Proto-syntax in wild Campbell’s monkeys: Affixation and call combination
Résumé
According to most current theories, one of the main differences between human speech and animal vocal communication concerns syntax. In searching for the origins of syntactic communication, we analysed the loud call behaviour of free-ranging Campbell's monkey males. Using two years of naturalistic observations and predator-simulation experiments, we were able to distinguish six basic alarm call types. Acoustic variation to discriminate between the different calls was achieved by frequency modulation (Boom, Krak, Hok) and affixation, i.e., the facultative addition of an acoustic suffix (Krak-oo, Hok-oo, Wak-oo). Moreover, when responding to a disturbance, males rarely produce signle calls, but usually generated sequences of 2 to 40 calls. From the analysis of 250 calling events, we identified six contextually-determined calling sequences, which varied in terms of call type composition, order of succession, and rhythm of delivery. These levels of organization appear to encode information about the type and urgency of the danger, as well as the caller's associated behaviour. Preliminary playback experiments confirmed that affixation and call combination were discriminated by females. Our findings suggest that early precursors of syntactic communication may already have been present in the common ancestor of modern non-human primates and humans.