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

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 call 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 call type (‘tcha-kow’) as a coordinated response to calls produced in non-predatory contexts (‘boom’) by associated male Campbell’s monkeys. Production of ‘kroo’ calls was also strongly affected by the alarm calling behaviour of male Campbell’s monkeys, suggesting that male lesser spot-nosed monkeys adjust their alarm call production to another species’ vocal behaviour. We discuss different hypotheses for this unusual phenomenon and propose that high predation pressure can lead to reliance on other species vocal behaviour to minimise predation. Predation can lead to the evolution of acoustically distinct, predator-specific alarm calls. However, there are occasional reports of species lacking such abilities, despite diverse predation pressure, suggesting that evolutionary mechanisms are more complex. We conducted field experiments to systematically describe the alarm calling behaviour of lesser spot-nosed monkeys, an arboreal primate living in the lower forest strata where pressure from different predators is high. We found evidence for two acoustically distinct calls but, contrary to other primates in the same habitat, no evidence for predator-specific alarms. Instead, callers produced one alarm call type (‘kroo’) to all predator classes and another call type (‘tcha-kow’) to non-predatory dangers, but only as a response to a specific vocalisation of Campbell’s monkeys (‘boom’). The production of both calls was affected by the calling behaviour of Campbell’s monkeys, suggesting that lesser spot-nosed monkey vocal behaviour is dependent on the antipredator behaviour of other species. Our study advances the theory of interspecies interactions and evolution of alarm calls.