Feeding Behavior
Last Review/Updated: May 24, 2002
Much of the following general summary of feeding behavior is taken from the published works of Stephen Herrero, University of Calgary, who explained the evolutionary divergence and resulting differences in feeding strategies of black bears and grizzly bears.
Evolving from small, carnivorous, tree-climbing mammals, the earliest bears were forest dwellers that became opportunistic omnivores; they ate meat when it was available, and when it was not, they subsisted on vegetable matter. Because plant foods were more dependable than animal foods, highly digestible forbs, leaves, roots and fruits came to dominate the diets of bears. Quite late in their evolution, the ancestors of the present grizzly bear began to adapt to more open habitats; digging for foods such as ground-burrowing rodents as well as the roots and tubers of plants became a specialization of this species.
 Interpreting the signs of a feeding bear can be interesting and educational. M. Gibeau photo |
Both species have a relatively unspecialized digestive system, which is essentially a carnivore's gut that has been lengthened. Bears have no cecum and their stomachs are too acidic to support the microflora and microfauna needed to digest cellulose. They, therefore, have difficulty in digesting the woody parts of plants, but both species can still survive on a very high proportion of plant foods in their diet. They choose food items that are easy to digest such as berries (sugar), roots (starch), meat, protein-rich, rapidly growing parts of plants, and fats (plants or animal).
Both species of bear choose the most digestible, nutritious foods that are available at a given time. When plant protein is a major item in the diet, its relatively low digestibility is counterbalanced by large intake. Grizzly bears effectively exploit foods in the soil; black bears in and around trees. However, there is still a great deal of dietary overlap between them. Grizzlies likely have a competitive advantage in open habitats, partly because of these feeding adaptations, larger size and greater defense of young.
Foods of the Black Bear
Research by Anne Holcroft (Kananaskis Country) and Brian Pelchat (Cold Lake) reveal that the diet of black bears varies with the seasons. On emergence from the winter den, a black bear is lucky if it discovers the carcass of a winter-killed animal; fortunate is the bear that finds a dead moose, but a floating beaver carcass or fish will do. In mountainous habitats, overwintered bear berries are favorite early foods. In the boreal forest, as the last snows melt in spring, the first green-up takes place far above the ground, and black bears climb tall poplars to feed on the sprouting new buds. Gradually, the green-up of the forest provides other choice foods such as horsetails and sedges in shallow retreating waters, and dandelions along openings and roadsides. Peavines and clovers become popular as the grazing black bear roams farther and farther from the winter den. Some individual bears, especially large males, will hunt for newborn calves of moose and caribou.
As spring turns to summer, an even wider variety of foods becomes available, including sarsaparilla, peavine, and especially ants and other insects. Wandering about the woods, bears overturn logs, tear apart stumps and occasionally try to catch a fish in streams or lake shallows. Berries are the staple during late summer and fall. In the mountains, the red buffaloberries are the usual favorites, but in the boreal forest, blueberry and other berry patches attract bears from mid-July to early October.
In years of berry failures, which are locally fairly common, black bears find it difficult to sufficiently fatten for the coming long period of hibernation. Then they wander great distances and may succumb to human-related foods and get into trouble with people.
Foods of the Grizzly Bear
Grizzly bears use a wide variety of foods that differ significantly between mountain and boreal forest habitats. Individual diets range from almost totally vegetarian to a heavy dependence on animal protein. Much of the following information on what grizzlies eat is taken from the works of Alberta grizzly biologists: David Hamer, Stephen Herrero, John Kansas, John Nagy and Dick Russell.
When the first grizzlies emerge from their winter dens in late March or April, finding something to eat is a challenge. Snow still lies deep in valley bottoms and on north-facing slopes. South-facing and wind-blown steep terrains at lower elevations provide overwintered bear berries, roots of Indian potato (Hedysarum) and the earliest green shoots of grass. Dick Russell, who conducted the first comprehensive field study of grizzlies in Alberta, and Jack Nolan found that the roots of Hedysarum were essential spring foods in Jasper National Park. As spring turns to summer, horsetails, grasses, sedges, and the young of elk and moose become important. Some individual grizzlies intensively search for newborn calves in late May and early June. As spring matures, grizzlies slowly follow the ripening of plants up the valley slopes.
Horsetails remain part of the diet during June. But, as summer progresses, grizzlies switch to ants in dry, open south- and west-facing forests and old burns and to succulents like cow parsnip and glacier lily, found on moist avalanche slopes and in other wet areas. During summer, early berries become the preferred food. Buffalo berry, huckleberry and blueberry are all favorites, but many other species are used. These berry plants are most common in burns and on open sunny slopes.
In fall, late-ripening berries such as crowberry and low-bush cranberry increase in importance. As well, grizzlies of the mountains dig for the roots of Hedysarum and for ground squirrels. These two favorites are not available in the boreal forests east and north of the Rocky Mountains. Here, the grizzlies often forage in areas where grasses and clover have been seeded along roads and pipeline rights-of-way.
"Black bears feed primarily on herbs, Equisetum sp. and monocots. Berries were consumed from spring through summer with heaviest use in July and August 1977. Bracted honeysuckle berries and high-bush cranberries occurred most often in the sample during that period. In contrast in 1976, berries were absent in the September-October scat sample. As with grizzlies, sweet clover replaced berries as the most widely consumed forage in autumn 1977. Sweet clover . . . representing 30% of the identifiable component of herbs found in scats collected in May, 83% in June, 55% in July and 83% during (fall)."
—John Nagy and Dick Russell, Ecological Studies of the Boreal Forest Grizzly Bear
