Use and Function of Multi-Articulator Versus Multi-Sensory Acts in the Close-Range Communication of Orang-Utans

From early infancy, human face-to-face communication is “multimodal”, comprising a plethora of interlinked articulators and sensory modalities. Although there is also growing evidence for this in nonhuman primates, the functions of integrating articulators (i.e. multiplex or multi-articulator acts) and channels (i.e. multimodal or multi-sensory acts) remain poorly understood. Here, we studied close-range social interactions within and beyond mother-infant pairs of Bornean and Sumatran orang-utans living in wild and captive settings, to examine to what extent species, setting and recipient-dependent factors affected the use of and responses to multi-sensory as well as multi-articulator communication. Results showed that both multi-sensory and multi-articulatory acts were more effective at eliciting responses (i.e. “apparently satisfactory outcomes”) than their respective uni-component parts, and generally played a larger role in wild populations. However, only multi-articulator acts were used more when the presumed goal did not match the dominant outcome for a specific communicative act, and were more common among non-mother-infant dyads and Sumatrans across settings. We suggest that communication through multiple sensory channels primarily facilitates effectiveness, whereas a flexible combination of articulators is relevant when social tolerance and interaction outcomes are less predictable. These different functions underscore the importance of distinguishing between these forms of multi-component communication.