Observations of intraspecific killing, cannibalism, and aggressive behavior among polar bears (Ursus maritimus) in the eastern Barents Sea and the Kara Sea

Abstract

The ability of the polar bear (U. maritimus) to kill and consume conspecifics is accepted, but the reasons behind this behavior and its prevalence remain unclear. Here, we report 14 cases of intraspecific killing, cannibalism, and aggressive behaviors among polar bears observed in seven instances in the Barents Sea and the Kara Sea on Frans-Josef Land, Novaya Zemlya, and Vize Island. All but one of the observations were made on land during the ice-free season. Perpetrators, when observable, were mostly thin. The hunger resulting from a failure to gain fat reserves in spring appears to motivate perpetrators, while bear aggregation on islands could facilitate successful intraspecific killing. Surprisingly, cases of intraspecific killing were observed even in years when sea-ice conditions in the previous spring were relatively good. In one case, the killing of a cub of the year in the spring by a large, fat male could hardly be explained by nutritional demands. This could be a case of sexually selected infanticide, as the killing of cubs during the mating season increases the male’s opportunity to breed.