Social animals have to establish and maintain social relationships with other group members throughout their lives. To do so, they rely on social grooming, a behavior which refers to the inspection and manipulation of the hair, skin, or oral structures with the hands or mouth of another individual (Boccia 1998). This behavior is often used as an index to evaluate the strength and quality of social relationships among pairs of individuals (Silk 2007) and it is broadly observed across animal taxa; from honey bees to bonobos (bats, Wilkinson 1986; primates, Goosen 1987; bonobos, Kano 1992; social insects, Schmid-Hempel 1998; ungulates, Mooring et al. 2004).

Conclusions

This chapter first provides long-term demographic data, based on simultaneous observations of multiple bonobo groups at Wamba, to investigate the timings and locations of individual female migrations. Emigration and immigration events involving nulliparous females were collected from two main study groups (E1 and PE) observed on a daily basis, as well as three neighboring groups (PW, BI, and IY) observed occasionally. Each event was retrospectively sub-classified into being temporary (breakaway/visit) and permanent (dispersal/settlement). The age distribution for each class was calculated as follows: breakaway (6.7 ± 1.5 years), dispersal (7.5 ± 0.8 years), visit (9.1 ± 1.7 years), and settlement (9.5 ± 2.2 years). Female bonobos demonstrated an average age gap of approximately 2.0 years between dispersal and settlement, although the duration from dispersal to settlement varied by individual. For example, four females (Puffy, Ichiko, Sachi, and Debby) directly settled down in E1 from their natal group, while one female (Ana) spent at least 3.5 years in E1 before reproducing in IY. Additionally, we reported that the estimated first parturition age of eight females, whose approximate birthdate was known, was 10.7 ± 0.8 years, which is younger than the previously estimated value of 14.2 ± 0.8 years.
In this chapter, we have classified the natal transfer process into four phases: (1) pre-dispersal, (2) dispersal, (3) pre-settlement, and (4) settlement to investigate the proximate mechanisms underlying each transfer phase in female bonobos. Below, we condense the discussions for each transfer phase.

1. Pre-dispersal phase: Previous studies have shown that weakened spatial relationships with mothers and sexual development related to the onset of puberty precede dispersal timings in female bonobos (Toda et al. 2021, 2022). Independence from maternal care may be associated with the decision to disperse and is considered one of the prerequisites for survival outside the natal group. Pubertal changes seem to intensify the attraction of female bonobos towards other groups, but it remains unknown whether the motivation for outbreeding incites dispersal.
2. Dispersal phase: This chapter is in line with previous bonobo studies at Wamba (Idani 1990; Kano 1992; Sakamaki et al. 2015), showing that female bonobos often migrate through intergroup encounters. The current data imply that female bonobos might be less likely to disperse to a group that they have frequently encountered during their early life. The decision of dispersal can be influenced by the novelty value of outgroup conspecifics, determined by spatial distance from and social relationship with the natal group.
3. Pre-settlement phase: The finding that immigrant females were more likely to settle down into E1 than in PE could be attributed to their preferences for the new group where they will reproduce. Here, we discuss two preliminary hypotheses: female bonobos might prefer settling down into a group with (A) a large number of mature males to reduce the future risk of secondary transfer, or (B) familiar conspecifics from the same natal group to acquire cooperative social partners from the begining of their immigration. However, there is scant evidence to date that supports these hypotheses in bonobos.
4. Settlement phase: The fact that immigrant females who had continuously stayed in the group for over a year all settled in that group suggests that a certain duration of association with resident conspecifics could cause female bonobos to be attached to the new group. If so, a significant interval between intergroup encounters in E1 might have led immigrant females to become established in this group before they had the next opportunity to encounter another group. Detailed demographic data on immigrant females in groups other than E1 and PE are needed to examine the relationship between the duration of stay and the outcome of settlement.

This chapter advances our knowledge about the natal transfer process of female bonobos. However, further research is necessary before drawing general conclusions, as the relevant data involve only a small sample size at Wamba and other field sites are even less represented. We expect the definitions for emigration and immigration events used in this chapter will serve as a foundation for future studies investigating female transfer processes in other bonobo populations and other primate species. We also highlight the importance of simultaneous data collection from multiple groups in a population. This approach will enable tracking of known individuals beyond their groups and identifying the contexts and consequences of their migration events (e.g., Strier et al. 2015; Walker et al. 2018; Manguette et al. 2020).