Lesser spot-nosed monkeys coordinate alarm call production with associated Campbell’s monkeys

Abstract Forest monkeys often form semi-permanent mixed-species associations to increase group-size related anti-predator benefits without corresponding increases in resource competition. In this study, we analysed the alarm calling system of lesser spot-nosed monkeys, a primate that spends most of its time in mixed-species groups while occupying the lowest and presumably most dangerous part of the forest canopy. In contrast to other primate species, we found no evidence for predator-specific alarm calls. Instead, males gave one general alarm call type (‘kroo’) to three main dangers (i.e., crowned eagles, leopards and falling trees) and a second alarm call type (‘tcha-kow’) to non-predatory alarm calls (‘boom’) of associated Campbell’s monkeys. Production of ‘kroos’ also varied according to Campbell’s monkeys’ vocal response, suggesting that lesser spot-nosed monkeys coordinate their alarm call production depending on an associated species’ alarm calling behaviour. We discuss different explanations for this unusual phenomenon and conclude that high predation pressure can prevent the evolution of predator-specific alarm calls and favour strategies relying on heterospecific vocalisations. Keywords Polyspecific association, Predation, Acoustic analyses, Playback experiments, Vocal communication, Cercopithecus petaurista