Grizzly Bears spend almost half of their lives in winter dens. On average, bears in the Rocky Mountains of Alberta spend 4.5 months of the year in or near their den sites. Den entry depends on both physiological and environmental factors. Pregnant females are usually the first bears to den in the fall and the last emerge in the spring. Male grizzly bears are almost always the last to enter their den in fall and the first to emerge in spring. These patterns in den use will vary depending on the age of the bear and the local climate.
The ESGBP has observed the nature and distribution of grizzly bear dens since the initiation of the project in 1994. In particular, research has focused on identifying the characteristics of den sites used by radio-collared grizzly bears. In the fall, den sites are located with the use of aerial telemetry. The dens are then visited in the spring after the bears have moved on to spring feeding sites.
Grizzly bears almost always dig their own dens but, on occasion, they will use a natural chamber such as a cave or hollow tree. One radio-collared female with three cubs used a natural rock cave for several winters. Grizzly bears often show a preference for a particular denning area. On two occasions, old den sites have been found within a few hundred meters of active den sites. Dens are usually dug horizontally into slopes, where the bear flings an incredible shower of rock and rubble out between its legs and down the mountainside. Each den contains a tunnel that opens up into a chamber lined with tree branches, grass or small twigs. Only once in a while will a grizzly bear re-use its den year after year. Most often, the dens collapse after the soil thaws in the spring, and the bear will choose a new den site for the coming winter.
A number of standard measurements are taken with each site visit so that den characteristics may be compared. We have found that, on average, the total length of dens, measured from the entrance to the back of the den chamber is about 2m. The average width of the den entrance is 74cm and the average height of the entrance is 56cm. The chamber of the den is usually just large enough for the bear to curl up and turn about. The den chamber can be as wide 1.3m with a maximum average height of 98cm.
The location of a den site is also described in great detail in the field. Dens within the study area have been found in the upper sub-alpine at elevations between 2000m and 2450 m. The subalpine region is influenced by extreme temperatures and heavy precipitation. Available habitat ranges from open subalpine forest to herb meadows and avalanche chutes. Bears often dig dens with a particular slope orientation or aspect. Studies completed in the mid 1970s revealed that the aspects of bear dens in Banff National Park range between a compass orientation of 22.5°(NNE) and 112.5°(ESE). ESGBP research to date has found that the aspects of surveyed dens range between 117″ (ESE) and 295″ (WNW). The compass orientation of dens can vary widely, according to local climate and terrain. Bears usually try to dig their dens in deep snow where the entrance is sheltered from strong winds. A thick blanket of snow accumulates over the entrance and provides a layer of insulation for the long winter.
Grizzly bears have a preference for den sites with a specific slope angle. Many of the study area dens have been dug into slopes which have an angle of roughly 30 degrees. Studies have shown that this slope angle is steep enough that there is plenty of soil or rock overhead to form a nice thick den roof that is unlikely to collapse during the winter (Vroom et al 1980). In addition, this slope angle is shallow enough for the den opening to be covered by a heavy blanket of snow (Vroom et al 1980).
Overall, the purpose of research on grizzly bear den sites is to discover the physical characteristics that are common to all den sites in the Rocky Mountains. This information provides for an understanding of the nature of suitable denning habitat for grizzly bears in the Central Rockies Ecosystem.
Suggested Readings:
Vroom, G.W, S. Herrero, R.T. Olgilvie. 1980. The Ecology of Winter Den Sites of Grizzly Bears in Banff National Park, Alberta.” In Bears-Their Biology and Management. C.M. Martinka and C. McArthur, editors. Proceedings 4th International Conference on Bear Research and Management. 321-330.
Herrero, S. 1985. Bear Attacks-Their Causes and Avoidance. New York: Lyons & Burford, Publishers.
McNamee, T. 1984. The Grizzly Bear. New York: Alfred A. Knopf.
