Grizzly bears enter their dens sometime during the first two weeks of October and remain there until mid-April in the case of most adults or mid-May in the case of sows with newborn cubs. The bears spend about half of the year avoiding the environmentally stressful winter period. Within ANWR, most dens are located on south-facing slopes in the mountains south of the Coastal Plain, where prevailing winds favor the accumulation of an insulating layer of snow. In contrast with den sites in more southerly areas, bear dens in ANWR are not surrounded by shrubby vegetation with roots that stabilize the soil. Because the soils tend to be coarse textured and poorly bonded, dens cannot be dug until the ground has frozen to a depth of about four inches. Dens in ANWR usually collapse with the spring thaw and cannot be reused in subsequent years.
Alaska polar bears spend little time on land. Throughout most of the year, most Beaufort Sea polar bears are closely associated with sea ice. Sea ice is the habitat of ringed seals which are the predominant prey of polar bears. Polar bears also prey on bearded seals and occasionally belugas and walrus. While some bears become stranded onshore during summer when the shore ice melts and pack ice blows far offshore, the bears generally stay with the ice when it retreats in the summer. In the fall, some bears move near shore to feed on remains of beached carcasses, especially of bowhead whales taken by whalers.
In late October or November, some of the pregnant females move near shore to dig dens in deep snow drifts on the fast ice, or on barrier islands and land, while the majority den on the drifting pack ice. The pregnant females give birth to one to occasionally three cubs in December or January, and remain in the dens until April. Mothers with new cubs remain near the dens for up to a couple of weeks, before venturing onto the sea ice to hunt seals. Cubs remain with their mothers for 28 months.
Wolves are widely distributed throughout Alaska and much of Canada. They are the largest wild members of the dog family, and adult males often weigh more than 100 pounds. They have a highly developed social structure which is manifested in the pack which is made up largely of family members. Within the pack, breeding is confined to the dominant male and the dominant female.
Wolves obtain most of their food by preying on large ungulates (caribou, Dall sheep, moose), particularly caribou. In some instances it is thought that wolf predation, particularly on calves, has been sufficient to cause caribou populations to decline. Part of the explanation for rapid growth of the Central Arctic Herd and high early calf survival of the Porcupine Herd may be the relative scarcity of wolves on their calving grounds.
With their squat silhouette and shaggy pelage, musk oxen look like a relic from the Ice Age. Their habit of forming a defensive circle in the face of danger resulted in near extinction of the species by hunters with guns; the animal’s instinctive defense mechanism protects them from wolves, but not from bullets. Musk oxen formerly were found throughout arctic Alaska, but they were wiped out after firearms were introduced into ANWR during the mid-1800s.
In 1930, the U.S. Fish and Wildlife Service purchased 34 musk oxen from Greenland and transported them to Nunivak Island just off the west coast of Alaska in the Bering Sea. The animals prospered there in the absence of predators or competitors for forage. In 1969 and 1970, musk oxen were transplanted onto the Coastal Plain. Since that time they have established themselves and the population has grown rapidly. In 1972, there were about 40 musk oxen and in 1985 there were nearly 400, divided among three herds.
In recent years musk oxen numbers on the Coastal Plain appear to have stabilized, but the population continues to expand to the east and west. At least 150 musk oxen now live in the northern Yukon Territory and approximately 200 between the Canning River and Prudhoe Bay. Musk oxen are also spreading to the west of Prudhoe Bay and even to the south side of the Brooks Range.
Although moose are commonly associated with the boreal forests, they range as far north as the Beaufort Sea, making them one of the most widely distributed large mammals in Alaska. Willow is a preferred food plant, and consequently moose are most common along the major river systems where willows are abundant. However, moose are rarely found in the foothills and on the Coastal Plain of ANWR.