White-faced capuchin monkeys show triadic awareness in their choice of allies

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 groupmatesʹ 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.