Abstract

Patterns of kinship among individuals in different groups have been rarely examined in animals. Two closest living relatives of humans, bonobos and chimpanzees share many characteristics of social systems including male philopatry, whereas one major difference between the two species is the nature of intergroup relationship. Intergroup relationship is basically antagonistic and males sometimes kill individuals of other groups in chimpanzees, whereas it is much more moderate in bonobos and copulations between individuals of different groups are often observed during intergroup encounters. Such behavioural differences may facilitate more frequent between-group male gene flow and greater between-group differentiation in male kinship in bonobos than in chimpanzees. Here we compared differences between average relatedness among males within groups and that among males of neighbouring groups, and between-group male genetic distance between bonobos and chimpanzees. Contrary to expectation, the differences between average relatedness among males within groups and that among males of neighbouring groups were significantly greater in bonobos than in chimpanzees. There were no significant differences in autosomal and Y-chromosomal between-group male genetic distance between the two species. Our results showed that intergroup male kinship is similarly or more differentiated in bonobos than in chimpanzees.

Highlights

  • Patterns of kinship among individuals in different groups have been rarely examined in animals

  • To clarify whether differences in intergroup interactions between the two species may be reflected in patterns of intergroup male kinship, the extent of between-group differentiation in male kinship should be directly compared between the two species using empirical data

  • The relationship between the dyad category and average relatedness value was significant between the two species

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Summary

Introduction

Patterns of kinship among individuals in different groups have been rarely examined in animals. Comparisons between bonobos and chimpanzees are effective in examining the relationships between intergroup interactions and kinship among individuals in different groups These two species share many characteristics of their social system including male philopatry, multi-male/multi-female group composition, and fission–fusion dynamics[5,6,7]. The number of available cases for male migration between groups is larger in bonobos than in chimpanzees[25,26,27] These differences between the two species suggest that between-group male gene flow may be more frequent in bonobos than in chimpanzees, which is implied by a previous study using mathematical simulations[28]. Adding data for male fixation index from a previous study[28], we compared the extent of between-group male genetic distance between the two species

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