Mountain Wildlife - Mountain Geography: Physical and Human Dimensions - page 418

hibernation period may last over eight months (Manville 1959). The timing of hiberna-
tion is greatly affected by environmental conditions, especially the length of the grow-
ing season. In the Montana Rockies, ground squirrels in mountain valleys emerge in
late April or early May, while those above timberline emerge in mid-June. Similarly, val-
ley populations disappear into their dens by mid-August, while those in alpine meadows
may stay active until the end of September, when snow is beginning to fall (Manville
1959). The higher populations apparently need this extra time in the fall to provide for
the sufficient maturation of the young, the accumulation of body fat to see them through
the winter, and the storage of enough food for the spring. Similar behavior patterns
have been observed among marmots (Pattie 1967; Hoffmann 1974; Barash 1989).
Among large mountain mammals, bears are hibernators, reflecting in their hibern-
ating strategies the effects of their Pleistocene histories. Grizzly bears are newcomers
to North America, spreading and multiplying only after the late-Pleistocene extinctions
of the native megafauna, including the huge, predacious short-faced bears ( Arctodus
simus ). In Europe, grizzly bears coevolved with the larger cave bear ( Ursus spelaeus ).
As its name implies, this huge bear occupied caves for hibernation, thereby foreclosing
caves as hibernating sites to the smaller brown bear, of which the grizzly is a mere
subspecies. Consequently, grizzly bears excavate and occupy their own hibernating
sites with great secrecy. They often choose gravelly moraines covered by willow bushes
where large-scale excavation is possible and the willow roots keep the cave from col-
lapsing. Here grizzlies retreat in late fall during a heavy snowfall that obliterates their
tracks. They need to be secretive because of the threat of a larger bear finding the hi-
bernating site and killing its occupant. Following a bear when it is heading for its den
in a snowstorm can be lethal!
Black bears, which evolved in North America among many powerful Pleistocene
predators, retained arboreal habits to escape their enemies. They are shy and exceed-
ingly clever, hibernating in shallow nests built in hollow trees, under uprooted trees,
in brush piles, or in dens carved in scree-slopes. In the mountains, small bears favor
higher-elevation cliffs and late hibernating dates when snow is on the ground, appar-
ently to minimize cannibalism. All bears awake from their winter sleep if disturbed, and
may abandon their dens. Females give birth to and suckle cubs in their winter dens,
living of large stores of body fat accumulated in fall. Bears have evolved a highly spe-
cialized physiology that allows both hibernating and gestating. Hibernation is triggered
not by temperature, but by food shortages (Stirling 1993).