Abstract

In wild populations, inbreeding tolerance is expected to evolve where the cost of avoidance exceeds that of
tolerance. We show that in a wild population of bottlenose dolphins found in East Shark Bay, Western
Australia, levels of inbreeding are higher than expected by chance alone, and demonstrate that inbreeding
is deleterious to female fitness in two independent ways. We found that inbred females, and females with
inbred calves, have reduced fitness (lower calving success). We further show that one of the costs of
inbreeding is extended weaning age, and that females’ earlier calves are more likely to be inbred. While
the exact causes of inbreeding remain obscure, our results indicate that one factor is female age, and
thus experience. Any inbreeding avoidance mechanisms such as female evasion of kin, or male dispersal,
do not seem to be completely effective in this population, which supports the view that inbreeding
avoidance does not always evolve wherever inbreeding incurs a cost.