Reputations and polyadic interactions among great apes

‘Reputation’ refers to information about the past behaviour of others, learnable either by direct observation, eavesdropping, or gossip. Reputations are thought to play a key role among human societies in ensuring that people behave cooperatively. When it is gained by eavesdropping, it is called ‘indirect reputation’. The aim of this thesis was to investigate the role that ‘indirect reputation’ plays in the lives of non-human great apes, with a special focus on chimpanzees. There were two projects: (1) an observational study of captive chimpanzees (Chester Zoo, U.K.), and (2) social preference experiments with chimpanzees, bonobos, gorillas, and orangutans (Leipzig Zoo, Germany). All results are put into the context of the evolution of cognitive abilities. The observational project focused on grooming behaviour. The aim was to examine how individuals observe the grooming of others when involved in polyadic (multi-individual) grooming cliques. I also provide detailed information on the social structure of the Chester Zoo chimpanzees, and a detailed introduction and catalogue of polyadic grooming cliques. For the purpose of studying reputation, there was an analysis of how individuals differ (according to age, sex, and social position) of how individuals behave in a ‘peripheral groomer’ position. This occurs when an individual (A) sees a pair of groomers (e.g. B <--> C) and joins them, forming a triad (e.g. A --> B <--> C). In this position, the peripheral is at a disadvantage because of the likelihood of being ignored. A number of analyses were conducted on the peripheral’s behaviour: partner choice, payoff (whether groomed in return), and how long the peripheral persisted in grooming (duration of attempt). The result relevant to reputation was that younger individuals gave up more quickly when the recipient of their grooming was socially closer to the third groomer than to them (AB < BC) than the reverse (AB > BC). This result implies that chimpanzees understand how their own relationship with the recipient (AB) compared with the relationship between third parties (BC). This result was corroborated in a psychology experiment that tested for 'image scoring' ('A monitors the giving behaviour of B towards C'). Here, apes were allowed to passively observe two play-acted incidents among humans: (1) a 'nice person' gave food to a human who begged for it, and (2) a 'nasty person' refused. After, the ape could approach one or both human actors (nice/nasty) sitting side-byside holding grapes (but neither offering grapes if approached). The subject’s expectation of which human was more likely to offer food was measured by comparing the proportion of time that subjects spent near each person. Chimpanzees spent significantly more time near the ‘nice’ person compared to ‘nasty’ (result not significant for other species). Both this and the observational work suggest that chimpanzees, at least, are cognisant of 'indirect reputation'.

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