Vocal-visual combinations in wild chimpanzees

Living organisms throughout the animal kingdom habitually communicate with multi-modal signals that use multiple sensory channels. Such composite signals vary in their communicative function, as well as the extent to which they are recombined freely. Humans typically display complex forms of multi-modal communication, yet the evolution of this capacity remains unknown. One of our two closest living relatives, chimpanzees, also produce multi-modal combinations and therefore may offer a valuable window into the evolutionary roots of human communication. However, a currently neglected step in describing multi-modal systems is to disentangle non-random combinations from those that occur simply by chance. Here we aimed to provide a systematic quantification of communicative behaviour in our closest living relatives, describing non-random combinations produced across auditory and visual modalities. Through recording the behaviour of wild chimpanzees from the Kibale forest, Uganda we generated the first repertoire of non-random combined vocal and visual components. Using collocation analysis, we identified more than 100 vocal-visual combinations which occurred more frequently than expected by chance. We also probed how multi-modal production varied in the population, finding no differences in the number of visual components produced with vocalisations as a function of age, sex or rank. As expected, chimpanzees produced more visual components alongside vocalizations during longer vocalization bouts, however, this was only the case for some vocalization types, not others. We demonstrate that chimpanzees produce a vast array of combined vocal and visual components, exhibiting a hitherto underappreciated level of multi-modal complexity. In humans and non-humans, acoustic communicative signals are typically accompanied by visual information. Such "multi-modal communication" has been argued to function for increasing redundancy as well as for creating new meaning. However, a currently neglected step when describing multi-modal systems and their functions is to disentangle non-random combinations from those that occur simply by chance. These data are essential to providing a faithful illustration of a species' multi-modal communicative behaviour. Through recording the behaviour of wild chimpanzees from the Kibale forest, Uganda we aimed to bridge this gap in understanding and generated the first repertoire of non-random combined vocal and visual components in animals. Our data suggest chimpanzees combine many components flexibly and these results have important implications for our understanding of the complexity of multi-modal communication already existing in the last common ancestor of humans and chimpanzees.