Influence of social interactions with nonmother females on the development of call usage in Japanese macaques

The goal of this study was to clarify whether social experience affects developmental changes in call usage in Japanese macaques, Macaca fuscata. We focused on vocalizations primarily used as greeting signals, such as grunts, girneys and coo calls. We hypothesized that the degree of interaction with adult females other than mothers would facilitate selective call usage towards less familiar individuals in young monkeys. To test this, we conducted observations on female monkeys from infancy to full adulthood in a free-ranging group, and found that young adult and full adult subjects produced calls more frequently when approaching unrelated, than related, adult females. In contrast, subadults, juveniles and infants did not produce calls selectively towards unrelated adults. Monkeys older than juveniles were more likely to engage in affiliative interactions after approaching unrelated females with than without calls, indicating that calling can be beneficial for signallers. Individuals who had more frequent interactions with unrelated adult females used calls more frequently towards unrelated adult females in subsequent observation years, an effect that was stronger in older animals. These findings suggest that calling towards unrelated adult females is facilitated through tangible interactions with these females. The interactions with related adult females did not influence calling towards relatives or nonrelatives, indicating that interaction with relatives did not facilitate call usage. We conclude that social experience with unrelated adult females modifies vocal greeting behaviours, which, in turn, may mediate social relationships between unrelated individuals.