Vervet monkeys greet adult males during high-risk situations

Many animal species produce ritualized signals during dyadic encounters but the functions of such 'greeting' behaviour vary considerably, or are often unknown. One established function is to acknowledge existing dominance relationships. At the same time, call rates often increase during social tension, suggesting additional functions, such as to appease higher-ranking individuals, or to maintain spatial proximity and friendly relations. For vervet monkeys, Chlorocebus pygerythrus, vocal behaviour has been studied extensively, but little research has been devoted to calls given during encounters between two individuals, i.e. grunts. Here, we examined how individual and relationship features affected the vocal greeting behaviour of wild vervet monkeys in different ecological and social situations. We used an information theory approach to investigate the functional hypotheses of vervet monkeys' vocal greeting signals. We found little support for the main functions proposed in the literature, that is, to signal submission, to avoid conflicts, to test social bonds or to coordinate group activity. Results supported the use of grunts to signal benign intent, and we found that grunts were mostly given to closely bonded males near rivers, suggesting that vervet monkeys use vocal greeting signals to recruit individuals in situations of danger to reduce predation risk.