populations of polar bears live at least part of their lives in coastal areas of Alaska. These are the Beaufort Sea population and the Chukchi/Bering Sea population of polar bears.

The polar bears in the Beaufort Sea population, which Steve monitors, total nearly 1,800 and live on off-shore pack ice in the Beaufort Sea. In the summer, these bears concentrate on pack ice in the central regions of the Beaufort Sea, while in the winter they radiate to the outer edges where the pack ice is more active and food more plentiful. Occasionally, Beaufort Sea polar bears will venture on shore, sometimes into the Arctic National Wildlife Refuge, to hunt and den. The majority of polar bears which build their dens on shore do so in the Arctic National Wildlife Refuge.

The approximately 2,000 individuals in the Chukchi/Bering Sea polar bear population will range as far as Russia's far east to breed in the spring. When the weather begins to cool in autumn, the pack ice advances in a southerly direction carrying these polar bears into the Bering Sea which separates Russia and Alaska. In the spring, when the weather begins to warm, these bears will migrate northward on pack ice into Russia's Chukchi Sea.

To Den Or Not To Den
During the month of November, female polar bears are busy searching out snow drifts in which to build a comfortably-sized 6'x 10â x4' den (or "igloovikus" in Inuit). Approximately 50% of the female polar bears in the Beaufort Sea population will build their dens on shore distributed across Alaska's North Coast. The other 50% will den on the large off-shore pack ice. Unlike other bear species, in which both males and females hibernate for long periods in dens, only female polar bears move into dens to hibernate. Why do you think this is so?

Other bear species, including grizzly, brown, and black bears, live in more temperate ecosystems. During the winter in these climates, it often takes more energy for bears to forage for food than there is energy in the food the bears find! Therefore, as a strategy to cope with lack of food, bears adapt by going into dens and lowering their metabolism. They enter into a period of deep sleep known as ãhibernation.ä

In contrast, polar bears can take advantage of abundant food supplies from the Arctic Ocean environmentyear around. There is no need to hibernate to compensate for lack of food. Female polar bears, however, do require a period of hibernation in order to conserve fat reserves during the later months of pregnancy with their cubs. Therefore, denning in polar bears is strictly a reproductive, rather than a feeding, strategy.

Photo credit: USFWS

Growing Up In The Arctic
In December, mother polar bears give birth to one or two tiny cubs. At birth, polar bear cubs are not much larger than a squirrel and weigh less than two pounds. Mothers will nurse the cubs inside the warm den for three months to allow the cubs to grow strong enough to enter the rugged arctic conditions outside the den. Both mother and cubs emerge from their dens in late-March and April.

By the time she finally comes out of the den, a mother polar bear has fasted for up to five to six months. She must immediately find food to replenish herself and to feed her hungry cubs. During this time of year, the vast expanse of ice still stretches as far as the eye can see and temperatures still dip well below 0 Fahrenheit (-17.8 C.). How can mothers possibly find enough food to support themselves and their cubs?

The Beaufort Sea is among the deepest and coldest ocean bodies on Earth. However, the areas directly over the outer continental shelf, which stretches underwater off the North Coast of Alaska, and also the juncture at which the continental shelf drops off into the deep ocean, support relatively abundant marine life. Bottom-to-top circular movements in the ocean produce currents called "upwelling." Upwelling stirs water and plankton from the lower reaches of the ocean to the surface levels. Plankton are microscopic marine organisms and are the building blocks of the marine food chain. These microorganisms are food for many fish species which, in turn, provide food for a wide diversity of other Arctic species including marine birds, seals, and walruses. On top of the ice, and on top of the food chain, sit these hungry polar bears.

Oil Tanker

Mapping Dens
Even in the remote Arctic habitat, polar bears face a number of dangers including increased construction of roads, refineries, and port facilities. Scientists question whether increased development related to oil production in Alaska's North Coast areas will impact polar bears.

Steve will return to the North Slope in late January to continue his research on polar bearsâ movements into their dens. Once the five polar bears carrying satellite collars have moved into their dens, he will fly over the area of these dens in a plane carrying an infrared sensor. The body heat of the collared bears in the dens will produce heat. Steve will test whether the infrared sensor on his plane will detect the heat coming from the dens. If it does, he will try to differentiate the pattern of infrared wavelengths coming from the polar dens from other heat sources on the ground. In so doing, he hopes to identify the locations of other polar bear dens. A map of these den locations will provide important information in planning future construction of oil facilities and other development.