Audience Effect in Wild Vervet Monkeys (Chlorocebus pygerythrus aethiops)

Although vervet monkeys warn their conspecifics about external events by producing distinct vocalizations, little is known about how their vocal behaviour is affected by the presence of an audience. Fake predator, playback and food experiments are used to investigate this in four contexts: encountering predators, discovering food, recruiting help and greeting another individual. While it is well known that alarm call production is influenced by the composition of the audience, few studies have been realized on how vervet monkeys can modify the acoustic structure of their call according to the presence of other individuals while discovering food. Moreover, significant acoustical differences were found between screams produced by victims or aggressors. As a consequence, variation of screams produced by vervet monkeys’ opponents might help bystanders to give support only when necessary. Lastly, the call production of vervet monkeys while encountering another individual seems to be influenced by the presence of an audience since they use these greeting signals towards males to minimise risks in dangerous situations. Getting a better understanding of the communication system of vervet monkeys and integrating it in a comparative study including baboons, mangabeys and chimpanzees might shed light on how the complexity of the communication system evolved through the primate lineage, and thus help us understanding part of the evolution of human language.