Responses of vervet monkeys in large troops to terrestrial and aerial predator alarm calls

Abstract

The extent to which animal vocalizations convey specific information about events in the environment is subject to continued debate. The alarm-calls of vervet monkeys have played a pivotal role in this debate as they represent the classic example of a predator-specific call production system combined with a set of equally specific responses by receivers. Here, we revisit the vervet alarm-calling system, and assess the hypothesis that these acoustically distinct calls trigger context- and predator-appropriate behavior. We investigated responses in 2 groups of free-ranging vervet monkeys (Chlorocebus pygerythrus) to both natural encounters with predators and experimental presentations of aerial and terrestrial predator alarm calls. Our results show that the modal natural and experimental response was not to initiate escape behavior, either immediately or in the 10s following an alarm call, but to look at the sound source. When monkeys did take evasive action, contextually inappropriate behavior (i.e., behavior that was not appropriate for evading the specific predator type) was as likely to occur as contextually appropriate behavior. The distance at which calls were heard was negatively correlated with the probability of evasive action. Larger group size, and the greater mean distance at which natural calls were heard, may explain why our animals displayed less predator-appropriate evasion or vigilance than expected. We conclude that the broader social and ecological framework in which calls occur, rather than a simple contextually regular linkage between call types and specific predators, shapes animals' responses to calls in this species.