The language void: The need for multimodality in primate communication research

Theoriesoflanguage evolutionoftendraw heavilyoncomparative evidence of thecommunicative abilitiesofextant nonhuman primates (primates). Many theories have argued exclusively for a unimodal origin oflanguage, usually gestural or vocal. Theories are often strengthened by research on primates that indicatesthe absence of certain linguistic precursors in the opposing communicative modality. However, a systematicreview of the primate communication literature reveals that vocal, gestural and facial signals have attracteddiffering theoretical and methodological approaches, rendering cross-modal comparisons problematic. Thevalidity of the theories based on such comparisons can therefore be questioned. We propose that thesea priori biases, inherent in unimodal research, highlight the need for integrated multimodal research. Byexamining communicative signals in concert we can both avoid methodological discontinuities as well asbetter understand the phylogenetic precursors to human language as part of a multimodal system