The grizzly bear, Ursus arctos horribilis, is an iconic North American subspecies of the brown bear. These massive mammals are highly adaptable, opportunistic omnivores that play an important role in their ecosystems. The grizzly’s diet is characterized by immense variability, constantly shifting to exploit the most available and energy-rich resources across its range. The specific foods a grizzly consumes depend almost entirely on the season and its geographical location.
Core Dietary Components
The baseline diet of the grizzly bear is surprisingly plant-heavy, sometimes comprising 80 to 90% of their total food intake. They consume a wide variety of plant material, including grasses, sedges, and broad-leaved herbs, which provide fiber and nutrients, especially in the spring. Grizzlies use their long claws and powerful shoulder muscles to dig for energy-dense roots and tubers, such as glacier lilies or horsetails, throughout the year. Berries, like huckleberries and salmonberries, are also valuable, providing concentrated sources of sugar and carbohydrates when in season.
Animal matter provides necessary protein and fat, though it is often a smaller percentage of the diet volume. Grizzlies consume numerous insects, including ants, larvae, and adult moths, which are found in large patches in high-altitude areas. They also hunt or scavenge small mammals like ground squirrels, marmots, and voles.
Larger prey, such as ungulates like moose, elk, or deer, are occasionally hunted, but they are more often taken as carrion or as vulnerable young calves. This meat provides a high-protein boost. Fish, primarily salmon during their spawning runs, offer an exceptionally rich, fatty food source.
Seasonal Shifts in Consumption
The grizzly bear’s feeding habits are strictly governed by the need to prepare for its long winter hibernation.
When bears emerge from their dens in the spring, their diet focuses on easily accessible, low-energy items to replenish lost stores. They primarily seek out emerging green vegetation, roots, and any available carrion, such as animals that perished over the winter.
As the weather warms and plant life flourishes, the diet shifts toward summer abundance. Bears begin to target insects and small mammals, but the largest change is the availability of high-sugar berries and fruits. Foraging for these concentrated carbohydrate sources is a major activity throughout the summer months.
The most intense feeding period, known as hyperphagia, occurs in the fall as the bear prepares for hibernation. During this time, grizzlies must rapidly accumulate body fat, often gaining up to four pounds of weight per day. This requires consuming an immense amount of calories, frequently exceeding 20,000 calories daily. Bears spend up to 22 hours a day eating, prioritizing the highest-fat foods like spawning salmon, nuts, and large quantities of late-season berries to maximize their fat reserves.
Geographical Dietary Specialization
While the general omnivorous nature of the grizzly remains consistent, local resource availability creates distinct dietary specializations between different populations.
Coastal Populations
Coastal grizzlies, often referred to as brown bears, have a diet heavily centered on marine resources. Their enormous size is largely attributed to the high intake of protein and fat provided by spawning salmon. These populations congregate near rivers during the salmon runs, sometimes consuming only the most calorie-dense parts of the fish, such as the brains and eggs. They also opportunistically scavenge for beached marine mammals, including seals or whales, which provide a massive influx of fat. The reliability of this coastal food source allows for much smaller home ranges compared to interior populations.
Interior Populations
Interior grizzlies, living far from major salmon runs, must rely more heavily on terrestrial resources to meet their energetic needs. In some mountain ecosystems, they travel to high elevations to feast on dense aggregations of army cutworm moths, consuming tens of thousands in a single day for their high-fat content. Other interior bears rely on a greater volume of vegetation, such as digging for roots and tubers, or consuming high-protein plants like clover. When available, they may utilize freshwater fish like kokanee, but the bulk of their protein comes from ungulates, often hunted as vulnerable calves in the spring.