Age differences in the responses of vervet monkeys, Chlorocebus pygerythrus, to terrestrial alarm calls

The high costs of predation and the opportunity costs associated with predator avoidance are likely to select for flexibility in the development of antipredator responses based on local socioecological conditions. As group size is hypothesized to vary across populations as a function of predation risk, the development of antipredator behaviours throughout ontogeny may be influenced by the size of an individual's social group. Here, we explore the development of alarm call responses in wild vervet monkeys living at the Samara Game Reserve, South Africa. Vervet monkeys at this site live in relatively large social groups, and adult responses to alarms have previously shown less uniformity than in other sites where group sizes are smaller. We presented monkeys playback recordings of terrestrial alarm calls produced by individuals of different age–sex classes. We then videorecorded and scored the responses of receivers along an ordinal maturity scale. We used a mixed-effects ordinal logistic regression model within a Bayesian framework to explore how response intensity is affected by the age–sex of the caller, and the age of receivers. Our analysis showed that younger monkeys (<2 years old) exhibit strong evasive responses to call stimuli, regardless of the age–sex class of the caller. The intensity of these responses decreases with age, with responses to nonalarm calls decreasing earlier in development than responses to alarm calls. Adult responses to alarm calls in this population are as likely to be characterized by a general increase in vigilance as they are to consist of an evasive response. We suggest that responses in younger individuals at Samara are mediated by generalized startle responses to loud, plosive noises. Development of more specific responses throughout ontogeny is likely to be dependent on learning from adult models, whose milder responses reflect local socioecological conditions specific to Samara.