The pivotal role of rank in grooming and support behavior in a captive group of bonobos (Pan paniscus)

We investigated dyadic grooming relationships in a captive group of bonobos (Pan paniscus) and questioned what social function grooming fulfils in the 'market of services and favors'. Hereto we examined which of two theoretic al models - grooming for support (Seyfarth, 1977, 1980) or grooming accordi ng to the similarity principle (de Waal & Luttrell, 1986) best accounted Fu r. the observed grooming distribution. Similarity in traits did not correla te with increased grooming or close proximity among the individuals. Theref ore, the similarity hypothesis was rejected. Seyfarth's model of rank-relat ed grooming was largely confirmed. The animals distributed their grooming a ccording to the rank of the receivers. We found an exchange between groomin g and receipt of support. There was more grooming up than down the hierarch y. However, not all predictions about rank-related competition over groomin g were confirmed. We found that dyadic grooming reciprocity indeed increase d with decreasing rank distance. Yet, there was no increase of grooming wit hin the dyad with decreasing rank distance and high ranking individuals wer e not competed over at the highest rates. The observed correlation between grooming and support received represents an important fit with Seyfarth's p rediction, but does not allow For conclusions about underlying causal proce sses. Other causal explanations, besides the 'groom to receive support' hyp othesis, that could explain a similar correlation are discussed.