Grizzly bears ( Ursus arctos horribilis ) are a captivating species, often misunderstood as purely carnivorous predators. While they belong to the Order Carnivora and Family Ursidae and share a close genetic relationship with the Kodiak bears of Alaska, their dietary habits are far more diverse and adaptable. Understanding what grizzly bears eat is crucial for appreciating their ecological role and ensuring their conservation.
Physical Characteristics and Adaptations
Grizzly bears are a subspecies of the North American brown bear and can be distinguished from black bears by several key features: longer, curved claws, a prominent hump on their shoulders, and a dished facial profile. This muscular hump provides immense strength for digging and foraging. Their fur varies in color from light tan to nearly black. In the lower 48 states, male grizzly bears typically weigh between 400 and 600 lbs, while females usually weigh between 250 and 350 lbs. Canadian and Alaskan grizzlies tend to be larger.
Omnivorous Nature and Dietary Flexibility
Despite their powerful stature, grizzly bears are not strict predators. Their omnivorous nature allows them to consume a vast assortment of foods, adapting their diet to whatever is readily available. Their opportunistic feeding habits make them expert scavengers, often foraging for both natural and human-sourced food, including garbage when accessible. They will consume almost any fresh, processed, frozen, canned, dried, boxed, or packaged foods sold for human consumption, including camp foods, groceries, beverages, grease, and garbage. However, while these foods can provide nutrition for bears, their use most often leads to bears' lethal removal from the ecosystem.
Grizzly bears eat what they can find, strongly depending on what is available. The grizzly’s diet consists of 80-90% vegetation! The majority of a grizzly bear’s diet is berries, grasses, nuts, and roots.
Seasonal Variations in Diet
The diet of grizzly bears varies significantly throughout the year, influenced by the availability of different food sources.
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Spring (March to May): Bears emerging from their dens are drawn to lower elevations where snow is retreating. They feed on emerging grasses, sedges, dandelion, clover, spring-beauty, and horsetail, as well as insects and grubs. Bears feed on ungulates primarily as winter-killed and wolf-killed carrion, but also through predation on deer fawns and elk calves. During this time, grizzlies will scavenge for carrion, like dried fish carcasses and other dead animals that winter left behind.
Summer: As summer unfolds, bears may ascend to higher elevations for cooler temperatures and more nutritious food sources. During this time bears continue to consume succulent grasses and sedges, dandelion, clover, spring-beauty, horsetail, and ants. In addition, thistle, biscuit root, fireweed, fern-leaved lovage, and army cutworm moths are eaten.
Fall: In the fall, bears enter a state called hyperphagia where they eat nearly nonstop to obtain enough fat reserves to survive winter hibernation. During this time, bears may consume up to 20,000 calories a day. Bears are drawn to areas with concentrated food sources in the fall, such as berry patches. Fall is also the time of year when most human-bear conflicts occur. In the fall, when berries are abundant, grizzly bears can eat 200,000 a day. In the late summer months, moths are plentiful in the high altitudes of the Rocky Mountains, which suits grizzlies just fine. These big guys can eat up to 40,000 moths a day.
Winter: Most bears hibernate or den during the winter months. The length of denning depends on location, and can vary from a few days or weeks to a few months or more. Bears make their dens in hollow trees or logs, under the root mass of a tree, or even in rock crevices. Grizzly cubs are born during winter hibernation and exist solely on the nutrition of their mother’s milk until the spring when they begin to try out solid foods. During this period, they do not eat, drink, urinate, or defecate. Over the course of the denning season, a bear may lose thirty percent of its body weight. All of this weight is stored as fat which is acquired during the 2 to 4 months prior to entering dens during a period called hyperphagia.
Key Food Sources in the Greater Yellowstone Ecosystem (GYE)
Research on grizzly bears in the GYE has been conducted continuously for over 50 years, likely making them the most studied bear population in the world. From a literature review, it was compiled a list of all the reported foods consumed by grizzly bears into one comprehensive document.
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GYE grizzly bears eat an incredibly diverse array of foods, from those as large as bison and moose, to those as small as ants and midges. Grizzly bears of the GYE prey on fast and agile animals like elk, but also on slow and relatively immobile species such as earthworms. Some prey species fight back and are quite formidable to subdue, which we have observed when grizzly bears kill adult black bears or even other grizzly bears. Some insects, such as yellow jackets, put up a stinging defense but are not much of a threat to bears. Other species like ladybird beetles and cutthroat trout are relatively defenseless.
In addition to prey species, grizzly bears eat many plants and mushrooms. Plants are consumed through grazing, digging, plucking, stripping, and peeling. Grasses, sedges, and clover are grazed; whereas, biscuitroot, yampa, and thistle roots are dug from the soil. Berries are plucked and stripped from branches and cambium is peeled from tree trunks. In all, more than 266 species in 200 genera from four different kingdoms (Plantae, Animalia, Fungi, and Protista) consumed by GYE grizzly bears.
- Plants: Grizzly bears consumed more than 162 different plant species (149 native and 13 non-native), including at least 85 forbs, 31 shrubs and berries, 25 grasses, 4 sedges, 2 rushes, 4 aquatic plants, and 4 different species of ferns and fern allies as well as cambium, catkins, and nuts from 7 tree species. The primary forbs eaten were clover, dandelion, thistle, horsetail, yampa, and biscuitroot. Frequently consumed berries included whortleberry, huckleberry, and strawberry. The most frequently consumed graminoids were Kentucky bluegrass, sedges, and brome grasses. Seven species of mushrooms were consumed, including false truffles, puffballs, and morels.
- Animals: Documented bears feeding on at least 26 mammal, 4 fish, 3 bird, and 1 amphibian species. Additional animal species are undoubtedly consumed opportunistically, but have not been documented. The primary mammals consumed were bison, moose, elk, mule deer, pocket gophers, voles, and ground squirrels; but mountain goats, marmots, mice, and pikas were also eaten.
- Invertebrates: Grizzly bears consumed at least 36 species of invertebrates: primarily ants, army cutworm moths, yellow jackets, and earthworms. Twenty-four different species of ants were eaten including mound, ground, and log-dwelling species.
- Other: Consumption of one algae and one soil type (geothermal) were also documented.
Energy Maximization and Nutrient Optimization
Because of their need to accumulate large fat reserves for hibernation, grizzly bears are energy maximizers. In grizzly bears, a diet of approximately 20% protein and 80% non-protein energy achieves maximum weight gain per unit of energy intake. To achieve this nutrient target and maximum weight gain, grizzly bears select mixed diets that maximize energy intake while optimizing macronutrient intake. Whenever available, grizzly bears seek foods of high caloric value that are concentrated on the landscape and can be efficiently foraged. Accordingly, frequently used foods included ungulates (bison, elk, moose, mule deer), cutthroat trout, army cutworm moths, and whitebark pine seeds.
Diet Flexibility and Adaptation
Grizzly bears in the GYE exhibit diet flexibility, consuming different foods depending on where their home ranges are located. Some of the highest-quality foods are not found within all parts of the ecosystem and thus are likely not available to all GYE bears. The diet flexibility demonstrated by Yellowstone grizzly bears greatly enhances their ability to occupy diverse habitats over a large geographic area. This diet flexibility likely also enhances their ability to cope with seasonal, annual, and longer-term perturbations in the abundance of high-calorie foods.
Identifying Bear Activity
When exploring bear habitats, recognizing signs of foraging activity can enhance safety and understanding of these creatures’ behaviors. For black bears, look for upturned rocks, torn-up logs, or overturned soil where they’ve sought insects and grubs. Tracks with prominent claw marks and claw-dug patches on the ground are also indicators. Grizzly bears might leave similar signs, but their larger size often results in more extensive damage to logs, rocks, and soil. Their claw marks may be deeper and more widely spaced. Scat, often containing berries and vegetation, is a common clue for both species.
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Grizzly Bear Interactions with Humans and Conservation
Grizzly bears typically avoid humans. However, grizzly bears can fiercely protect themselves, their young, and their food when they feel threatened. Because of their shifting range, grizzlies are having closer encounters with humans and livestock in their search for adequate resources.
Knowledge of the dietary breadth of grizzly bears helps managers of grizzly bears and their habitat document future changes in patterns of food consumption. This increased understanding provides managers with a strong foundation for making decisions about future grizzly bear management in the GYE.