Relationships between denning of polar bears and conditions of sea ice

We examined shelter and maternity dens used by 97 adult female polar bears (Ursus maritimus) in relation to conditions of sea ice. Obligate use of mat ernity dens for pregnancy, birth, and lactation varied little with latitude or area. In contrast, timing of facultative use of shelter dens switched f rom autumn in the southern area (<70<degrees>N) to winter in the northern a rea (>75 degreesN). For the southern area, 13 of 16 female polar bears used shelter dens in autumn versus winter (median dates of entry and exit, 11 S eptember and 2 November; total = 56 days), whereas in the northern area, 11 of 17 bears used shelter dens in winter versus autumn (median dates, 24 De cember and 2 March; total = 65 days). Difference in facultative use of shel ter dens was associated with conditions of sea ice, Southern regions had no sea ice when polar bears used shelter dens. In contrast, northern areas ha d more constant ice conditions that included presence of ice throughout the year. Southern regions seem to have greater primary productivity and more seals as a result of a pronounced seasonal cycle of annual ice. Polar bears in northern areas responded to the more constant ice conditions and less p roductive environment with use of shelter dens during the period with lowes t seal accessibility.