A dynamic interaction between aggression and grooming reciprocity among female chacma baboons

Abstract

We used data from a natural experiment on adult female chacma baboons, Papio cynocephalus ursinus, to test the hypothesis that variation in aggression through time influences patterns of grooming reciprocity within a social group. Owing to a change in the baboons' competitive regime, we were able to compare data from periods when aggression was high (period 1) and low (period 2). During period 2, the slope of the relationship between aggression and rank was significantly shallower than during period 1 and less aggression was directed at the lowest-ranking females suggesting there had been a reduction in the dominance gradient. We attributed this to reduced effectiveness of dominance as a means of excluding other females from feeding resources. The reduction in aggression during period 2 was accompanied by an increase in grooming reciprocity between dyads suggesting that high-ranking females no longer attracted grooming by subordinates in exchange for tolerance, and that grooming in period 2 was exchanged for its intrinsic benefits. The loss of rank-related effects on grooming reciprocity in period 2 compared with period 1 further confirmed this. These findings show that female baboons are able to respond flexibly and swiftly to changes in their social circumstances and that a dynamic approach to primate social interactions is worth pursuing. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.