Grizzly (brown)Â bears are typical of all bears physiologically, behaviorally, and ecologically. They are slow growing and long-lived (20 to 25 years). Their ability to store and use fat for energy makes long denning periods (5 to 7 months) possible. During denning they enter a form of hibernation in which their respiration rate (approximately 1 per minute) and heart rate (as low as 10 beats per minute) are greatly reduced. Their body temperature remains just a few degrees below normal; they do not eat, drink, defecate, or urinate, and their dormancy is continuous for 3 to 7 months. The adaptive value of winter denning relates to survival during inclement weather, when reduced food availability, decreased mobility, and increased energy demands for thermoregulation occur.
In most populations, brown bears breed from mid-May to mid-July. Both males and females are polygamous, and although males attempt to defend females against other males, they are generally unsuccessful. Implantation of the fertilized ova is delayed until the females enter their dens, from late October to November. One to three (usually two) cubs are born in January in a rather undeveloped state. They require great care from their mothers, which leads to strong family bonding and transfer of information from mothers to offspring. Brown bears may not produce young until 5 to 6 years of age and may skip 3 to 6 years between litters. Because of their low reproductive potential, bear populations cannot respond quickly to expanded habitats or severe population losses.
During the breeding season, male and female grizzly bears spend considerable time together, and family groups break up. The young females are allowed to remain in the area, taking over a portion of their mother’s range. They are not threatened by the males, even though they are still vulnerable without their mother’s protection. The young males, however, must leave or be killed by the adult males. Many subadult males disperse into marginal bear habitats while trying to establish their own territories. This often leads to increased human-bear conflicts and the need for management and control actions.
Home ranges vary in size, shape, and amount of overlap among individuals. Abundance and distribution of food is the major factor determining bear movements and home range size. Home ranges are smallest in southeastern Alaska and on Kodiak Island. The largest home ranges are found in the Rocky Mountains of Canada and Montana, the tundra regions of Alaska and Canada, and the boreal forest of Alberta. In areas where food and cover are abundant, brown bear home ranges can be as small as 9 square miles. Where food resources are scattered, the ranges must be at least ten times larger to provide an adequate food base.
Some bears establish seasonal patterns of movement in relation to dependable high-calorie foods sources, such as salmon streams and garbage dumps. Such movements are likely to place bears in close contact with humans. In addition to finding food, bears spend considerable time in attempting to detect people, evaluating situations, and taking corrective actions to avoid conflict with humans. People, on the other hand, typically go noisily about their business, often without ever knowing that a bear is nearby.