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Across multiple species of social mammals, a growing number of studies have found that individual sociality is associated with survival. In long-lived species, like primates, lifespan is one of the main components of fitness. We used 18 years of data from the Lomas Barbudal Monkey Project to quantify social integration in 11 capuchin (Cebus capucinus) groups and tested whether female survivorship was associated with females’ tendencies to interact with three types of partners: (1) all group members, (2) adult females, and (3) adult males. We found strong evidence that females who engaged more with other females in affiliative interactions and foraged in close proximity experienced increased survivorship. We found some weak evidence that females might also benefit from engaging in more support in agonistic contexts with other females. These benefits were evident in models that account for the females’ rank and group size. Female interactions with all group members also increased survival, but the estimates of the effects were more uncertain. In interactions with adult males, only females who provided more grooming to males survived longer. The results presented here suggest that social integration may result in survival-related benefits. Females might enjoy these benefits through exchanging grooming for other currencies, such as coalitionary support or tolerance.

 

Cultural evolution researchers still debate whether humans are unique among species in having social norms, i.e. moralized, group-specific, socially learned, shared understandings of the rules by which social life should be conducted, maintained via moral emotions that inspire impartial third parties to punish violators of these rules. I sought to establish what behaviors spark outrage in capuchins by recording the details of social context whenever a capuchin aggressed against or screamed at another monkey. Food theft, certain types of sexual interaction, and branch-breaking displays were situations that elicited outrage often enough to warrant documentation of which other monkeys witnessed these events, and how they responded. Three decades of long-term data on ten groups were used to measure degree of maternal kinship and relationship quality (using focal follow data and ad libitum data) between the bystander monkeys and the monkeys involved in the putative norm violation. This population fails to meet three of my operational criteria for social norms: (1) There is very little between-group variation in the patterning of social behaviors relevant to the putative social rules identified. (2) The rate at which third party bystanders aggress against putative norm violators is low (0.6-7.0%). (3) Using a logistic regression modeling framework, the most salient predictor of whether third party bystanders punish putative rule violators is the quality of bystanders’ relationships with those violators, suggesting that bystander behavior is driven more by grudge-holding against particular individuals with whom they have poor-quality relationships than by altruistic enforcement of a group-wide behavioral standard. 

Many white-faced capuchin monkey dyads in Lomas Barbudal, Costa Rica, practise idiosyncratic interaction sequences that are not part of the species-typical behavioural repertoire. These interactions often include uncomfortable or risky elements. These interactions exhibit the following characteristics commonly featured in definitions of rituals in humans: (i) they involve an unusual intensity of focus on the partner, (ii) the behaviours have no immediate utilitarian purpose, (iii) they sometimes involve ‘sacred objects’, (iv) the distribution of these behaviours suggests that they are invented and spread via social learning, and (v) many behaviours in these rituals are repurposed from other behavioural domains (e.g. extractive foraging). However, in contrast with some definitions of ritual, capuchin rituals are not overly rigid in their form, nor do the sequences have specific opening and closing actions. In our 9260 h of observation, ritual performance rate was uncorrelated with amount of time dyads spent in proximity but (modestly) associated with higher relationship quality and rate of coalition formation across dyads. Our results suggest that capuchin rituals serve a bond-testing rather than a bond-strengthening function. Ritual interactions are exclusively dyadic, and between-dyad consistency in form is low, casting doubt on the alternative hypothesis that they enhance group-wide solidarity. This article is part of the theme issue ‘Ritual renaissance: new insights into the most human of behaviours’.