Longitudinal Structure of a Unit-group of Bonobos: Male Philopatry and Possible Fusion of Unit-groups

Bonobos and chimpanzees have a male-philopatric social structure (Nishida 1979, Itani 1985, Goodall 1986, Wrangham 1986, Pusey and Packer 1987, Kano 1992, Wallis 1997, Reynolds 2005, Furuichi 2006). Demographic data from long-term research sites show that all males remain in their natal groups throughout life, while most females leave their natal groups and join neighboring groups. Itani (1977, 1985) argued that female or male philopatry is a rigid, species-specific social structure. However, some researchers have reported cases in which male chimpanzees or bonobos joined non-natal groups (Nishida and Hiraiwa-Hasegawa 1985, Sugiyama 1999, 2004, Hohmann 2001). Thus, it is not clear how consistent male philopatry is in chimpanzees and bonobos, and under which circumstances male transfer occurs in these species.