Prolonged maximal sexual swelling in wild bonobos facilitates affiliative interactions between females

in Behaviour
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Perineal sexual skin swelling in relation to menstrual cycle occurs in a variety of primate taxa. However, sexual swelling with exaggerated size and colour is found only in some Old World monkeys and the two Pan species. Although several hypotheses have been proposed (e.g., reliable indicator hypothesis and graded signal hypothesis), it seems unlikely that a single explanation can account for the significance of the sexual swelling in all of these species. Bonobos (Pan paniscus) provide an excellent opportunity for studying sexual swelling since they have the most prolonged maximal swelling periods among primates. In this study we propose a new hypothesis that sexual swelling in female bonobos increases their attractiveness to other females and thereby facilitates affiliative social interaction with them. We found that free-ranging female bonobos with maximal sexual swelling engaged in affiliative social interactions with other females, including genito-genital rubbing, staying in close proximity and grooming, more frequently than females without maximal swelling. These tendencies suggest that females with maximal swelling were attractive to other females. The results also suggest that the benefits of maximal swelling might vary among females depending on their life-history stage. In particular, young females may get more benefits from prolonged maximal swelling through increased grooming reciprocity and staying in close proximity to other females. Thus our study supported the hypothesis that one function of prolonged maximal swelling in bonobos is to increase attractiveness to other females, thereby enhancing affiliative relationships between females in a male-philopatric social system.

Prolonged maximal sexual swelling in wild bonobos facilitates affiliative interactions between females

in Behaviour



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    Mean number of copulations and g-g rubbing interactions per hour with standard error. (a) Copulations in relation to swelling status and age. (b) G-g rubbing interactions in relation to swelling status and age. This figure is published in colour in the online edition of this journal, which can be accessed via

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    Mean number of females within 1 m, 5 m and party. (a) Number of females within 1 m in relation to swelling status and age. (b) Number of females within 5 m in relation to swelling status and age. (c) Number of females in 1-h party in relation to swelling status and age. This figure is published in colour in the online edition of this journal, which can be accessed via

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    Grooming frequency and reciprocity. (a) Proportion of focal sessions in which grooming interaction was observed. (b) Grooming reciprocity index. Each box indicates upper quartile and lower quartile and dots indicate outliers. This figure is published in colour in the online edition of this journal, which can be accessed via

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