Grizzly Bear
Grizzly bear, Ursus arctos
Grizzly bears, or brown bears, are a relatively recent addition to North American fauna, having migrated to this continent in the late Pleistocene. These large, omnivorous bears were once widely distributed across North America; however, during the 20th century they were extirpated from most of the U.S. and central Canada. Grizzlies are currently only found in Alaska, western and northern Canada, and in very restricted portions of the continental U.S. To a large extent, the systematic eradication of the grizzly bear in the lower 48 states stemmed from public fear of the animal and habitat loss as human populations expanded into the western U. S. and human-bear interactions increased (Schwartz et al., 2003).
Grizzly bear in field at Yellowstone National Park. Credit: US Fish and Wildlife Service, Terry Tollefsbol.
Height: 1.5-2.5 m long (5-8 ft); 1 m shoulder height (3.35 ft)
Weight (avg): female: inland populations – 98-152 kg (216-335lbs); coastal populations – 160-226 kg (353-498lbs); male: inland populations – 145-269 kg (320-593 lbs); coastal populations – 257-357 kg (567-787 lbs)
Physical Description:
Grizzly bears have a massive head with a distinctive, dished (concave) facial profile, prominent nose, and small ears and eyes. Typically, grizzly bears are brown in color, with silver or white tipped guard hairs, although they can vary in color from blond to nearly black. In some regions, it has been observed that a grizzly bear's coloring correlates with its habitat, such that lighter colored bears tend to be found in open, tundra habitats (e.g., in the Arctic), while bears who occupy forested habitats have darker fur (Reynolds, 1987). As with other bears, the grizzly bear has large, plantigrade (flat) feet with five toes that end in long claws. Their foreclaws are much larger than those of black bears or polar bears, and theys are used primarily for digging, rather than for climbing trees or capturing and holding prey (Schwartz et al. 2003). These bears have a pronounced hump of muscle between their shoulders, which can be useful for distinguishing them from black bears. As with many animals, grizzly bears exhibit sexual dimorphism, with males reaching a size that is roughly 1.2-2.2 times larger than females (Schwartz et al. 2003). Their body mass also fluctuates throughout the year, with peak weight reached in the fall in preparation for hibernation. Grizzlies can lose as much as 43% of their body mass during hibernation, depending on their condition prior to entering hibernation, how long they hibernated, their age, and whether they are male or female.
Grizzly bears are fairly adaptive and able to utilize a wide variety of habitats, which may be one reason they were able to survive the widescale extinctions at the end of the Pleistocene. Currently, the range of habitats available to these animals is restricted by human settlements and landscape alterations, and thus is somewhat different than those utilized during the late Pleistocene, or even historic times. Grizzlies may utilize forested, mountainous, coastal, or tundra habitats, and their home range will typically encompass portions of more than one of these habitats. Grizzlies prefer to dig their dens into high, steep mountain slopes on which deep snow will fall and insulate them until spring. Sometimes, they will place their dens under the roots of large trees, digging out a large enough space within the roots and dirt. In spring, they will move to lower elevations where emergent plant foods are available, as well as carrion from animals that didn't survive the winter.
Grizzly bears are opportunistic omnivores that will utilize any potential food resource available, including a wide range of vegetation, fungi, insects, fish, birds, and mammals. They will prey on large mammals, such as moose, deer, sheep, elk and caribou, when it is available, as well as fish such as salmon, trout and bass. They also scavenge food left behind by other predators. However, the bulk of their diet consists of plant foods, such as nuts, berries, fruits, bulbs, leaves and roots.
Grizzly bears are typically solitary animals, aside from females and their cubs. They will den in caves, when they are available, but most grizzly bears dig winter dens into the side of a hill, many times under a tree. Females give birth in these dens during the winter months. As with other bears, grizzly bears enter a period of winter dormancy, lasting up to seven months, during which their heart rate and body temperature drop significantly, and they do not eat, drink, urinate or defecate. Most researchers agree that this dormancy state should be classified as hibernation, but other researchers argue that winter denning is not true hibernation since the animal's body temperature does not drop below 15°C (Schwartz et al. 2003).
Grizzly bears are fairly late arrivals to North America, having migrated from Eurasia towards the end of the late Pleistocene (ca. 50,000-25,000 years ago). During this time they were concentrated in Alaska and the northwest portion of Canada, although they rapidly expanded south and east as the continental ice sheets retreated at the end of the Pleistocene (Kurtén and Anderson 1980; Schwartz et al. 2003)
Not only did the grizzly bear survive the mass extinctions of the terminal Pleistocene, but they were able to expand into most of North America as the continental ice sheets retreated to the north. By the early Holocene, they ranged as far east as Ontario, Ohio and Kentucky and as far south as Mexico. This distribution retreated north and west, somewhat, as environmental conditions continued to fluctuate through the Holocene, and by historic times, they ranged through most of western North America and into the central Plains of the U.S. (Schwartz et al. 2003).
Despite their relatively recent arrival, remains of grizzly bears have been recovered from late Pleistocene sites in the central Midwest. Most notably, remains have been recorded at Polecat Creek in eastern central Illinois, the Overpeck Ursus find south of Dayton, Ohio, and in Welsh Cave, southwest of Lexington, Kentucky.
- The white tips on most grizzly bear hairs gives the animal a grizzled appearance, similar to an older man's beard. This is where the name "grizzly bear" comes from.
- Grizzly bears shed and replace their hair annually, in a process called molting.
- Although the stance is perceived as threatening by humans, grizzly bears stand when they are curious or surveying their surroundings.
Kurtén, B., Anderson, E. 1980. Pleistocene Mammals of North America. Columbia University Press. New York.
Reynolds, H. 1987. The brown/grizzly bear. In Bears of Alaska in Life and Legend, edited by J. Rennicke, pg 41-42. Roberts Rinehart, Boulder, CO.
Schwartz, C., Miller, S., Haroldson, M. 2003. Grizzly Bear. In Wild Mammals of North America, edited by Feldhamer, G., Thompson, B., Chapman, J., pp. 556-586. The Johns Hopkins University Press, Baltimore, MD.
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