Discussion

Like chimpanzees, bonobos have a male-philopatric social structure. Males remain in their natal unit-group, whereas females leave their natal unit-group before sexual maturity and transfer between groups (Wrangham 1986, Kano 1992, Furuichi, 2006). In our study group, E and E1, no cases of male transfer were observed between 1976 and 1996, though there were many cases of female emigration and immigration during the same period (Furuichi 1989, Kano 1992, this study). However, after an interruption of the study from 1996 to 2002, at least two males joined E1 from other groups.

In early studies of primate social systems, Itani (1977, 1985) argued that each primate species has a specific basic social structure that is strongly affected by its phylogenetic position, and male or female philopatry is a basis for primate social structures. In general, this claim is still valid, but there are some exceptional cases of transfer by males or females. For example, some researchers reported temporary visits of out-group males in male-philopatric unit-groups of bonobos and chimpanzees. Hohmann (2001) reported that 2 strange adult males visited and stayed in his bonobo study group at Lomako for 12 months, and that one of them developed friendly social relationships with resident males. In addition, one juvenile male chimpanzee at Mahale encountered members of another group when his mother temporarily joined the M group (Nishida and Hiraiwa-Hasegawa 1985). At Bossou, 2 strange adult male chimpanzees joined a semi-isolated group of chimpanzees and stayed there for several days, and another adult male joined and stayed for several months (Sugiyama 1999). Moreover, most adolescent and young adult males disappeared from Bossou, and at least some of them must have emigrated (Sugiyama, 2004).

Three cases of female transfer have been reported for Japanese monkeys, which have a matrilineal social structure. Takahata et al. (1994) reported that 2 females transferred to an adjacent troop when their troop rapidly decreased in size, leaving them as the last 2 surviving members. Sugiura et al. (2002) also reported 2 cases of transfer of a female when she became the last member of a declining troop.

Gibbons have monogamous social groups in which both males and females leave their natal groups and form new ones without joining other groups. However, a young adult male and a young adult female joined non-natal monogamous groups and settled there after their forested habitat was fragmented by forest fire (Oka & Takenaka 2001).

Since most of the cases described above occurred under unusual circumstances, immigration of the strange males to E1 in our study might have also occurred under these circumstances. When we resumed research on E1, we found that their home range had expanded into the eastern area previously used by the Kofola and Bokela groups, probably because the disappearance of these groups left their home ranges vacant. When we first observed Nord, Yuki and her infant, and Jacky and her infant in E1, the group was ranging in the eastern area. Even after they joined E1, for several months they remained in the eastern area when the main members of E1 went back to the west. We also first observed Dai when E1 was ranging in the eastern area. Not only immigration of males, but also that of females with infants was unusual. We observed no permanent immigration of adult females with infants in the first 20 years of study of E and E1. Thus, this case might be better understood as aggregation of declining groups, rather than a strict intergroup transfer of adult males. Exactly what caused the immigration of Jeudy or Mori in our absence during the war is unclear. However, it is possible that something similarly unusual happened when the local population of bonobos was severely impacted by human activities.

Although some cases of male transfer have been reported in chimpanzees, there is no record of permanent immigration of adult males. This fact may reflect the intolerant relationships between males from different groups of chimpanzees (Nishida 1985, Goodall 1986, Wrangham and Peterson 1996, Reynolds 2005). Contrarily, bonobos sometimes display affinitive relationships between different groups (Idani 1990). Different groups of bonobos sometimes forage together for as long as a week, and members of these groups exhibit affiliative social interactions. Though further observation of the new immigrants is needed, the high tolerance between different groups of bonobos might have enabled the permanent aggregation of fragmented groups, as observed in the above-mentioned cases of Japanese macaques.

Acknowledgments We thank Drs. Takayoshi Kano and Toshisada Nishida for their continued support of the study at Wamba and for research guidance. We also thank Dr. Tetsuro Matsuzawa and Ms. Sally Coxe for aiding resumption of the study; Drs. Shin Nakamura and Akiko Takenaka for their support of our laboratory work; Dr. Mwanza and members of the Research Center for Ecology and Forestry (CREF) of the Democratic Republic of Congo for their support of our field work; and Dr. Shigeo Uehara and members of the Primate Research Institute, Kyoto University, for valuable discussion and advice. We are grateful to Mr. Nkoi Batolumbo and other local staff and villagers for their support during and after the war. This study was supported by the National Geographic Fund for Research and Exploration (#7511-03 to Furuichi), the JSPS core-to-core program HOPE (#15001 to Matsuzawa), JSPS Grant-in-Aid for Scientific Research (#17570193 to Hashimoto, #12575017 and 17255005 to Furuichi), and Japan Ministry of Environment Global Environment Research Fund (#F-061).

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