Determinants of home range size for polar bears (Ursus maritimus)

The mean home range size of female polar bears (Ursus maritimus, 125 100 km (2) +/- 11 800; n = 93) is substantially larger than the predicted value (5 14 km(2)) for a terrestrial carnivore of similar weight. To understand this difference, we correlated home range size and sea ice characteristics. Hom e range size was related to (i) the ratio of land vs. sea within a given ho me range (42% of explained variance), and (ii) seasonal variation in ice co ver (24%). Thus, bears using land during the ice-free season had larger hom e ranges and bears living in areas of great seasonal variation in ice cover also had larger home ranges. In another analysis we investigated how varia tion in a bear's environment in space and time affects its choice of home r ange. We found that polar bears adjusted the size of their home range accor ding to the amount of annual and seasonal variation within the centre of th eir home range. For example, polar bears experiencing unpredictable seasona l and annual ice tended to increase their home range size if increasing hom e range size resulted in reducing variation in seasonal and annual ice. Pol ar bears make trade-offs between alternate space-use strategies. Large home ranges occur when variable ice cover is associated with more seals but als o a more unpredictable distribution of those seals.