Impartial Third-Party Interventions in Captive Chimpanzees: A Reflection of Community Concern

et al. (2012) Impartial Third-Party Interventions in Captive Chimpanzees: A Reflection of Community Concern. PLoS ONE 7(3): e32494. doi:10.1371/journal.pone.0032494 Impartial Third-Party Interventions in Captive Chimpanzees: A Reflection of Community Concern Claudia Rudolf von Rohr 0 Sonja E. Koski 0 Judith M. Burkart 0 Clare Caws 0 Orlaith N. Fraser 0 Angela 0 Ziltener 0 Carel P. van Schaik 0 Mark Briffa, University of Plymouth, United Kingdom 0 1 Anthropological Institute & Museum, University of Zurich , Zurich , Switzerland , 2 University Research Priority Program in Ethics, University of Zurich , Zurich , Switzerland , 3 School of Natural Sciences and Psychology, John Moores University , Liverpool , United Kingdom , 4 Department of Cognitive Biology, University of Vienna , Vienna , Austria Because conflicts among social group members are inevitable, their management is crucial for group stability. The rarest and most interesting form of conflict management is policing, i.e., impartial interventions by bystanders, which is of considerable interest due to its potentially moral nature. Here, we provide descriptive and quantitative data on policing in captive chimpanzees. First, we report on a high rate of policing in one captive group characterized by recently introduced females and a rank reversal between two males. We explored the influence of various factors on the occurrence of policing. The results show that only the alpha and beta males acted as arbitrators using manifold tactics to control conflicts, and that their interventions strongly depended on conflict complexity. Secondly, we compared the policing patterns in three other captive chimpanzee groups. We found that although rare, policing was more prevalent at times of increased social instability, both high-ranking males and females performed policing, and conflicts of all sex-dyad combinations were policed. These results suggest that the primary function of policing is to increase group stability. It may thus reflect prosocial behaviour based upon ''community concern.'' However, policing remains a rare behaviour and more data are needed to test the generality of this hypothesis. - Funding: This study was funded by the University Priority Research Program in Ethics at the University of Zurich and the A. H. Schultz-Foundation of the Anthropological Institute and Museum at the University of Zurich as well as the Liverpool John Moore University and the Chester Zoo, both in the UK. Furthermore, the authors thank the Lucie Burgers Foundation in the Netherlands for funding data collection in Arnhem. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Group living and hence sociality is a widespread phenomenon among animals. Because groups are often composed of individuals of different age, sex and relatedness, conflicts arise concerning reproduction or access to resources. Such conflicts may dramatically increase when groups are perturbed from the outside or undergo changes in composition through dispersal or immigration. Conflicts may disrupt group stability, so if individual fitness is dependent on group stability [1], evolution should favour mechanisms that decrease disruption [2,3]. Researchers have identified several mechanisms through which social animals, especially nonhuman primates, manage conflicts, including dominance [4], reconciliation [3], bystander affiliation to recipients and/or initiators of aggression [5,6,7,8,9], mediation [10], punishment [11,12] and policing [1,13]. The focus of this paper is on events of policing, which we define as impartial interventions by third parties in ongoing conflicts. Being impartial, these interventions never include aggression directed specifically at one of the contestants. Such policing is different from the common partial bystander involvement in conflicts, which involves agonistic support of one of the contestants. It is also different from punishment [11], which concerns aggression directed specifically at the wrongdoer. To emphasize the impartiality of the performers of policing, we call them arbitrators. Policing has been reported in chimpanzees [14], bonobos [15], mountain gorillas [16,17,18] and in captive Bornean orang-utans [19,20]. Other species include the golden monkeys [21], hamadryas baboons [22], and several macaques species such as Barbary (A. Bissonnette; unpublished data), rhesus [23,24], Japanese [25], pigtailed [13,26] and Tonkean macaques [27]. Policing is risky because it requires approaching two or more fighting contestants, which may lead to becoming the recipient of aggression [13]. Additionally, arbitrators may incur energy and opportunity costs. To be favoured by natural selection, therefore, policing should bring fitness benefits for the arbitrator. However, the impartiality makes it difficult to recognize such direct fitness benefits. Various hypotheses have been proposed for the function of policing in nonhuman primates. However, because policing is a relatively rare behaviour among primates, the cross-species data thus far has not consistently supported any functional hypothesis. In this paper, we discuss the proposed hypotheses and develop predictions for policing in chimpanzees, where conflicts often arise among females over access to food [28] or among males over access to females [29] and may result in severe dyadic or even polyadic agonistic interactions [10]. The most popular hypothesis claims that policing brings only indirect benefits to the arbitrator, because it serves to increase group stability [30] by reducing the number of conflicts [13,30] and by allowing all individuals build up larger and more diverse social networks [1]. Increased group stability may indirectly increase the arbitrators fitness via the reproductive benefits of living in a stable network of beneficial relationships [26]. In terms of proximate causation [31,32], policing might be motivated by a concern about the conflicts of others and thus might reflect a basic community concern [33]. As a result, policing has been seen as a precursor of human morality [33,34], and therefore may inform of its evolution. If the increase of group stability is the primary function of policing, it should occur most often when group stability is weakened (see Table 1 for predictions). Group instability may arise due to major relationship changes, such as rank reversals near the top of the hierarchy, death of an important social player, or immigration/emigration. In such unstable circumstances, relationships are easily damaged and thus group stability is at stake. First, the group stability hypothesis predicts that arbitrators are highranking individuals because high-ranking individuals have the power to effectively stop aggression [13] and simultaneously have a lower risk of receiving aggression upo (...truncated)