Diana monkey long-distance calls: messages for conspecifics and predators

Primate long-distance calls have typically been interpreted as communication signals between conspecific groups (the ‘resource defence hypothesis’), but their potential role as anti-predator alarm calls has received comparably little attention. Male diana monkeys,Cercopithecus diana dianain the Taı̈ forest of Côte d’Ivoire often utter long-distance calls, either spontaneously or in reaction to a variety of stimuli, including predators and non-predators. The present study focuses only on predation contexts and provides evidence for communication to both predators and conspecifics. Males called only in response to predators whose hunting success depends on unprepared prey, that is, leopards and crowned hawk eagles, but not in response to pursuit hunters, such as chimpanzees and humans, which can pursue the caller in the canopy. Calling was regularly combined with approaching the predator. Both observations suggest that male long-distance calls are used to signal detection to the predator (‘perception advertisement hypothesis’). Analysis of male long-distance calls given to leopards and eagles showed that they differed according to a number of acoustic parameters. The two call variants were played to different diana monkey groups; conspecifics responded to them as though the original predator were present. We conclude that, in addition to their function in perception advertisement, diana monkey long-distance calls function as within-group semantic signals that denote different types of predators.