Hibernation is an adaptation to a seasonal shortage of food, low environmental temperatures, and snow cover on the ground (Crowed and Crowed 1972; Tietje and Ruff 1980). Bears hibernate during the winter months in most areas of the world. Duration of winter denning is dependent upon latitude and varies from a few days or weeks for black bears in Mexico to 6 months or more for bears in Alaska (Kolenosky and Strathearn 1987). The denning period in YNP is approximately 5 months.

For many years bears were not considered to be true hibernators. Mammals considered true, or deep hibernators, such as chipmunks and ground squirrels, experience a drastic decrease in body temperature during hibernation. Body temperature for a hibernating bear remains above 88° F (31° C) which is within 12° F (11° C) of their normal body temperature of 100° - 101° F (37.7° - 38.3° C) (Bagget 1984). This allows bears to react to danger quicker than hibernators whose body temperature may be less than 40° F (4° C) and who have to warm up before they can move quickly (Bagget 1984). Many researchers now consider bears to be super-hibernators. Due to the highly insulative pelts of bears and their lower surface area to mass ratio than smaller hibernators, body heat is lost slowly which enables bears to cut their metabolic rate by 50-60% (Crowed and Crowed 1972; Rogers 1981). Respirations in bears decrease from 6-10 breaths per minute normally, to 1 breath every 45 seconds during hibernation. They experience a drop in heart rate from 40-50 beats per minute during the summer to 8-19 beats per minute during hibernation. Mammals that experience lower body temperatures during hibernation, such as chipmunks and ground squirrels, must awaken every few days to raise their body temperature, move around, urinate, and eat (Rogers 1981). Grizzly bears and black bears generally do not eat, drink, defecate, or urinate during hibernation. Bears live off of a layer of fat built up during the summer and fall months prior to hibernation. Waste products are produced, however, instead of disposing of their metabolic waste, bears recycle it. The urea produced from fat metabolism (fatal at high levels) is broken down and the resulting nitrogen is used by the bear to build protein which allows them to maintain muscle mass and organ tissues (Rogers 1981). Bears lose fat and may actually increase lean-body mass while hibernating due to this nitrogen recycling (Wickelgren 1988). Bears may loose 15-30 % of their body weight during hibernation (Rogers 1981). It was once thought that bears ate roughage prior to den entrance to scour their digestive tract and form a plug in the anus to prevent them from eating any more food that fall. Actually, the plug, made up of feces, dead intestinal cells, hair, and bedding material, forms during hibernation and not before (Rogers 1981). Bears continue to produce some feces during hibernation yet they do not defecate (Rogers 1981). It is possible this plug may keep the bear from defecating inside the den during hibernation as fecal plugs are found just inside or outside the dens of bears that have just emerged (Rogers 1981).

It was once believed that bears obtained nutrients from sucking their paws during hibernation. This idea most likely arose from observations of bears licking the bottom of their paws during the last half of the denning period when their old, calloused foot pads slough off (Rogers 1977). The sucking and licking action apparently helps toughen the new foot pads so bears can walk on them without pain or difficulty when they emerge from the den and begin searching for food (Beecham et al. 1983).

In the Yellowstone ecosystem, grizzly bears tend to dig or locate dens on the mid to upper one-third of 30° - 60° slopes with northern exposures between 6,562 - 10,006 ft,  =8103 ft (2,000 - 3,050 meters,  =2,470 m) in elevation (Judd et al. 1986). Black bears locate or excavate dens on 20° - 40° slopes ( =27.8°) with northerly aspects between 5,800 - 8,599 ft,  =7,346 ft (1,768 - 2,621 meters,  =2,239 m) in elevation (Mack 1990). There are several different types of dens utilized by bears. Black bears tend to excavate dens, den under windfalls, in hollow trees or caves, and in previously occupied dens (Jonkel 1980). Grizzly bears tend to excavate dens at the base of large trees often on densely vegetated north-facing slopes. This is advantageous in the Yellowstone ecosystem due to prevailing SW winds which accumulate snow on northerly slopes and insulate dens from temperatures which often drop as low as -40° F to -60° F (-40° C - -51° C) (Crowed and Crowed 1972; Jonkel 1980; Vroom et al. 1980). Grizzly bears in YNP usually dig new dens but on occasion, dens (especially natural cavities) are re-utilized. (Crowed and Crowed 1972; Judd et al. 1986; Miller 1990). Most dens are dug in sandy loam soils with some occurring in clay loams and rocky silt soils (Judd et al. 1986). Reuse of excavated dens is rare but does occasionally occur. Usually excavated dens collapse the spring after they are dug due to runoff and are unusable. Some grizzly bears excavate dens long before the onset of hibernation while other bears tend to wait to almost the last minute to construct dens (Crowed and Crowed 1972). Major den excavation is completed in 3-7 days during which a bear may move up to a ton of material (Brown 1993; Crowed and Crowed 1972). After completion of a den (which consists of an entrance, a short tunnel, and a chamber) bears will cover the chamber floor with bedding material ranging from spruce boughs to duff. The bedding material has many air pockets which trap body heat and form a microclimate around the bear helping to keep it warm (Crowed and Crowed 1972). These bedding materials are related to availability at the den site and not on the bears preference (Judd et al. 1986). The den entrance is usually just large enough for the bear to squeeze through. This minimal opening size helps prevent heat loss during hibernation since a smaller opening will be covered with snow more quickly than a large opening. The tunnel is dug straight into a hillside or at a slightly upward angle to keep heat in the den chamber. The chamber is dug slightly larger than the bear allowing for efficient heat retention. Males and females with young usually dig the largest dens.

Movement to dens is correlated to weather and snow conditions with most movement usually occurring from late October to mid November (Judd et al. 1986). However, Crowed and Crowed (1972) found hibernation onset varied by as much as one month depending on weather conditions. Bears will remain in the area of their den for a few weeks and enter a state of lethargy during which they eat nothing and sleep frequently (Crowed and Crowed 1972). According to Crowed and Crowed (1972) and Servheen and Klaver (1983), final den entry occurs during severe snowstorms. In theory the fresh snow will hide any tracks or other evidence of where the bear's den is located. Solitary females usually enter dens first, followed by females with young, subadults, and lastly, adult males. Grizzly and black bears breed from May through July but the embryonic implantation does not occur until around December, about 1 month after solitary females den. The cubs are then born in late January or early February and are naked, blind, and helpless (Roger 1981). The measure only about 8 inches (20 cm) long and weigh from 8-12 ounces (224-336 g). The cubs do not hibernate. They sleep next the the sow, nurse, and grow rapidly. When black bear cubs emerge from the den at about three moths of age, they weigh about 4-8 pounds (1.8-3.6 kg) and are able to follow the sow around in search of food (Rogers 198). At ten weeks of age, grizzly bear cubs weigh about 10-20 pounds (4.5-9.0 kg) (Brown 1993),

When temperatures warm up and food is available in the form of winter-killed ungulates or early spring vegetation, bears emerge form their dens. First to emerge are adult males in mid March (= number of days denned =113), followed by subadults, and solitary females in late March to early April (= days denned =132). The last to emerge are females with young in early to mid April (number days denned = 170) (Judd et al. 1986, Beecham et al. 1983). The males, subadults, and solitary females usually leave the vicinity of their den within a week of emergence while females with young remain in the general vicinity for 2-3 weeks (Lindzey and Meslow 1976).

Several physiological processes bears undergo during hibernation are of interest to medical researchers. When bears are hibernating and metabolizing body fat, their cholesterol levels are twice as high as during the summer and twice as high as the cholesterol levels of most humans (Bagget 1984). Bears, however, do not suffer from hardening of the arteries (arteriosclerosis) or gallstones, conditions which result from high levels of cholesterol in humans. The bear's liver secretes a substance that dissolves gallstones in humans without surgery. Another mystery of hibernation is that bears do not lose bone mass during hibernation. All other mammals which maintain non-weight bearing positions for an extended period of time suffer form osteoporosis, or a weakening of the bones (Wickelgren 1988). When the substance for this phenomenon is discovered it may help people who suffer from weak bones.

* Information on this page provided by the NPS.

Literature Cited:

Bagget, J.A. 1984. Hibernation. Science World. 40(10):8-11.

Beecham, J.J., D.G. Reynolds, and M.G. Hornocker. 1983. Black bear denning activities and den characteristics in west-central Idaho. Int. Conf. Bear Res. and Mange. 5:79-86.

Brown, G. 1993. The great bear almanac. Lyons and Burford, publishers. New York, N.Y. p. 146-155.

Crowed, F.C. Jr., and J.J. Crowed. 9172. Grizzly bear prehibernation and denning activities as determined by radiotracking. Wildl. Mongr. 32. 25pp.

Jonkel, C. 1980. Black, Brown (Grizzly), and Polar Bears. Pages 227-248 in Big Game of North America, Ecology and Management. Stackpole Books, Harrisburg, Pa.

Judd, S.L., R.R. Knight, and B.M. Blanchard. 1986. Denning of Grizzly Bears in the Yellowstone National Park Area. Int. Conf. Bear Res. and Manage. 6:111-117.

Kolenosky, G.B., and S.M. Strathearn. 1987. Winter denning of black bears in east-central Ontario. Int. Conf. Bear Res. and Manage. 7:305-316.

Lindzey, F.G., and E.C. Meslow. 1976. Winter dormancy in black bears in southwestern Washington. J. Wildl. Manage. 40(3):408-415.

Mack, J.A. 1990. Black bear dens in the Beartooth face, south-central Montana. Int. Conf. Bear Res. and Manage. 8:273-277.

Miller, S.D. 1990. Denning ecology of brown bears in southcentral Alaska and comparisons with a sympatric black bear population. Int. Conf. Bear Res. and Manage. 8:279-287.

Rogers, L.L. 1977. The ubiquitous American black bear. Pages 28-32 in North American Big Game. Boone And Crockett Club. Parker and Nesbitt (eds).

_____ 1981. A bear in its lair. Natural History Magazine. 70(10):64-70.

Servheen, C., and R. Klaver. 1983. Grizzly bear dens and denning activity in the Mission and Rattlesnake Mountains, Montana. Int. Conf. Bear Res. and Manage. 5:201-207.

Tietje, W.D., and R.L. Ruff. 1980. Denning behavior of black bears in boreal forest of Alberta. J. Wildl. Manage. 44(4):858-870.

Vroom, G.W., S. Herrero, and R.T. Ogilvie. 1980. The ecology of winter den sites of grizzly bears in Banff National Park, Alberta. Int. Conf. Bear Res. and Manage. 4:321-330.

Wickelgren, I. 1988. Bone loss and the three bears: A circulating secret of skeletal stability. Science News. 134(26):424-425.

YELL 710
INFORMATION PAPER No. BMO-10
Mark J. Biel
Bear Management Office Biological Technician
Yellowstone National Park May 1996