Except for mating and caring for their young, grizzly bears in primarily lead solitary lives, spending most of their time foraging, or looking for food. The grizzly is North America 's largest omnivore, meaning it eats both plants and animals. About 80-90% of the grizzly's food is green vegetation, wild fruits and berries, nuts, and bulbs or roots of certain plants. Grizzlies also eat a great deal of insects, sometimes tearing rotten logs apart and turning over heavy stones in search of the insects or their larvae.
The remaining percentage of food is meat and that mostly comes from animal carcasses, or carrion, of big game animals. However it will sometimes prey on elk or moose calves or smaller mammals. The grizzlies along the west coast of Canada and in Alaska have salmon which is an important food source. In Yellowstone National Park , spawning trout also play a crucial role in the diets of some bears. Bears are also inclined to find other sources of nutrition including army cutworm moths but have traditionally eaten pine nuts or seeds from white bark pine trees as important sources of food. This has decreased due to disease that heavily impacts these plants.
During the six months of winter in its den, it is crucial that grizzlies must eat enough to store huge amounts of fat needed to sustain it. The grizzly's ability to eat large quantities of rich food and store fat without suffering from heart disease or cholesterol problems is of great interest to medical scientists. By determining how this is accomplished, scientists could find information that may be useful in preventing human heart disease.
Adult grizzly bears have little to fear from other wild animals as they are at the top of the food chain. Their only enemy is man where habitat loss and mortalities caused by humans are the greatest threats to the bear's survival today.
Hibernation
Interestingly enough, grizzly bears begin looking for a proper place to dig their dens, and may travel many miles before finding a suitable area in the early fall. Generally they seek a high remote mountain slope where deep snow will lie until spring to serve as insulation. They often dig beneath the roots of a large tree to create their dens by chewing up obstructing roots and thrusting loose rocks and earth through the narrow entrance by its powerful strokes.
By October or November, the grizzly will enter its den and will get no water or nourishment of any kind but will use up its accumulated fat for the next 5 to 6 months. However, some grizzlies have been observed throughout the year, including during severe winters, if there is sufficient food to maintain them. In particular, grizzlies have been observed in the North Fork of Glacier National Park during winter for the last several years. They seem to be following wolf packs and mountain lions in remote regions, using those species as "providers" for meals of deer and elk.
By March or April, male grizzly bears usually emerge from the den while females emerge in late April and May. The first food it eats is sometimes carrion from animals that did not survive the winter. A grizzly bear will usually travel to lower elevations to reach vegetated, snow-free areas. This can bring the bear into direct conflict with humans who traditionally build their homes in lower elevations, along the very creeks and rivers that bears need to visit in order to find spring "green-up".