With or without you: common marmoset, Callithrix jacchus, personality expression is mediated by social setting

Animal personality has been studied in many solitary and socially living taxa, but the effect of conspecifics on the individuals' personality expression remains largely unknown. Studies assessing the personality of the same animals in individual and social settings are also lacking. Common marmosets are callitrichid primates that spend their lives in family groups, yet they also face problems on their own (e.g. during foraging). Marmosets show consistent interindividual behavioural differences; however, whether and how their personality profiles are affected by conspecifics remains unknown. In this study, we aimed to answer this question by replicating a personality test battery previously done in an individual setting. A total of 25 captive subjects were tested across five experimental set-ups in two sessions in a social setting, assessing their general activity, reactions to a novel object, food and predator model, and their foraging under risk. Most of the behavioural variables showed temporal and contextual consistency. The resulting personality structure was similar to the previously obtained captive and wild personality structures. When examining the personality structure containing the same behavioural variables across settings, two principal components emerged. Although individual scores of ‘Avoidance/Shyness’ were consistent across solitary and social settings, ‘Stress/Activity’ scores were considerably modified by the presence of group members: more stressed/active individuals in the individual setting became less stressed/active when tested with conspecifics, and vice versa. The personality traits were further linked with subjects' age and group membership. These findings indicate that some personality traits may be more plastic than others and that the social environment can be an important modifier of individual behaviour. Furthermore, our findings promote the implementation of social setting in test designs for animal behaviour and cognition as an equally rigorous yet, in some cases, ecologically more relevant empirical choice. • The personality structure of common marmosets is similar across studies. • Personality shows similar traits and structure across settings. • The presence of conspecifics affects the personality expression of marmosets. • Conspecifics influence ‘Stress/Activity’ scores but not ‘Avoidance/Shyness’ scores.