Polar Bears In Depth
Denning
Page 1 of 8
Across most of their range, pregnant female polar bears excavate dens in snow and ice in early winter (Harington 1968; Lentfer and Hensel 1980; Ramsay and Stirling 1990; Amstrup and Gardner 1994). They give birth in those dens during midwinter (Kostyan 1954; Harington 1968; Ramsay and Dunbrack 1986) (see section on reproduction), and emerge from dens when cubs are approximately 3 months old. Because neonates are so altricial, the period of denning is essential to their early survival. Recognizing it as a critical phase in the polar bear life cycle, scientists have devoted much attention to aspects of maternal denning.
Distribution of Denning. Historically, polar bear dens were thought to represent the "core areas" of their ranges (Harington 1968). In those core areas, large numbers of polar bears repeatedly and predictably concentrated their denning within relatively small geographic regions (see Fig. 27.1). Examples of concentrated denning areas include particular islands of the Svalbard Archipelago north of Norway (LØnØ 1970; Larsen 1985); Franz Josef Land, Novaya Zemlya, and Wrangel Island in Russia (Uspenski and Chernyavski 1965; Uspenski and Kistchinski 1972); and the west coast of Hudson Bay in Canada (Harington 1968; Jonkel et al. 1975; Stirling et al. 1977b; Ramsay and Andriashek 1986; Ramsay and Stirling 1990).
Concentration or core areas were easily detected by aerial or ground surveys, and often had been known from reports of early explorers or Native people residing in the area. Early knowledge of concentrated denning led to the view that all polar bears must use such core areas, and that areas without them did not significantly contribute to polar bear reproduction. Harington (1968 :8) implied as much when he stated, "No major denning areas seem to exist in Alaska." It was assumed geographic regions without such areas mainly were populated by visiting polar bears. That concept led Native people of northern Canada to believe that harvests in Alaska were being sustained by polar bears produced in Canada (Stirling and Andriashek 1992). Over much of their range, we now know, polar bears den in a more diffuse pattern where individual dens are scattered over broad reaches of habitat at low density (Lentfer and Hensel 1980; Stirling and Andriashek 1992; Amstrup 1993; Amstrup and Gardner 1994; Messier et al. 1994; Ferguson et al. 2000a; Durner et al. 2001, 2003).
Successful denning by polar bears requires accumulation of sufficient snow that a pregnant female can create a snow cave early in winter and subsequently be covered over. A variety of weather and topographic conditions meet that requirement. Bears denning in the concentration areas of the Svalbard Islands or the large islands north of the Russian coast meet their needs in rugged mountains and fjordlands (Uspenski and Chernyavski 1965; LØnØ 1970; Uspenski and Kistchinski 1972; Larsen 1985). One famous concentration area, however, is in the relatively flat tundra along the west coast of Hudson Bay in Canada (Ramsay and Andriashek 1986; Ramsay and Stirling 1990). There, dens are concentrated along relatively low banks and ridges supporting small trees. In the regions where denning is dispersed, the topography ranges from mountainous to essentially flat (Harington 1968; Lentfer and Hensel 1980; Stirling and Andriashek 1992; Amstrup 1993; Amstrup and Gardner 1994; Messier et al. 1994; Ferguson et al. 2000a; Durner et al. 2001, 2003).
Distribution of Denning. Historically, polar bear dens were thought to represent the "core areas" of their ranges (Harington 1968). In those core areas, large numbers of polar bears repeatedly and predictably concentrated their denning within relatively small geographic regions (see Fig. 27.1). Examples of concentrated denning areas include particular islands of the Svalbard Archipelago north of Norway (LØnØ 1970; Larsen 1985); Franz Josef Land, Novaya Zemlya, and Wrangel Island in Russia (Uspenski and Chernyavski 1965; Uspenski and Kistchinski 1972); and the west coast of Hudson Bay in Canada (Harington 1968; Jonkel et al. 1975; Stirling et al. 1977b; Ramsay and Andriashek 1986; Ramsay and Stirling 1990).
Concentration or core areas were easily detected by aerial or ground surveys, and often had been known from reports of early explorers or Native people residing in the area. Early knowledge of concentrated denning led to the view that all polar bears must use such core areas, and that areas without them did not significantly contribute to polar bear reproduction. Harington (1968 :8) implied as much when he stated, "No major denning areas seem to exist in Alaska." It was assumed geographic regions without such areas mainly were populated by visiting polar bears. That concept led Native people of northern Canada to believe that harvests in Alaska were being sustained by polar bears produced in Canada (Stirling and Andriashek 1992). Over much of their range, we now know, polar bears den in a more diffuse pattern where individual dens are scattered over broad reaches of habitat at low density (Lentfer and Hensel 1980; Stirling and Andriashek 1992; Amstrup 1993; Amstrup and Gardner 1994; Messier et al. 1994; Ferguson et al. 2000a; Durner et al. 2001, 2003).
Successful denning by polar bears requires accumulation of sufficient snow that a pregnant female can create a snow cave early in winter and subsequently be covered over. A variety of weather and topographic conditions meet that requirement. Bears denning in the concentration areas of the Svalbard Islands or the large islands north of the Russian coast meet their needs in rugged mountains and fjordlands (Uspenski and Chernyavski 1965; LØnØ 1970; Uspenski and Kistchinski 1972; Larsen 1985). One famous concentration area, however, is in the relatively flat tundra along the west coast of Hudson Bay in Canada (Ramsay and Andriashek 1986; Ramsay and Stirling 1990). There, dens are concentrated along relatively low banks and ridges supporting small trees. In the regions where denning is dispersed, the topography ranges from mountainous to essentially flat (Harington 1968; Lentfer and Hensel 1980; Stirling and Andriashek 1992; Amstrup 1993; Amstrup and Gardner 1994; Messier et al. 1994; Ferguson et al. 2000a; Durner et al. 2001, 2003).