Personality assessment of synanthropic rhesus macaques: implications and challenges

″What makes animals thrive in human-dominated environments?″ is a question that has been extensively researched transcending disciplines, but findings remain inconclusive. Consistent inter-individual differences or personalities can potentially explain the functional significance of habitat-specific traits and their variations that help animals successfully coexist with humans. Rhesus macaques (Macaca mulatta) are the most successful non-human primate in the Anthropocene, living in diverse climatic and environmental conditions. Studying the personalities of synanthropic rhesus macaques can provide insights into the biological traits that facilitate their success in human-dominated environments. We employed a multi-method ′bottom-up′ approach of behavioral observations and novelty experiments, standardized for assessing captive non-human primates, to evaluate the personalities of synanthropic adult rhesus macaques (N=52). To our surprise, novelty experiments encountered significant challenges, limiting their effectiveness. However, behavioral observations in the form of focal sampling revealed two repeatable traits: sociability and cautiousness. We found an effect of sex on sociability, where males were more sociable than females. In an additional analysis, we found that individuals who obtained food through contact provisioning had higher cautiousness scores than individuals who obtained food through non-contact provisioning. We discuss how the observed personality traits and their variations potentially offer adaptive advantages in human-dominated environments, where rhesus macaques face both benefits, such as anthropogenic subsidies and reduced predation, and costs, like exposure to anthropogenic stressors. We also emphasize that protocols designed for captive conditions may not be directly applicable to free-living animals. Thus, the study underscores the need to reconsider experimental designs to obtain comparable empirical evidence between captive and non-captive populations to enhance the ecological validity of personality assessments. Nevertheless, empirically identifying traits using observations in synanthropic species like rhesus macaques can still provide valuable insights into the mechanisms that enable certain animals to thrive amidst a rapid expansion of anthropogenic activities.