The brown bear (Ursus arctos) is one of the most omnivorous animals in the world and has been recorded to consume the greatest variety of foods of any bear.Carnivores of the World by Dr. Luke Hunter. Princeton University Press (2011), Throughout life, this species is regularly curious about the potential of eating virtually any organism or object that they encounter. Certainly no other animal in their given ecosystems, short perhaps of other bear species and humans, can claim to feed on as broad a range of dietary opportunities. Food that is both abundant and easily obtained is preferred. Their jaw structure has evolved to fit their dietary habits. Their diet varies enormously throughout their differing areas based on opportunity.McDonald, D., & Norris, S. (2001). The new encyclopedia of mammals. Oxford University Press. In spring, winter-provided carrion, grasses, shoots, sedges and forbs are the dietary mainstays for brown bears from almost every part of their distribution. Fruits, including berries, become increasingly important during summer and early autumn. Roots and bulbs become critical in autumn for some inland bear populations if fruit crops are poor. The dietary variability is illustrated in the western United States, as meat made up 51% of the average year-around diet for grizzly bears from Yellowstone National Park, while it made up only 11% of the year-around diet for grizzlies from Glacier National Park a few hundred miles to the north.
Plants and fungi Despite their reputation, most brown bears are not highly carnivorous, as they derive up to 90% of their dietary food energy from vegetable matter. Brown bears often feed on a variety of plant life, including berries, grasses, flowers, acorns (Quercus ssp.) and pine cones as well as mosses and fungi such as mushrooms. In total, over 200 plant species have been identified in their foods. Arguably the most herbivorous diets have come from the warmer temperate parts of Eurasia as more than 90% of the diet may be herbivorous. These include countries and regions such as Spain, Slovakia, most of the Balkans including Greece, Turkey, the Himalayas and presumably the Middle East.Rigg, R., & Adamec, M. (2007). "Status, ecology and management of the brown bear (Ursus arctos) in Slovakia" . Slovak Wildlife Society, Liptovský Hrádok. In many inland portions of North America the diet of grizzly bears is between 80 and 90% plant-based, but animal meat can be much more important in some areas.Murie, A. (2012). The grizzlies of Mount McKinley. University of Washington Press. It has been found that being restricted to a largely vegetarian diet puts constraints on the growth and size of bears who live off them, largely because their digestive systems do not process plants as well as animal fats and proteins.
Among all living bears, brown bears are uniquely equipped to dig for tough foods, such as roots and shoots. They use their long, strong claws to dig out earth to reach the roots and their powerful jaws to bite through them. For the most part, the consumed plant life in spring, predominantly roots immediately post-hibernation and grasses later in spring, is not highly nutritious for bears and mainly staves off hunger and starvation until more nutritious food is available. Brown bears have difficulty digesting large quantities of tough, fibrous foods.Servheen, C. (1987). Grizzly bear compendium. Interagency Grizzly Bear Committee. Hedysarum roots are among the most commonly eaten foods from throughout the range and can become important substitutes if stable foods such as fruits become unavailable. Corms and bulbs are important when available, as they are one of the greater sources of protein in plant life, as are hard masts such as acorns. Brown bears are restricted in their access to hard masts compared to American and Asian black bears because of the limited climbing abilities of grown bears and therefore are confined largely to masts fallen to the ground, pirated from other creatures or within a reach of about that the bears can stretch to with their paws extended and standing on their hindlegs.Seryodkin, I. V. (2016). Behavior of Brown Bears During Feeding in the Sikhote-Alin. Achievements in the Life Sciences. Hard masts can become the most important food (although consumed mainly in late summer and fall) where available in large quantities such as on Hokkaido, Italy and Spain. One of the most important foods in the Rocky Mountains region of the United States is the whitebark pine nut (Pinus albicaulis), which is attained perhaps a majority of the time by raiding the once-abundant caches of American red squirrels (Tamiasciurus hudsonicus) rather than direct foraging. The decline of whitebark pine nuts due to the inadvertent introduction by man of the invasive, virulent fungi Cronartium ribicola has in turn required grizzlies to seek alternative food sources, many of which are carnivorous. In a Greek food study, soft masts were found to outrank hard masts as a food source, with about a quarter of the year-around diet consisting of the legume Medicago.
Fruits and berries are indispensable for brown bears in most areas as a high-energy food stuff for bears, which is necessary to survive the hibernation cycle. The variety of fruits consumed is high, with most of the well-known, wild fruiting plants in temperate North America and Eurasia attracting brown bears in late summer and fall.Ewer, R. F. (1973). The carnivores. Cornell University Press. Among the most prominent fruits found in their foods from through the range include many Prunus species including prunes and cherries, crowberries (Empetrum nigrum), pears (Pyrus ssp.), crabapples (Malus ssp.), brambles (Rubus fruticosus), raspberries (Rubus idaeus), bearberries (Arctostaphylos ssp.) (reportedly named for bears' fondness for them), blueberries (Vaccinium ssp.), lingonberries (Vaccinium vitis-idaea) and huckleberries (Vaccinium parvifolium). Fruit appears to become more secondary in the diet in areas where hard masts and animal protein are abundant in late summer and fall, as these more protein-rich foods appear to be more nutritious for bears than carbohydrate-rich fruits are, despite their fondness for fruit. Even where fruit are commonly eaten, other foods must be eaten to meet nutritional requirements. It is estimated that a small female brown bear may need to eat nearly 30,000 berries each day in late summer/fall in order to subsist on a purely fruit-based diet.
Invertebrates Brown bears will also commonly consume animal matter, which in summer and autumn may regularly be in the form of insects, larvae such as grubs and including beehives. Most insects eaten are of the highly social variety found in colonial nests, which provide a likely greater quantity of food, although they will also tear apart rotten logs on the forest floor, turn over rocks or simply dig in soft earth in attempts to consume individual invertebrates such as bugs, beetles and earthworms.Whitaker, J. O., & Elman, R. (1996). National Audubon Society Field Guide to North American Mammals (p. 992). New York: Knopf.Prater, S. H., & Barruel, P. (1971). The book of Indian animals (Vol. 3). Bombay: Bombay Natural History Society. Honey bees and wasps are important supplemental foods in Eurasia from the furthest west of their range, in Spain, to the furthest east, in Hokkaido. Bears in Yellowstone and Montana eat an enormous number of moths during the summer, sometimes consuming as many as 40,000 army cutworm moths (Euxoa auxiliaris) in a single day, and may derive up to half of their annual food energy from these insects. In Europe, a variety of species of ants have been found to factor heavily into the diet in some areas such as Scandinavia and eastern Europe. In Slovenia, for example, up to 25% of the dry mass consumed by brown bears was ants. Locally heavy consumption of ants has been reported in North America as well, as in west-central Alberta, 49% of scat contained ants. Brown bears mainly feed on ants with a passive response to the colony being dug out and low levels of formic acid, therefore carpenter ants (Camponotus ssp.), which are accessed through rotten logs rather than underground colonies, are preferred where available. Other important insect aggregations that brown bears feed heavily on in some regions include ladybird beetles and caddisfly and cicada nymphs. Brown bears living near coastal regions will regularly eat crabs and clams. In Alaska's Katmai National Park and Preserve, brown bears along the beaches of estuaries regularly dig through the sand for soft-shell clam (Mya arenaria) and Pacific razor clam (Siliqua patula), providing a more nutritious source of dietary energy in spring than plant life before fish become available there.Bear Anatomy and Physiology from Gary Brown's The Great Bear Almanac, Lyons & Burford, Publishers, 1993 The zarigani (Cambaroides japonicus), a type of crayfish, of Hokkaido is also an important, protein-rich dietary supplement for brown bears there.
Fish
By far the closest dietary relationship between brown bears and fish occurs between the salmon and trout of the genus Oncorhynchus, particularly in coastal areas, but also in some inland areas of North America. In the Kamchatka peninsula and several parts of coastal Alaska, including Kodiak Island, brown bears feed largely on spawning salmon, whose nutrition and abundance explain the enormous size of the bears in these areas. Sockeye salmon (O. nerka) and pink salmon (O. gorbuscha) are the two most commonly preyed upon, but many coho (O. kisutch), Chinook (O. tshawytscha), masu (O. masou) and chum salmon (O. keta) are also taken.Quinn, T. P. (2011). The behavior and ecology of Pacific salmon and trout. UBC press. Even in the coastal ranges of the Pacific, a diverse omnivorous diet is eaten, with the salmon spawning reliably providing food only in late summer and early fall. Exceptionally, salmon may come to inland rivers as early as June in the Brooks River when other coastal Alaskan bears are in their dietary "lean period" and provide food for bears sooner than normal. On Kodiak island, it appears the availability of alternative food sources is high, as berry crops are often profuse, marine organisms often wash up and ungulates both wild and domesticated are available.Troyer, W.A. & Hensel, R.J. (1969). The Brown Bear of Kodiak Island. Kodiak National Wildlife Refuge Report. U.S. Fish and Wildlife Service. Kodiak, Alaska, USA. The fishing techniques of bears are well-documented. They often congregate around falls when the salmon are forced to breach the water, at which point the bears will try to catch the fish in mid-air (often with their mouths). They will also wade into shallow waters, hoping to pin a slippery salmon with their claws. While they may eat almost all the parts of the fish, bears at the peak of spawning, when there is usually a glut of fish to feed on, may eat only the most nutritious parts of the salmon (including the eggs (if the salmon is female) and the head) and then indifferently leave the rest of the carcass to scavengers, which can include red foxes (Vulpes vulpes), bald eagles (Haliaeetus leucocephalus), common ravens (Corvus corax) and gulls. Despite their normally solitary habits, brown bears will gather rather closely in numbers at good spawning sites. The largest and most powerful males claim the most fruitful fishing spots and bears (especially males) will sometimes fight over the rights to a prime fishing spot. Despite their aggressive defensive abilities, female brown bears usually select sub-optimal fishing spots to avoid male bears that could potentially threaten their cubs.