Age Differences in the responses of vervet monkeys to terrestrial alarm calls

Here, we explore the development of alarm call responses in wild vervet monkeys (Chlorocebus pygerythrus) 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 played back recordings of terrestrial alarm calls produced by individuals of different age/sex classes. We then video-recorded 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 (<2yrs) 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 non-alarm calls decreasing earlier in
development compared to alarm calls. Adult responses to alarms in this population are equally 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 a 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 specific to local socioecological conditions at Samara.

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