The face is central to primate multimodal signals

The face is the most salient part of any primate. A wealth of experimental and observational evidence suggests that faces have become increasingly important in the communication system of primates over evolutionary time, and that both the static and moveable aspects of faces convey considerable information. Therefore, whenever there is a visual component to a multimodal signal the face is potentially relevant. The role of the face in multimodal signals, however, has been overlooked. We propose that the face has been neglected for two reasons: first, due to methodological difficulty. Examination of multimodal signals in primates is difficult, and so scientists tend to examine a limited number of signals in combination. Detailed examination of the subtle and dynamic components of facial signals is particularly hard to achieve in studies of primate multimodality. Second, due to a common assumption that the face contains ‘emotional’ content. A priori categorisation of facial behaviour as ‘emotional’ ignores the potentially communicative and predictive information present in the face that might contribute to multimodal signals. In short, we propose that the face is central to multicomponent signals and suggest future directions for investigating this phenomenon.