A longitudinal assessment of behavioral development in wild capuchins: Personality is not established in the first 3 years

Animal personality is defined as consistent individual differences across time and situations, but little is known about how or when those differences are established during development. Likewise, several studies described the personality structure of adult capuchin monkeys, without assessing the ontogeny of these personality traits. We analyzed the behavioral repertoire of 12 wild infants (9 males, 3 females) yellow-breasted capuchin monkeys (Sapajus xanthosternos), in Una Biological Reserve (Bahia, Brazil). Each infant was observed and filmed weekly from birth until 36 months, through daily focal sampling. We analyzed the behavior of each individual in 10 developmental points. By means of component reduction (principal component analysis), we obtained four behavioral traits: Sociability, Anxiety, Openness, and Activity. We investigated whether there were developmental effects on those traits by fitting regression models for the effect of time on behavioral traits, controlling for monkey identity, sex, and cohort. Sociability (decreasing) and Anxiety (increasing) changed significantly along development. By means of repeatability analysis, we did not find intra-individual consistency across time in those traits, so we cannot discriminate stable personality traits in early ontogeny. Our results show that the personality structure of capuchin monkeys is not established during early development, in agreement with the literature on human personality.