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Sociosexual interactions of non-human primates have multiple functions, and information on partner choice could help us to determine the major purpose of these behaviors. Female bonobos (Pan paniscus) frequently engage in genito-genital (GG) rubbing, which is categorized as a sociosexual behavior. The functions of GG rubbing may vary across allopatric bonobo populations, especially in relation to its use in social bonding. Thus, we aimed to examine the use of GG rubbing in the formation and maintenance of social bonds by examining partner choice in this context in the habituated bonobo population at Wamba, Democratic Republic of the Congo. We examined the effect of female age (and correlated dominance rank) on the proportion of solicited GG rubbing, and the effects of age difference, proximity index, and grooming index on the successful GG rubbing occurrences. Our results showed that female age significantly affected the proportion of solicited GG rubbing, indicating that older and higher-ranking females solicited this activity more frequently. Individuals of female-female dyads who were close in age and dominance rank frequently engaged in GG rubbing. The more the females in a dyad were in physical proximity, the more they engaged in GG rubbing. No correlation was observed between grooming and GG rubbing. These results indicate that partner choice in GG rubbing is highly dependent on physical proximity, and suggest that characteristics of female gregariousness might be important with respect to this choice among bonobos.

Among non-human primates, male dominance rank is not necessarily a good indicator of mating success, and relationships between male dominance rank and mating or reproductive success are affected by female behavior and sexual states implying their probability of conception. Although comparisons of the behavior of male chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) suggest that the effect of male dominance rank on mating success is expected to be less prominent in bonobos, recent genetic studies have shown that high-ranking male reproductive skew is higher in bonobos than in chimpanzees. One possible reason for the higher reproductive skew in bonobos could be that female behavior and their sexual states may have much more influence on male mating and reproductive success in bonobos than in chimpanzees. In the current study on bonobos, we conducted focal animal observation of females and analyzed the influence of female sexual swelling, the number of days after parturition, and dominance rank of males on female associations, and copulation with adult males. Our results showed that females with maximum swelling (MS) had more proximity with high-ranking males and copulated more frequently with higher-ranking males than with lower-ranking males. Females for whom longer time had elapsed since parturition, and therefore had higher probabilities of conception, had 5-m proximity with adult males more frequently than females whom shorter time had elapsed since parturition, but did not have more copulation with adult males. Females with MS had proximity and copulated with high-ranking males frequently, which partly explains why the reproductive skew is so high in bonobos. These results are discussed in relation to previous hypotheses on the influence of long-lasting mother-son relationships on mating success of males and on the contribution of female receptivity during non-conceptive nursing periods to moderate intermale aggression in bonobos.