The bear family, Ursidae, inhabits environments from Arctic ice to temperate forests globally. Their diet is varied and opportunistic, depending heavily on the specific species and local resources. While most bear species are omnivores with flexible diets, some demonstrate pronounced specialization in their feeding habits. The need for sufficient energy drives their foraging behavior, distinguishing them between generalist and specialized feeders.
The Omnivorous Diet of Black and Brown Bears
The American Black Bear (Ursus americanus) and the Brown Bear (Ursus arctos), including the Grizzly, rely on a diet that is overwhelmingly plant-based for much of the year. Up to 80 to 90 percent of their caloric intake, particularly during summer, comes from vegetation and fruits. Although originating from a carnivorous lineage, their digestive systems are adept at processing fibrous and sugary plant matter. They use large molars for grinding vegetation and a complex stomach to break down bulk food quickly.
These bears are skilled generalists, adapting their menu to seasonal abundance. They forage extensively for soft mast, such as huckleberries and blueberries, and target hard mast like acorns and pine nuts in the autumn. To access protein, they consume insects, tearing apart decaying logs to find grubs and ant colonies. Their long, non-retractable claws are adapted for digging up roots, tubers, and small burrowing mammals.
Animal matter provides concentrated fat and protein, though it is a smaller proportion of their diet by volume. Along coastal regions, brown bears feast on spawning salmon, often consuming only the nutrient-dense brains and eggs when fish are plentiful. They are also proficient scavengers, using their powerful sense of smell to locate carrion or winter-killed animals. This wide array of food sources allows these species to thrive in diverse habitats.
The Highly Specialized Diet of Polar Bears
In contrast to their omnivorous relatives, the Polar Bear (Ursus maritimus) is an obligate carnivore, subsisting almost entirely on fat from marine mammals. The harsh Arctic environment offers no reliable plant-based calories, necessitating a high-energy, protein-rich diet. Their unique metabolism efficiently processes this fat, which is essential for maintaining the thick blubber layer required for insulation. Females must accumulate enough fat reserves to sustain themselves and their cubs during denning.
The primary prey is the ringed seal, followed by the bearded seal, hunted from the sea ice. They employ still-hunting, a patient, energy-saving technique where they wait motionless by a seal’s breathing hole, or aglu, until the animal surfaces. Their acute sense of smell detects these openings even when covered by snow. A single seal, particularly its blubber, can provide a massive caloric reward, often exceeding 100,000 calories.
Another specialized hunting method involves detecting and pouncing on ringed seal birth lairs, which are snow-covered dens where seal pups are born. When food is abundant, a polar bear may only consume the energy-dense blubber and skin, leaving the rest for scavengers like the Arctic fox. This diet is necessary for the species to maintain its large body mass and energy requirements in the sparse Arctic ecosystem.
Seasonal Changes in Foraging Behavior
The feeding patterns of black and brown bears shift dramatically throughout the year based on resource availability and metabolic demands related to hibernation. Upon emerging from dens in early spring, bears have lost 16 to 37 percent of their body weight. They focus on a protein-lean diet of new plant growth, roots, and carrion to rebuild lost muscle mass and replenish basic energy stores.
As summer progresses into autumn, bears enter hyperphagia, a period of intense, non-stop feeding where satiety signals are suppressed. During this time, a bear may spend up to 20 hours a day foraging, consuming over 20,000 calories daily to maximize fat accumulation. The diet switches to high-calorie foods like hard mast and fatty salmon to build necessary fat reserves for winter dormancy.
For female bears, reaching a minimum of 20 percent body fat is necessary to ensure successful reproduction while hibernating. The body undergoes a metabolic shift in the fall, increasing its ability to assimilate fats and carbohydrates. This ability to gorge on energy-dense food is a survival mechanism that determines the bear’s ability to survive the winter fast and its reproductive success the following spring.
The Impact of Human Food Sources
The intersection of bear habitat and human development introduces unnatural, high-calorie food sources that alter bear behavior. Bears are drawn by their powerful sense of smell to anthropogenic attractants.
Anthropogenic Attractants
These attractants include:
- Unsecured residential garbage
- Pet food left outdoors
- Barbecue grills coated in grease
- Backyard bird feeders
These easily accessible, energy-rich meals offer a significant caloric reward with minimal effort, outweighing the difficulty of natural foraging.
This repeated food reward causes a bear to become food-conditioned, leading to habituation. The animal loses its natural wariness of humans and begins to associate people with food. Such bears may become bold, approach campsites, or break into homes, increasing the risk of dangerous human-bear encounters.
The phrase “a fed bear is a dead bear” refers to the reality that these habituated animals often must be captured and euthanized by wildlife management agencies. Relocation is rarely a successful long-term solution, as food-conditioned bears often return to human areas or repeat the behavior elsewhere.
Bears that frequently visit human settlements also experience higher mortality rates from vehicle collisions and poaching. Securing all attractants is the most effective way to prevent this learned behavior, ensuring bears remain reliant on their natural diet and maintain a safe distance from people.