This report includes my observations on grizzly bears (Ursus arctos L.) in Mount McKinley National Park from 1922 to 1970; studies were most intensive from 1959 to 1970.
Grizzlies range throughout the park, but favor particular areas where food is abundant. Density in a 400 square mile area along the road where most work was done was estimated at one to two bears per 10 square miles. Mean litter size was 1.85 for spring cubs and 1.70 for all age-classes of cubs.
Home ranges were documented for 2, 3, or 4 years for a number of bears, primarily families that I recognized from year to year based on characteristics of females and cubs. Bears tended to occupy the same general area every year. Observed ranges, usually 5 to 12 miles in length and 1 to 5 miles wide, do not represent total home ranges because rough terrain limited observability. Bears occupy different portions of their home ranges as food availability and food habits shift from season to season. Home ranges overlap extensively and territoriality was not evident. A sort of "peck order" based on size, and perhaps reproductive status and past experience, determined the outcome of encounters between bears. Ordinarily, bears avoid close proximity to others.
The breeding season extends from mid-May to mid-July, with a peak in June. In spring, males wander widely in search of receptive females. A male attends one, or occasionally two, females for 1 to 3 weeks. Initially, females are intolerant of males, often attempting to evade their attentions, but later become tolerant and permit the male to mount. The minimum breeding interval for females is 3 years, but is usually at least 4 years. Presumably, cubs are born in January and February. They remain with their mother until 2-1/2 years of age, continuing to nurse into the spring and summer of their third year. Occasionally, a single cub stays with its mother into its fourth summer of life. Breakup of the family usually was initiated by the mother. After separating, twin and triplet cubs often remain together, at least in loose association, for up to three summers.
Grizzly bears are omnivorous, but rely mainly on a vegetarian diet that changes as summer progresses. During May and early June, digging for roots is the predominant feeding activity. Bears graze on grasses and herbs in late June, July, and to some extent in early August. Berries become a major food in August, and rooting activities increase again, especially in years when berry crops are poor. In September, digging for roots and ground squirrels are the most frequent feeding activities. Carrion is eaten whenever available, and bears occasionally capture young calves of moose and caribou in early summer. A large carcass often attracts several bears, but the largest bear in the area has priority.
Grizzly bears are potential or actual predators on a number of mammals sharing their range. Caribou and moose are wary of bears during their calving periods when bears actively prey on newborn animals. Caribou calves soon mature enough to outrun grizzlies, and caribou herds then pay less attention to passing bears. Cow moose with calves are usually able to defend their offspring from bears. Dall sheep are not vulnerable to bear predation most of the time when in their usual rugged and rocky haunts. During short migrations across valleys from winter to summer ranges, ewes and lambs are more subject to predation; bears occasionally catch a sheep then, usually by surprise in gentle terrain.
Of the smaller mammals, only ground squirrels are captured routinely by grizzlies. Bears are always alert for opportunities to surprise a ground squirrel away from its burrow, and in the fall may concentrate on digging them out for days at a time. Marmots and beaver rarely are captured. Porcupines are well protected against bears; their quills can cause temporary lameness to imprudent bears.
Bears meet a variety of other animals at carrion. Magpies and ravens obtain a small share with little problem. Wolves, however, have little chance to feed at a carcass if a bear is present, but are able to take their turn after a bear has temporarily had his fill.
Wild grizzlies in McKinley National Park, conducting their affairs undisturbed, are the essence of wilderness spirit.
Last Updated: 06-Dec-2007