During a study of the reproductive ecology of polar bears (Ursus marit
imus) in western Hudson Bay (Canada), we documented a case of litter a
doption. In an eight-month period, a ten-year-old adult female lost a
litter of two cubs-of-the-year and adopted three other cubs-of-the-yea
r. This is the first reported case of natural offspring adoption in po
lar bears, and its significance as a reproductive strategy is unknown.
Nevertheless, the observation raises questions regarding the social c
ircumstances under which adoption may occur and the benefits or costs
to maternal fitness in a solitary mammal such as the polar bear.