Vocal production and auditory learning in the genus Chlorocebus

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The human language faculty provides our species with exceptional means of communication that are unparalleled in the animal kingdom. Much research effort has been devoted to shed light on evolutionary precursors to linguistic abilities in nonhuman primates. With regard to the vocal-auditory domain, it is well established that the acoustic structure of nonhuman primate vocalizations is largely hard-wired, while call usage is more flexible. On the recipients’ side, auditory learning is crucial to establish what specific calls stand for. Yet, important gaps in understanding remain. First, in light of the restricted call structure, it has been proposed that critical information may be encoded in the composition of the call sequences. Previous attempts to characterize the structure of such sequences made the assumption that calls can be clearly assigned to specific categories although most nonhuman primate vocal repertoires are highly graded. Moreover, these attempts considered the entire sequence to assign it to a specific context, which does not seem adaptive given that listeners are forced to make rapid decisions in predator contexts. We will develop a biologically grounded computational Bayesian perception and decision-model that takes into account the uncertainty with which calls can be assigned to different contexts to provide a lean account of monkeys’ responses to calls. Second, there is ample evidence that calls vary in relation to the context in which they are given, but it is not yet clear which factors specifically drive the variation in call structure. The second aim of this proposal is to examine the degree to which stimulus type, stimulus location, and experience with a stimulus affect call structure, to shed light on how monkeys categorize different kinds of danger and how this in turn affects their communication. Third, our previous work has shown that monkeys are able to attach meaning to sounds after single exposure to a threatening stimulus in the wild. This observation stands in stark contrast to the difficulties that nonhuman primates reveal when tested in auditory discrimination tasks in operant conditioning settings. We will test the conjecture that the valence of the stimulus strongly affects the learning trajectory by conducting field experiments to assess how rapidly the monkeys learn to distinguish different sounds in predator and food contexts. We will use the alarm call system in the genus Chlorocebus. The computational model will be specified based on existing acoustic analyses on the graded structure of vocalizations in this genus; field experiments will be conducted in Senegal and at two sites in South Africa on a total of 11 social groups of green monkeys and vervets. With this research, we aim to contribute to a better understanding of the principles characterizing nonhuman primate communication.