White-faced capuchin monkeys show triadic awareness in their choice of allies SUSAN PERRY*†‡, H. CLARK BARRETT†‡ & JOSEPH H. MANSON*†‡ *Max Planck Institute for Evolutionary Anthropology, Leipzig yDepartment of Anthropology, University of California, Los Angeles zCenter for Behavior, Evolution and Culture, University of California, Los Angeles (Received 1 November 2002; initial acceptance 6 January 2003; final acceptance 27 April 2003; MS. number: 7517) The social intelligence hypothesis, which holds that social challenges have selected for increased intelligence and social skills, has been supported by evidence that, in catarrhine primates, individuals know about the characteristics of groupmates’ social relationships. Evidence for such ‘triadic awareness’ has not been sought for platyrrhine primates, although two platyrrhine genera, capuchins, Cebus, and squirrel monkeys, Saimiri, are among the most highly encephalized primates. We examined patterns of coalitionary recruitment in wild white-faced capuchins, C. capucinus. Analyses have shown that more dominant individuals are more likely to join aggressive coalitions than low-rankers, and that individuals preferentially support those with whom they have stronger affiliative relationships. Data from 110 fights, analysed using simulation techniques that produced distributions of results expected under null hypotheses, revealed that contestants preferentially solicited prospective coalition partners that (1) were dominant to their opponents, and (2) had better social relationships (higher ratios of affiliative/ cooperative interactions to agonistic interactions) with themselves than with their opponents. Further analyses showed that soliciting dominant partners could be explained by either of two simpler rules, ‘Solicit an ally that outranks yourself’ or ‘Solicit the highest-ranking available individual’. However, soliciting partners with better social relationships appears to indicate triadic awareness, because subjects did not preferentially solicit the nearby individual with whom they shared the highest-quality social relationship. Effects of relative relationship quality on coalition solicitation decisions were independent of effects of dominance rank. Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. According to the social intelligence hypothesis (Jolly 1966; Humphrey 1976; Byrne & Whiten 1988; Cheney & Seyfarth 1990), the challenges of navigating a complex political landscape have been a major selective force in the shaping of the primate mind. Primates living in complex social groups need to understand the nature of the social relationships between other group members, so that they can use such information to their advantage when selecting social partners for particular purposes. Since the social intelligence hypothesis was originally proposed, a database has slowly accumulated supporting the idea that Old World monkeys and apes understand the nature of other individuals’ social relationships (reviewed in Tomasello & Call 1997). However, New World primates have been largely overlooked in this endeavour. Because capuchin monkeys, Cebus, along with squirrel monkeys, Saimiri, have the largest encephalization quotients of any nonhuman primate (Stephan et al. 1988), they represent a particularly valuable source of data in the quest to understand the selective factors affecting the evolution of intelligence. Experimental studies of Old World monkeys provide the most convincing source of evidence of ‘triadic awareness’ (i.e. animal A understands some aspect of the quality of relationship between animals B and C) in primates. For example, Dasser (1988a, b) showed that juvenile long- tailed macaques, Macaca fascicularis, understand the kin- ship structure of their groups, successfully matching mothereoffspring and sibling pairs in a picture-sorting task. Cheney & Seyfarth (1990) performed playback experiments on vervet monkeys, Chlorocebus aethiops, in Correspondence: S. Perry, Max Planck Institute for Evolutionary Anthropology, Deutscher Platz 6, 04103 Leipzig, Germany (email: perry@eva.mpg.de). H. C. Barrett is at the Department of Anthropology, UCLA, 405 Hilgard Avenue, Los Angeles, CA 90095-1553, U.S.A. 165 0003e3472/03/$30.00/0 Ó 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2004, 67, 165e170 doi:10.1016/j.anbehav.2003.04.005