Primate societies are diverse in terms of social organization and structure (Smuts et al. 1987; Boesch et al. 2002). The society, or social unit, is the set of conspecific animals that interact regularly and more commonly with each other than with members of other societies (Kappeler and van Schaik 2002). Traditionally, social organization, which has been the basis for characterizing primate societies, can be classified into three types: solitary, pair-living, and group-living (Kappeler and van Schaik 2002). Social units, including mature males, fertile females, and their offspring, function as the reproductive unit and are linked via two processes (Itani 1985). First, sex-biased dispersal across social units is a near-ubiquitous feature of the life history of mammals and birds (Handley and Perrin 2007). Second, different groups in a population occasionally come together during intergroup encounters.

Conclusion

Recent studies have shown that high fruit abundance or reduced food competition is a prerequisite for frequent intergroup encounters and subsequent associations at Wamba and Kokolopori (Sakamaki et al. 2018; Lucchesi et al. 2020), whereas similar frequent intergroup encounters have not been observed at Lomako or LuiKotale (Hohmann and Fruth 2002; Fruth and Hohmann 2018; Moscovice et al. 2022). A comparative study involving study sites where bonobos in multiple groups have been fully habituated is needed to examine the associations between short- and long-term competition over food and mates and patterns of intergroup encounters and interactions. From the food competition hypothesis, we assume at least three degrees of long-term feeding competition in bonobo habitats, “rich” sites where prolonged intergroup associations are observed such as at Wamba and Kokolopori, “moderate” ones where prolonged intergroup associations are rare such as at LuiKotale and Lomako, and “poor” ones in forest-savanna mosaic habitats (Serckx et al. 2015; Pennec et al. 2020), where only brief encounters and avoidance between groups may occur. Furthermore, one key factor in food conditions may be related to the Landolphia fruits because the lianas provide relatively large patches that may prolong intergroup associations (Landolphia hypothesis) (Kano and Mulavwa 1984; Sakamaki et al. 2018). The density of Landolphia spp. may vary considerably in bonobo habitats because several species of Landolphia woody vines have been cut down in some areas for natural rubber in the past.
The female cooperation hypothesis is a leading one among potential benefits derived by intergroup associations in bonobos. Bonds formed between females of different groups may be the foundation for a regionally structured society above the group level, which is a marked feature of human societies (Chapais 2011; Hill et al. 2011; Furuichi 2020). A future challenge is to elucidate the long-term benefits that females would gain through intergroup cooperation, including improvements in female reproductive success, dominance status, and feeding priority. It also remains unclear whether female cooperation can occur between two groups that infrequently encounter each other and whether such intergroup bonds promote encounters between the two groups. Furuichi (2020) hypothesizes that female dispersal and kinship between groups may be a foundation for affiliative intergroup relationships. However, young females typically visit several different groups for several years and finally settle in a new group beyond their neighboring groups (Kano 1992; Sakamaki et al. 2015). Recently, some young females settled in the E1 group from its neighboring groups at Wamba, but this is mainly because the E1 group rarely encountered any neighboring groups in the far eastern areas (Fig. 2; Toda et al. 2022). Future genetic studies will elucidate the associations between kin structure among multiple neighboring groups and intergroup affiliation (Ishizuka et al. 2018, 2020).
Chronological changes of intergroup relationships such as “affiliative” and “avoidance” described in previous studies at Wamba (e.g., Kano 1992; Furuichi 2020) were mainly based on the encounter frequency. Concerning encounter frequency with patterns of intergroup interactions, we need to carefully deal with the data collected during the process of habituation for 5–10 years. If we could control the impact of habituation at sites where long-term food competition was relatively low, all intergroup relationships would be “affiliative,” or every group would experience frequent intergroup associations. Further studies on fully habituated bonobos in populations with different food conditions will elucidate the effects of food competition and female cooperative bonding on the patterns of intergroup associations and interactions. These observations will reveal whether intergroup female bonds depend on frequent intergroup associations under reduced food competition and/or close relatedness between groups and whether female intergroup bonds result in regionally structured societies beyond the social group in bonobos.