Should I stay or should I go?: Natal dispersal in the brown bear

We studied the causes of natal dispersal of male and female brown bears, Ursus arctos, in two study areas in Sweden. Males had a higher dispersal probability ( 94%) than females ( 41%). For males, we found no difference in dispersal probability or mean age of dispersal between the study areas, in spite of differences in population density and sex ratio. Male-male competition did not seem to influence subadult male dispersal probability significantly. These results support the inbreeding avoidance hypothesis as the cause of male natal dispersal. For females, dispersal probability decreased with increasing maternal age and decreased with increasing body size, and an interaction between maternal age and body size suggested that the importance of body size decreased with increasing maternal age. Nondispersing females were closer to their mother than their dispersing sibling sisters were in the period between weaning and dispersal. Female littermates seemed to compete for philopatry, suggesting that a dominance hierarchy among female littermates based on body size may cause the subdominant sister to disperse. If juvenile females are born into matrilineal assemblages, surrounded mostly by related females, the competition for philopatry may not be as severe as when they are born into an area surrounded by mostly nonkin females. This hypothesis is supported by the decreasing importance of body size for dispersal with increasing maternal age. We suggest that natal dispersal in juvenile female brown bears can be explained by the resident fitness hypothesis. (C) 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.