Pregnant female polar bears enter their dens in the autumn after drifts large enough to excavate a snow cave are formed. Because polar bears (in most areas) den only in ice and snow rather than in the soil under the snow, the annually variable snow and ice conditions mediate when and where bears enter their dens each autumn. Polar bears depart dens in the spring when their cubs are able to survive in the outside climate. Until the advent of effective radiotelemetry, little was known about the chronology of denning. Larsen (1985) reported that most dens on Svalbard were opened in late March and vacated by mid-April. Lentfer and Hensel (1980) reported Alaskan polar bears came ashore to den in late October and early November and left their dens in late March and early April. L�n� (1970) concluded dens on Svalbard were entered in November and December and abandoned between 10 and 25 April. At the far north of Svalbard, he speculated that bears entered dens as early as late October. Observations in other High Arctic areas suggest abandonment between mid-March and mid-April (Uspenski and Chernyavski 1965; Kistchinski 1969; Belikov et al. 1977). Hansson and Thomassen (1983) suggested the first dens were opened in the first week of March and most were abandoned by mid-April. Kolenosky and Prevett (1983) and Ramsay and Andriashek (1986) reported emergence from dens in the Hudson Bay area in late February and early March. Polar bears are largely food deprived while on land in the ice-free period. During this time, they survive by mobilizing stored fat. Pregnant females that spend the late summer on land and then go right into dens may not feed for 8 months (Watts and Hansen 1987; Ramsay and Stirling 1988). This may be the longest period of food deprivation of any mammal, and it occurs at a time when the female must give birth and nourishment to her new cubs.
Satellite telemetry has now confirmed that the chronology of denning varies somewhat around the world. In the Beaufort Sea, mean dates of den entry were 11 and 22 November for land (n = 20) and pack-ice (n = 16) dens, respectively (Amstrup and Gardner 1994). Female bears continued foraging right up to the time of den entry. Then they denned near where they happened to be foraging. On average, Beaufort Sea polar bears emerged from their dens with new cubs on 26 March if they were on the pack ice (n = 10) and 5 April if they were on land (n = 18). Dates of entry and exit varied somewhat among years depending on sea-ice, snow, and weather conditions.
Messier et al. (1994) reported the mean entry into maternal dens in the Canadian Arctic was 17 September (SE = 3 days; range 27 August � 12 October) and mean emergence was 21 March (SE = 3 days; range 4 March � 7 April). Females and their cubs remained near dens for a mean 13 (SE = 3) days in the spring before leaving the denning area.
Those data may indicate an earlier and more protracted denning period at higher latitudes than in the Beaufort Sea. Ferguson et al. (2000a), on the other hand, observed that bears denning at higher latitudes entered their dens a bit later than those to the south, but that exit times did not differ by latitude. They reported a mean den entry of 15 September (1 September�7 October), a mean exit of 20 March (15�28 March), and a mean 180 days in dens (163�200 days)
As noted, initiation of denning depends on sufficient snow accumulation to allow excavation of a den cavity. For bears denning on sea-ice or moving from sea-ice to land denning habitat, timing of sea-ice consolidation can alter the onset of denning. Sea-ice dens must be in ice stable enough to stay intact for up to 164 days while being pushed by currents for hundreds of kilometers.
Whereas only pregnant female polar bears enter dens for the entire winter, any bear may enter shelters for shorter periods to avoid storms, extreme cold or heat, or periods of poor hunting. Sheltering is best known along the west coast of Hudson Bay. Because the ice in Hudson and James Bays disappears entirely, the whole population there is forced onto land in summer. Feeding opportunities are minimal, and many animals take shelter in earthen dens, where it is cooler and they minimize insect harassment. When the ice forms in fall, most of the bears in earthen shelters go out on the new ice to hunt. Pregnant females, however, remain in the dens and eventually move into snow that drifts over their earthen structures (Stirling et al. 1977b; Derocher and Stirling 1990).
Use of shelter dens also occurs at higher latitudes. Messier et al. (1992) reported that long periods of "sheltering" were common among all classes of female bears (except those in maternal dens) wintering in Viscount Melville Sound. Females entered shelters on average on 18 December (SD = 7 days) and stayed an average of 53 days (SD = 9 days). The duration of sheltering ranged from 25 to150 days. Messier et al. (1992) attributed this behavior to the poor foraging conditions in the Viscount Melville Sound region. In Baffin Bay and the eastern Canadian High Arctic, Ferguson et al. (2000a) reported a bimodal incidence of sheltering. Autumn sheltering occurred from mid-September to early November, whereas winter shelters were occupied mainly from late December to March. Autumn shelters were occupied for a mean 56 days (range = 50�70), whereas winter shelters were occupied for a mean 65 days (range = 35�86). At higher latitudes, the frequency of winter sheltering increased and the frequency of autumn sheltering decreased.
Although more female polar bears have been followed for longer periods there than anywhere, sheltering for protracted periods has not commonly been observed in the Beaufort Sea region (Amstrup and Gardner 1994; S. C. Amstrup, unpublished data). Yet, the latitude of the Beaufort Sea is in the middle of the range reported by Ferguson et al. (2000a). Clearly, use of sheltering in the eastern Canadian High Arctic and Baffin Bay is not so much a function of latitude as of sea-ice and other ecological conditions. Sea-ice formation and ablation, weather, and prey availability, although influenced by latitude, are as much controlled by the shapes of coastlines, presence of islands, water depths, currents and other factors. In areas of Baffin Bay and Davis Strait, seasonal absence of sea-ice forces polar bears onto land in autumn as it does in Hudson Bay. While on land, the best strategy is to conserve energy (Nelson et al. 1973; Guppy 1986). Conversely, ice is available in the High Arctic year-round, and autumn sheltering is less necessary. The winter increase in sheltering at higher latitudes is probably an adaptation to avoid the harshest of winter weather and heavy ice. In the Beaufort Sea, the sea-ice substrate is available year-round, but winter ice conditions may not be as harsh as in the eastern High Arctic. Those realities may ameliorate the need for sheltering. Messier et al. (1994) also concluded that variations in the availability of satisfactory hunting conditions may encourage a facultative approach to use of shelters.
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