The rules of disengagement: Takeovers, infanticide, and dispersal in a rainforest lemur, Propithecus edwardsi

Dispersal is a critical issue for understanding various aspects of animal biology and for conserving endangered species. We investigated the conditions that determine dispersal in four groups of sifakas inhabiting Ranomafana National Park, Madagascar. Following this population for 22 years, we collected and analyzed data on birth, relatedness, immigration, dispersal and death of 77 sifakas. We predicted that dispersal would function to prevent inbreeding and occur seasonally to aid establishment in a group before the breeding season. We further predicted that immigrants would evict same-sex residents and immigrant males would commit infanticide to shorten interbirth intervals. Although most dispersal theories predict male-biased dispersal in mammals, both male and female sifakas dispersed at equal rates from their natal and breeding groups. Behaviourally and genetically, there was no evidence for inbreeding within this population; individuals never produced offspring with close relatives. Dispersal matched the strong seasonal pattern of reproduction. Immigration events were always associated with the disappearance of the group infant(s). As a result of infanticide by male immigrants, mothers came into estrus sooner; infanticide by female immigrants caused mothers to disperse from their groups. This study reveals a clear pattern of dispersal that is condition, not sex, dependent.