The brown bear, Ursus arctos, is an opportunistic omnivore whose diet is highly varied, shifting significantly depending on the local geography and the time of year. This species has the most diverse diet of any bear, capable of utilizing everything from tiny insects to large ungulates. While plant matter often provides the caloric bulk, their survival hinges on the strategic consumption of whatever resources are most abundant and easiest to obtain within their wide-ranging habitats.
The Seasonal Shift in Foraging
A brown bear’s feeding strategy is a structured cycle, driven by the need to accumulate enough body fat to survive the winter denning period. The year begins with a phase of scarcity in early spring after emerging from the den. During this time, bears focus on lean mass growth, consuming low-nutrient foods like fresh grasses, forbs, and carrion that survived the winter.
The feeding cycle shifts into a period of relative abundance during the summer months when a variety of plant and animal resources become widely available. Bears capitalize on these easily digestible, high-energy foods to maximize their intake, providing necessary nutrients for growth and maintenance.
The final phase is hyperphagia, beginning in late summer and continuing through the fall, where bears experience a profound, biologically programmed hunger. This state of excessive eating is essential for building the massive fat reserves required for hibernation. The availability of high-calorie fall foods dictates the bear’s behavior and its long-term survival prospects.
Primary Plant-Based Foods
Plant matter consistently forms a large part of the brown bear’s diet, often accounting for 80% to 90% of its total food energy, particularly in inland populations. Bears consume a variety of herbaceous vegetation in the spring, including grasses, sedges, and flowering plants known as forbs. They use their long, powerful claws to dig for subterranean plant structures like roots, bulbs, and tubers, which provide necessary carbohydrates and roughage.
Berries represent a significant seasonal food source, providing concentrated sugar and antioxidants during the summer and fall. Coastal bears in the Pacific Northwest rely heavily on salmonberries and huckleberries, while those in other regions seek out blueberries and cranberries. These calorie-dense foods require minimal energy expenditure to gather.
In regions lacking a reliable salmon run, hard mast—such as acorns, pine nuts, and beechnuts—becomes a high-value food source in autumn. These nuts are rich in fat and protein, making them an excellent fuel for hibernation. Bears actively forage for these resources, which can sustain them well into the late fall.
Animal Protein Sources
Animal protein is a highly sought-after component of the diet, offering concentrated energy and lean mass building components. Coastal brown bears, especially in Alaska, capitalize on the Pacific salmon spawning runs. They often “high-grade” their catch, selectively eating the fattiest parts of the fish, such as the brain, skin, and eggs, leaving the less calorie-dense muscle tissue for scavengers.
Insects provide a consistent source of protein and fat throughout the active season. Bears dig up ant colonies to consume the larvae and pupae, known as ant brood, and tear apart rotten logs to find beetle grubs. Small mammals, including ground squirrels, voles, and marmots, are also readily excavated and consumed.
True predation on larger animals is less common than scavenging, but brown bears are capable of taking down vulnerable prey. They opportunistically target ungulate calves, such as young moose or caribou, especially in the spring when the young are less mobile. The consumption of carrion, or dead animals, is a regular occurrence, providing an easy meal with no hunting effort required.
Preparing for Winter Fat Storage
The physiological state of hyperphagia drives brown bear foraging behavior in autumn. To survive months of dormancy, a bear must accumulate a thick layer of fat, requiring a daily caloric intake that can exceed 20,000 calories. This massive energy demand forces the bear to prioritize the most calorie-dense foods available.
Late-season runs of high-fat salmon provide the ideal combination of protein and fat for rapid weight gain. In areas without abundant fish, bears focus intently on hard mast and late-ripening, sugary berries. The success of this autumn feeding campaign directly determines the bear’s health and the reproductive success of pregnant females upon emerging in the spring.