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College of Forestry and Conservation

Grizzly Bear Biology

Species Description

Grizzly bears (Ursus arctos horribilis) belong to the Order Carnivora and Family Ursidae. The grizzly bear is a member of the brown bear species (U. arctos) but the subspecies U. a. horribilis is only found in North America.

Grizzly bears are generally larger and more heavily built than other bear species. Grizzly bears can be distinguished from black bears by longer, curved claws, humped shoulders, and a face that appears concave. A wide range of coloration from light brown to nearly black is common. In the lower 48 States, male grizzly bears generally weigh between 400 and 600 lbs whereas females usually weigh between 250 to 350 lbs. Grizzly bears typically live to be around 25 years old.

Identifying Bears: Telling the difference between a grizzly bear and a black bear can be tricky. Size and color alone are not sufficient to make a positive identification. Learn how to tell the difference between black bears and grizzlies then test your knowledge at the Montana Fish, Wildlife, and Parks Bear Identification Website


Grizzly bears are opportunistic omnivores that will eat fish, berries, grasses, leaves, insects, roots, carrion, small mammals, fungi, nuts, and ungulates.

Grizzly bears have home ranges (not territories) and they will share this habitat with other grizzly bears although social systems influence movements and interactions among resident bears. Home range sizes for adult female grizzlies vary from 50 to 150 square miles whereas an adult male can have a home range size of up to 600 square miles.

Grizzly bears in the lower 48 States spend 4 to 6 months in dens beginning in October or November. During this period, they do not eat, drink, urinate, or defecate. Over the course of the denning season, a bear may lose thirty percent of its body weight. All of this weight is stored as fat which is acquired during the 2 to 4 months prior to entering dens during a period called hyperphagia. During hyperphagia, bears increase their food intake dramatically and may gain as much as 3.64 lb/day.

Grizzly bears typically avoid humans. However, grizzly bears can fiercely protect themselves, their young, and their food when they feel threatened. If grizzly bears learn to seek human sources of food such as garbage, pet food, bird seed, vegetables, or fruits; they may lose their tendency to avoid humans and become aggressive instead.

Reproductive Biology

Mating occurs from May through July, with a peak in mid-June. The age at which females produce their first litter varies from 3 to 8 years old with litter size varying from 1 to 4 cubs. Cubs are born inside the den in late January or early February and remain with the female for 2 to 3 years. Grizzly bears have one of the slowest reproductive rates among terrestrial mammals. Grizzly bear females cease breeding successfully some time in their mid- to late 20s.


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