Vocal-gestural combinations in infant bonobos: new insights into signal functional specificity.

Comparing the communicative abilities of humans and great apes is a commonly used research strategy to understand the evolutionary foundation of modern human language. The ability to combine signals from different communicative modes (vocal, gestural, facial, etc.) is a universal and important feature of human language that is shared with great apes, although rarely investigated. Research on apes' signal combination has usually studied the frequency, distribution, effectiveness, and function of combinations in comparison with single signals, but only seldom have studies addressed their functional specificity, i.e., their ability to convey specific signallers' goals within contexts. Here, I suggest a new methodological approach to the investigation of functional specificity of signals, which consists of focusing on one call type and exploring its various patterns of production when combined with gestures. I illustrate the different methodological steps with a case study on infant bonobos' combinations of 'Pout moan' calls with gestures in the context of infant-to-mother requests. The descriptive results indicate that at least three combinations seem functionally specific, i.e., specific to the infants' desired goals. It suggests that combinations can be functionally specific from a young age and that learning how to combine signals efficiently might occur early in ontogeny. This suggested approach might provide insight to the on-going debate regarding the mechanisms underpinning the learning process of successful signal production and potentially further our understanding of the evolution of the combinatorial characteristics of human communication.