Christoph von Borell, Lars Kulik and Anja Widdig

Growing into the self: the development of personality in rhesus macaques

Animal Behaviour

Although personality has been widely studied among animal species, only a few studies have investigated the long-term development of personality during early ontogeny. In fact, no study of nonhuman primates has consistently mapped personality development from birth to adulthood. Our study aimed at closing this gap by examining the development of personality among free-ranging rhesus macaques, Macaca mulatta, using longitudinal behavioural data of 24 subjects (3758 h) collected from birth to 7 years of age on the island of Cayo Santiago, Puerto Rico, U.S.A. In our analyses we combined different frameworks of animal personality research to discuss behavioural differences in terms of latent personality models, behavioural syndromes and behavioural characters. The results showed that a core model of rhesus macaque personality, comprising three latent factors (Fearfulness, Aggression, Sociability), can already be established over the course of the first 7 years of life. However, only Fearfulness emerged consistently throughout development. While the factor of Sociability diffused during maturation, Aggression stabilized towards adulthood after having inconsistent loadings during infancy. When assessing correlations among behaviours separately on the within- and between-individual level, again only Fearfulness showed significant results averaged over the entire study period and can therefore be classified as behavioural syndrome or behavioural character. We discuss differences in correlations, interactions between sex and age and the effect of maternal rank as potential source of differences in stability of latent traits. Furthermore, we assessed plasticity of behaviour with regard to first maternity in females and natal dispersal of males. While the latter was accompanied by an increase of fearful behaviour and decrease of physical aggression, first maternity was marked by a mixed pattern of changes. Overall, our results suggest that rhesus macaques are not born into their personality, but grow into it.