It has been calculated that their caloric needs require one ringed seal every six and a half days. Arctic foxes live on the sea ice in winter by scavenging polar bear kills. Since their prey is available year-round, polar bears do not hibernate like brown bears, except pregnant females, who spend about five months in dens to give birth to their cubs. The female must greatly increase her weight, mostly in fat, to carry off a successful pregnancy and denning. Cubs generally remain with their mother for two and a half years.
Females are therefore able to bear young only every three years.
This low rate of reproduction is balanced by a long life and low rates of natural mortality. Moving in autumn from drifting ice to suitable denning sites requires a remarkable and little understood navigational ability. An important denning area for the Beringia population is on Wrangel Island. Denning also occurs on the northeastern coast of Alaska, although a majority of the Beaufort population dens on sea ice. Once they learned this life-style they spread quickly along Arctic coasts.