The Grizzly Bear (Ursus arctos horribilis) is a highly adaptable subspecies of brown bear, thriving across diverse North American landscapes. Classified as a true omnivore, its survival depends on consuming a wide spectrum of nutritional sources, from insects to large mammals. The bear’s diet is opportunistic and constantly shifts to exploit the most abundant resources, preparing it for winter hibernation. This flexible approach allows the Grizzly to persist in habitats ranging from dense forests to high alpine meadows.
The Foundation: Primary Vegetation Sources
Despite the Grizzly Bear’s reputation as a powerful predator, the bulk of its diet by volume is often composed of plant matter. In some areas, vegetation can constitute up to 90% of the bear’s total food intake, particularly during the spring and early summer. Bears seek out new growth like fresh grasses and sedges as soon as they emerge after the snowmelt, providing fiber and easily digestible carbohydrates.
Grizzlies also rely heavily on subsurface plant materials, using their powerful claws to excavate roots, tubers, and bulbs. Specific examples include the starchy roots of the yellow hedysarum and the biscuitroot, which are dug up to access concentrated nutrients. As the season progresses into summer, the focus shifts to various berries, providing a significant source of sugar and energy. Huckleberries and buffaloberries are particularly important, consumed in vast quantities to begin fat storage.
Protein Sources: Aquatic and Terrestrial Prey
While plants provide bulk, animal matter delivers the fat and protein required for growth and muscle mass. The Grizzly Bear’s protein sources are diverse, starting with small invertebrates readily available across the landscape. They consume large numbers of insects, such as ants, grubs, and the calorie-rich army cutworm moths, which they gather in high-elevation colonies.
Small burrowing mammals also represent a common protein source, including voles, marmots, and ground squirrels. The bear rapidly digs up rodent burrows to access both the animals and their stored food caches. Larger prey is typically taken as calves or weakened individuals, such as newborn elk, moose, and deer, which are vulnerable targets in the spring. Grizzlies are effective scavengers and consume significant amounts of carrion, including winter-killed ungulates or the leftovers from other large carnivores.
The most energy-dense animal food source for many populations is fish, particularly the seasonal spawning runs of salmon in coastal regions. Salmon provide a high concentration of fat and protein, delivering an immense caloric boost that inland populations often lack. This rich marine diet is directly responsible for the larger size of coastal brown bears compared to their interior grizzly counterparts. A single salmon provides the bear with essential omega fatty acids and considerable energy needed for fat deposition.
Seasonal Dietary Shifts and Hyperphagia
The Grizzly Bear’s diet shifts annually to prepare for hibernation. Following emergence from the den in spring, bears initially focus on protein, consuming carrion and early green vegetation to replenish lost muscle mass. As summer arrives, their nutritional strategy focuses on increasing carbohydrate intake with the ripening of berries and other plant foods.
The most intense feeding period, known as hyperphagia, occurs during the late summer and fall to accumulate fat reserves. During this time, a bear may consume upwards of 20,000 calories per day, sometimes gaining up to four pounds of body weight daily. This high-calorie diet involves a shift toward the most energy-dense foods available, such as whitebark pine nuts, high-fat berries, and spawning salmon. The proportional composition of the diet changes dramatically during hyperphagia, favoring fat and sugar over fiber and lean protein.
This intense feeding window is necessary for survival, as the stored fat must sustain them through the five to seven months of winter dormancy. The bear’s ability to exploit calorie-rich foods in the autumn directly determines its body condition and reproductive success the following year.
Foraging Tactics and Acquisition Methods
Grizzly Bears are equipped with specialized physical tools and learned behaviors that allow them to acquire their diverse menu. Their large, muscular forelimbs and the prominent shoulder hump serve as a powerful engine for excavation. The long, slightly curved claws are perfectly adapted for digging deep into the soil to unearth roots, tubers, and burrowing animals like ground squirrels.
To gather insects, a bear will use its strength to flip over large rocks and logs, exposing concentrated colonies of ants or beetles underneath. These powerful forelimbs are also employed for fishing, often seen standing in a river or stream, either swatting at passing salmon or pinning them with a rapid movement of the paw.
The bear’s acute sense of smell, estimated to be many times greater than that of a bloodhound, allows it to detect food sources from miles away. This keen nose is instrumental in locating buried caches of roots, carrion, and hibernating small mammals beneath the ground.