Abstract
Individual dispersal is seen throughout the biological world. The present article discusses the dispersing sex of chimpanzees and humans with special reference to their common ancestors from the point of view of their biological characters, which are basically common with other mammalian species, concerning gestation, parturition, lactation and nursing. Many mammalian species have male-dispersal and female-philopatric (or short-distance dispersal) societies. However, chimpanzees are said to have male-philopatric and female-dispersal societies. Comparing with those of Bossou, Guinea, where males as well as females during adolescence may disperse, I hypothesized that male chimpanzees of other research fields allow each other in a group and form ‘male-bond’ to defend their resources from conspecific competitors and predators. The necessity for hunting cooperation may be a subsidiary factor. However, the problem why non-related males cannot be accepted and all related males cooperate with each other in their natal group remains to be solved. On the other hand female dispersal is not unusual among mammals in a habitat having few predators. However, the problem why young pregnant females and mothers with dependent infants transfer to an adjacent group in spite of the possibility of receiving infanticide also remains to be solved. Many scholars hypothesize that common ancestor of chimpanzees and humans also had a male-philopatric and female-dispersal society just as most present day human societies have. However, the transfer of a human female to a distant community or region does not depend on her own decision but that of her parents in exchange for treasure or for political purposes. Instead of a rigid sex-biased dispersal society I hypothesized the existence of a fluent and flexible bilateral society as our common ancestors.
