Although male-biased transfer is more prevalent among group-living mammals (Greenwood 1980; Pusey and Packer 1987), female-biased transfer exists in some primate taxa (Moore 1984; Lee and Strier 2015). Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), two of the four great ape species in Africa, diverged approximately 1.0 million years ago (Takemoto et al. 2015). Chimpanzee and bonobo females leave their natal group and transfer to another group before the first parturition (Nishida and Kawanaka 1972; Pusey 1979; Kano 1982). In contrast, males remain in their natal group and thus maintain the group’s home range across generations. Therefore, chimpanzee and bonobo societies show a structure that can be called a patrilineal basic social unit (Itani 1980).

Conclusion

In general, female chimpanzees maintain strong social bonds with their mothers when they remain in their natal group (Kahlenberg et al. 2008b; Langergraber et al. 2009) and maternal presence/rank enhances their reproductive rates (Foerster et al. 2015; Walker et al. 2018). To the best of our knowledge, this is not the case for female bonobos (Moscovice et al. 2017). From 3 to 4 years of age, female bonobos begin to exhibit weaker spatial relationships with their mothers compared to female chimpanzees (Lee et al. 2020) and they become estranged from their mothers before emigration. Given these characteristics, mothers may be less important for female bonobos than for female chimpanzees (Toda and Furuichi 2022).
In chimpanzee societies, like bonobos, migrating adolescent females follow females of new groups (Hasegawa, personal communication). However, in the case of chimpanzees, adolescent females do not overtly and persistently follow specific individuals and it seems that there are no SSF-like figures. This difference between chimpanzees and bonobos is likely due to the different positions of females in each society. Chimpanzee females are highly dispersed in their group and often behave hostile toward newer females, and when male chimpanzees reach full maturity, even sons overtake their mothers in social status (Nishida 1989). On the other hand, because male dominance rank in bonobos changes over time during which old females remain dominant and at the top hierarchy of the group (Furuichi 1997), it may be more efficient for immigrant females to build a close relationship with high-ranking females rather than with high-ranking males. Immigrant females provide affiliative grooming more frequently to higher-ranking females compared to lower-ranking females, and because lower-ranking natal females also prefer to groom high-ranking females, it is possible that all adolescent female bonobos—regardless of immigration status—target higher-ranking females as social partners to facilitate integration into the adult social group (Toda and Furuichi 2022).
Bonobo females engage in various social interactions such as frequently grooming each other, and most importantly, genital rubbing which is unique to this species (Kuroda 1980; Idani 1990; Hohmann et al. 2009; Tan and Hare 2013; Tan et al. 2017; Moscovice et al. 2019; Tokuyama et al. 2019). The social bond between females is strong, and it can be said that females are the core of the group. However, for newly immigrated adolescent females, a new group is unfamiliar and stable social relationships with other members have not yet been established. Therefore, adolescent females engage in various behaviors to be accepted by group members (especially parous females) and to actively form relationships with them.
In summary, adolescent female bonobos gradually separate themselves from the members of their natal group as they reach sexual maturity, until one day they disappear in search of a new group. They may migrate to several groups until they permanently immigrate (i.e., settle) into one group. In their new group, they form special relationships with older females using various strategies and are eventually accepted as members of the group.