Flexible use of simple and combined calls in female Campbell's monkeys

Call combinations allow animals to expand the communicative power of small repertoires with acoustically inflexible elements. In Campbell's monkeys, Cercopithecus campbelli, males possess a small repertoire of calls that can be merged to an acoustically invariable suffix and which are concatenated into various sequences, mainly in response to external disturbances. The vocal repertoire of adult females has been less well studied although it is much richer, containing both alarm and various social calls. In particular, females possess a low-pitched contact call, produced either alone or merged with a high-pitched, arched unit. Combined contact calls are identity-richer and easier to detect than simple calls. Here, we investigated the socioecological factors that determined the production of single and combined utterances and found that combined utterances were more common when identity was relevant such as in mixed-species associations and during socially important vocal exchanges. In contrast, single calls were used mainly when predation risk was high, as part of this species' generally cryptic antipredator strategy. We discuss these finding in the light of current theories regarding the evolution of combinatorial signalling.