Spotted hyenas 10, chimpanzees 11 and also humans 12 engage in more cooperative interactions with friends. In bottlenose dolphins, strong bonds between males increased mating chance 8, while male-female bonding increased the lifespan of juvenile males 9. Similarly, in marmosets, breeding pairs and breeder-helper dyads with stronger bonds contributed more in offspring care 7. For example, offspring survival is higher in social female yellow baboons 5 and in feral horses with more and stronger female-male bonds 6. These interactions occur non-randomly and often result in lasting and stable social bonds, also called friendships 3, that can improve individual fitness 4. Populations of group-living species comprise individuals who differ in the level of interaction they have with others 1, 2. In addition, we suggest that future studies in closely related species like chimpanzees should implement identical methods for assessing bond strength to shed further light on the evolution of this phenomenon. The results of this study expand our understanding of the mechanisms underlying social bond formation in anthropoid apes. While similarity in Sociability resulted in higher relationship values, lower relationship compatibility was found between bonobos with similar Activity scores. We show that, aside from relatedness and sex combination of the dyad, relationship quality is also associated with personality similarity of both partners. Dimension reduction analyses on individual and dyadic behavioral scores revealed multidimensional personality (Sociability, Openness, Boldness, Activity) and relationship quality components (value, compatibility). We used behavioral observations on 39 adult and adolescent bonobos housed in 5 European zoos to study the role of personality similarity in dyadic relationship quality. Recent studies in other species also stress the importance of personality, but this relationship remains to be investigated in bonobos. Often age, sex or kinship similarity have been used to explain social bond strength variation. In bonobos, strong bonds have been documented between unrelated females and between mothers and their adult sons, which can have important fitness benefits.
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