Probable community transfer of parous adult female chimpanzees in the Budongo Forest, Uganda

Female chimpanzees with dependent offspring generally avoid border areas of their community's home range because they risk aggression and infanticide from extracommunity males. Typically, only nulliparous females risk crossing the boundary areas to transfer between communities; while immigration of parous females occurs, it is extremely rare and dangerous for the females and their offspring. In the Budongo Forest, Uganda, where researchers have continuously studied the Sonso community since 1990, our field data provide strong indications that >= 5 adult females with offspring have immigrated into the community. If the interpretation is correct, then it has fundamental implications for our understanding of female chimpanzee social strategies and dispersal patterns. Further, the identification of such a large number of new individuals within a short time frame is remarkable and suggests a major event, such as the breakup of a neighboring community or major habitat disturbance. We explore the evidence that points to the events as immigrations and discuss the implications for understanding the chimpanzee social system.