Morphologically structured vocalizations in female Diana monkeys

Social complexity is often thought of as a driving force in the evolution of communication and cognition, but this is at odds with the fact that nonhuman primates generally display only very limited flexibility in vocal production. Some primates partially overcome their limited vocal flexibility by combining two or more acoustically inflexible calls into complex sequences. Equally relevant is that some primate calls consist of separable morphological elements whose combinations create different meanings. Here, we focus on the vocal system of wild female Diana monkeys, Cercopithecus diana, which produce three call units (R, L, A) either singly or merged as RA or LA call combinations. Previous work has shown that R and L convey information about external events, while A conveys information about caller identity. We tested this hypothesis experimentally, by broadcasting artificially combined utterances to eight adult females. To test the significance of the R and L 'event' units, we merged them with the A 'identity' unit of a group member. To test the significance of the 'identity' unit, we merged an R 'event' unit with an 'identity' unit from a group member or a neighbouring individual. Subjects responded in ways that suggested that both event and identity units were relevant, suggesting that Diana monkeys' social calls possess morphosemantic features. We discuss this finding in relation to the coevolution of communication and social complexity in primates.