Conclusion

Differences in the behavior of male bonobos and chimpanzees have often been shown, mainly in terms of aggression and relationships with their mothers. However, in this chapter we have shown that there are many similarities between males of these species who live in similar male-philopatric societies. Males of both species try to stay with their mothers as much as possible after weaning, and even full adult males range with their mothers when possible. Males remain in their natal groups throughout their lives. Intergroup transfer occurs rarely, confirmed in only two cases of bonobos in Wamba and under an unusual circumstance where neighboring groups became extinct. Although there is no difference in genetic distance between females within and between populations, the genetic distance between males is significantly greater between groups in both bonobos and chimpanzees. Although bonobos and chimpanzees tend to have different group sizes, when comparing parties of bonobos that always have receptive females to parties of chimpanzees on the days when receptive females are present, the rate and number of males participating in these parties are very similar between the two species. With regard to intragroup affiliative relationships, the mother–son relationship is prominent in bonobos. If we exclude this, the frequency of grooming is highest among males, which is similar to what is observed in chimpanzees. Considering encounters between groups, peaceful encounters and mixed ranging with members of different groups are frequently observed in bonobos, but males often show aggressive behavior, especially during the beginning of encounters, and grooming between males of different groups is rare. For aggressive behavior between males within a group, there is no notable difference in the frequency of aggressive interactions between the two species, but the frequency of physical aggression is lower in bonobos. In addition, aggressive interactions among male bonobos predominantly occur between the sons of the two highest-ranking females. If these dyads are excluded, the frequency of aggressive interactions between males is much lower in bonobos than in chimpanzees. Although female chimpanzees rarely participate in aggressive interactions among males as aggressors, mothers and other bonobo females participate in the aggressive interactions between males and supported their sons. No difference in mating behavior is observed between the two species, either in terms of alpha males tending to mate most frequently or in the proportion of mating monopolization by alpha males.

Analyses of the factors responsible for the large differences between bonobos and chimpanzees observed in intermale relationships reveal the significant effects of female behavior. The strong lifelong relationship between mothers and sons in bonobos is affected by the tendency of bonobo females to attend mixed-sex parties irrespective of their sexual state. Male aggression by bonobo males is lower than that of chimpanzees during intergroup encounters. This, however, is likely related to the fact that females take the initiative to mingle during intergroup encounters and that male aggressive behavior is dampened by females from the other group. Bonobo females also show receptivity during postpartum infertility. This means that there is always more than one receptive female with a maximal sexual swelling in the group. This too may also suppress aggression between males. Another distinctive feature of bonobos is that alpha male status is strongly affected by the dominance status and support of mothers. This is thought to occur because of the long-lasting and close mother–son relationship and the high dominance status of females. It is surprising that alpha male bonobos monopolize paternity more than do male chimpanzees. This may also reflect the reproductive strategy of females, in which high-ranking females promote aggregation of females around themselves and place their sons in positions for to obtain more opportunities to produce offspring. To understand differences in socio-sexual characteristics between bonobos and chimpanzees, it is essential to understand the evolution of female behavior and physiology. Why do bonobo females show high levels of aggregation? Why are they receptive during postpartum infertility? And why do they possess social statuses comparable to or even higher than that of males? These questions are discussed by Furuichi (2023).