Brown bears consume a wide variety of plant materials, which form a significant portion of their diet. They forage for diverse plant parts, including tender grasses, broad leaves, and the nutrient-rich roots and tubers. These plant sources provide essential carbohydrates and fiber, supporting their large body mass. Bears often use their strong claws to dig for underground plant structures like the bulbs of camas lilies or the rhizomes of horsetails.
Berries are a particularly important plant food, with species like blueberries, huckleberries, raspberries, and salmonberries providing concentrated sugars and antioxidants during the summer and early fall. Nuts, such as acorns and pine nuts, also offer high caloric density, especially in preparation for hibernation. Fungi, including various mushrooms, contribute to their diet, though typically in smaller quantities.
Animal-Based Nutrition
Brown bears supplement their plant-based diet with a range of animal protein sources, crucial for their growth and energy needs. Fish, particularly salmon in regions like the Pacific Northwest, are a primary and highly nutritious food source, with bears expertly catching them during spawning runs. They may employ various techniques, from pouncing on fish in shallow streams to scooping them directly from the water.
Insects like ants, their larvae, and beetle grubs provide accessible protein, with bears often overturning rocks or logs to find these small invertebrates. Small mammals such as voles, ground squirrels, and marmots are also hunted, typically by digging them out of their burrows. Carrion, or the remains of dead animals, is readily consumed, providing an opportunistic and energy-efficient food source. Occasionally, brown bears prey on larger ungulates, such as deer fawns or elk calves, especially during their vulnerable early stages.
Dietary Adaptations Across Seasons and Habitats
A brown bear’s diet exhibits remarkable flexibility, adapting significantly to the changing seasons and the specific characteristics of their habitat. In spring, as they emerge from hibernation, their diet primarily consists of newly sprouted grasses, sedges, and the roots of early-growing plants, along with any available carrion from winter-killed animals. This helps them recover from fasting. As the year progresses into summer, the availability of high-calorie berries increases, becoming a major food item.
Coastal brown bear populations demonstrate a strong reliance on annual salmon runs, consuming vast quantities of fish to build up fat reserves. Conversely, inland bears may depend more heavily on the seasonal availability of nuts, such as acorns or whitebark pine nuts, which offer dense caloric value in the fall. This period of intense feeding, known as hyperphagia, is critical for accumulating the necessary fat to sustain them through the next hibernation cycle.