Bonobos and chimpanzees have a male-philopatric social structure. 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 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. Thus, it is not clear how consistent male philopatry is in chimpanzees and bonobos, and under which circumstances male transfer occurs in these species. We have been conducting research on wild bonobos at Wamba since 1974 and have studied various aspects of bonobo ecology and behavior. Since the original identification of all members of the main bonobo study group in 1976, all natal females have disappeared before maturity, and no males have immigrated into the study group (Furuichi 1989); thus, we had been confident that male philopatry was a rigid social structure for wild bonobos. Our long-term research was interrupted by political disorder in 1991. When we resumed research in 1994, we found that some individuals of our study group had disappeared (Furuichi et al. 1998). In 1996, our research was again interrupted by civil war. When we visited Wamba to observe security conditions in 2002 during a ceasefire, we confirmed that the main study group, E1, had survived the war (Furuichi and Mwanza 2003)...