Denning Chronology. 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. LØnØ (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 September­7 October), a mean exit of 20 March (15­28 March), and a mean 180 days in dens (163­200 days)