Individual differences in learning, personality, and social success in brown capuchin monkeys (Sapajus sp.)

Abstract

This thesis examines the relationship between individual differences in learning, personality, and social success in two groups of brown capuchin monkeys (Sapajus sp.) housed at the “Living Links Centre for Human Evolution” at Edinburgh Zoo, UK. Being able to learn quickly and efficiently likely helps primates achieve social success (defined here in terms of centrality within a social network), such as acquiring knowledge of others or learning social skills. Therefore, individuals that are better at learning were predicted to have greater social success than other group members. This prediction, however, contrasts with hypotheses generated from two other disciplines at the individual level: 1) the study of behavioural innovation, and 2) the study of individual differences, i.e. “personality”. In terms of behavioural innovation, better learners should have less social success than other group members because they are expected to rely more on problem-solving, rather than physical combativeness or status, to gain access to socioecological resources. In terms of personality, learning should have little or no direct relationship with social success because other individual differences, like sociability and fearfulness, should mediate primates’ social decision making. This thesis investigates each of these hypotheses. Personality was assessed in 127 capuchins from 7 international sites using the Hominoid Personality Questionnaire, and then validated at Living Links (LL) using behavioural codings; this was the first-ever description of personality structure in brown capuchins. Brown capuchins have five personality dimensions: Assertiveness, Openness, Sociability, Neuroticism, and Attentiveness. Ratings were consistent across observers, and predicted relevant behaviours among the LL capuchins over a year later (e.g. scores on Sociability predicted time spent in close proximity to others). “Social success” in the LL capuchins was assessed in terms of centrality in spatial proximity networks. Individual scores on social network centrality were significantly correlated with scores derived from a Principal Components Analysis of eight affiliative and agonistic behaviours among the LL capuchins, indicating that spatial proximity is a reliable measure of the quality of subjects’ social embeddedness within their groups. Social rank and two personality traits (Assertiveness and Sociability) were positively related to network centrality, while another personality trait (Neuroticism) was negatively related to centrality. Sociability was a significant predictor of network centrality even after controlling for social rank and the other personality traits, highlighting the importance of this personality trait in shaping the social success of capuchins beyond that of basic social rules (e.g. kinship, sex, and rank). Individual learning was assessed in the LL capuchins by administering two operant tasks to subjects under conditions of free choice participation. In Task 1, thirteen monkeys participated, and eight individuals met learning criteria (i.e. >80% trials correct over 3 consecutive sessions). In Task 2, fifteen monkeys participated, and five individuals met learning criteria; the monkeys that learned this second task were also among those individuals that learned Task 1. For monkeys that regularly participated in both tasks (i.e. >50% of sessions), their average performances (i.e. % trials correct) were significantly correlated with individual scores on Assertiveness, but not the other four personality traits, or individual differences in attention span during testing, the percent of sessions subjects participated during testing, the amount of scrounging events subjects directed towards others within their social group, or the percent of observation time subjects spent feeding within their main indoor/outdoor enclosures. In terms of social success, relatively better learners had lower social rank and network centrality compared to relatively poor learners. Also, compared to poorer learners, better learners were generally less likely to direct affiliative acts (e.g. grooming, food sharing, coalitionary support) to other group members. Controlling for Assertiveness (i.e. the only variable related to individual differences in subjects’ average learning performance), individual differences in learning performance were no longer significantly related to social rank, network centrality, or the amount of affiliative acts subjects initiated with others. Collectively, such findings contrast the hypothesis that better learners should (concurrently) be more socially successful than poorer learners, and instead are more reflective of hypotheses pertaining to behavioural innovation and/or the study of individual differences. Social rank and certain traits of personality (Assertiveness, Openness, Neuroticism, and Sociability) appear to interact with capuchins’ patterns of social interaction, and one personality trait (Assertiveness) may mediate how individual differences in learning are associated with differences in social success.