The brown bear, Ursus arctos, is a highly adaptable omnivore defined by opportunistic foraging rather than dietary specialization. Consuming a wide range of foods depending on local availability, the brown bear has the most varied diet of any bear species. This flexible approach allows them to thrive across their vast northern hemisphere range, from coastal Alaska to the interior mountains of Eurasia. Their diet is primarily driven by the need to accumulate sufficient fat reserves for their long period of winter dormancy.
The Foundation: Vegetation, Insects, and Small Prey
Despite their powerful build and classification in the order Carnivora, brown bears derive a significant portion of their diet from plant matter. In many populations, vegetation makes up 70% to 90% of their total food intake by volume, serving as the nutritional backbone. This plant-based diet includes grasses, sedges, and forbs, which are particularly important food sources during the spring. Their strong claws and powerful shoulders are uniquely adapted for digging, allowing them to unearth calorie-dense roots, bulbs, and corms, such as those from glacier lilies and sweet-vetch.
Insects provide an accessible source of protein and fat, which is often crucial when other high-energy foods are scarce. Bears will actively seek out and consume ant larvae, beetles, and moth aggregations. In some regions, a single brown bear can consume as many as 40,000 army cutworm moths (Euxoa auxiliaris) in a day, which can account for up to half of their annual energy intake.
While less common than plants and insects, animal protein is also a regular part of the brown bear’s diet. This protein often comes from scavenging winter-killed animals, known as carrion, especially in the early spring. They are also skilled at preying on smaller animals, digging up rodents like ground squirrels and marmots from their burrows. When the opportunity arises, they may target vulnerable young ungulates, such as moose or elk calves.
The Annual Cycle: Seasonal Dietary Shifts and Energy Strategy
The brown bear’s active season is characterized by profound dietary shifts centered on gaining weight for hibernation. The primary energy strategy is hyperphagia, a state of excessive eating that occurs in the late summer and fall to maximize fat storage. Females must accumulate at least 20% of their body weight as fat to successfully support gestation and give birth in the den.
When bears emerge in the spring, food is scarce, forcing them to rely on low-energy foods like grasses, newly sprouted shoots, and carrion. They typically lose weight until well into June, focusing energy on turning over rocks for insects and digging for surviving roots. This initial phase is characterized by a scramble for survival while they wait for the landscape to become productive.
The summer brings an explosion of high-calorie plant foods, marking a shift toward efficient foraging. Bears dedicate significant time to consuming soft mast, such as huckleberries, blueberries, and salmonberries, which are critical for early fat gain. In some areas, an individual bear has been documented eating over 200,000 buffalo berries in a single day, highlighting the intensity of this summer foraging.
The fall culminates in hyperphagia, where a bear may spend up to 22 hours a day eating and can gain up to four pounds daily. During this frantic phase, bears must consume 20,000 calories or more per day to prepare for winter. High-fat, easily digestible sources like late-ripening berries, acorns, and nuts (hard mast) become the most sought-after foods. The physiological goal is to maximize fat input, temporarily overriding the normal mechanisms that trigger a feeling of fullness.
Geographic Diversity: Coastal vs. Interior Diets
The brown bear’s diet is heavily influenced by its geography, resulting in two distinct feeding profiles: coastal and interior. Coastal brown bears, such as those found along the Alaskan Peninsula, have access to a uniquely rich marine resource base. Their diet is dominated by the seasonal abundance of Pacific salmon runs, which provide a massive influx of high-fat, easily accessible protein.
The high nutritional value of salmon, combined with other coastal foods like clams, marine carcasses, and nutrient-rich sedges, allows coastal bears to grow significantly larger than their inland counterparts. This reliance on marine protein means coastal bears have a different foraging schedule, often congregating at river mouths to fish during the spawning season.
Interior grizzly bears, in contrast, must rely on more terrestrial and scattered food resources due to the lack of major fish runs. Their diet is characterized by intensive digging for roots and tubers, a greater reliance on hunting larger prey, and exploiting ephemeral, high-energy resources. Inland bears are known to target high-elevation moth aggregations and hunt young ungulates, such as moose and caribou calves, for protein. These inland populations are typically smaller, reflecting the lower protein and energy density of their primarily herbivorous diet.