Understanding the variables that shape the use and evolution of vocal communication in non-human primates can inform understanding of how language evolved. Social complexity might drive communicative complexity and one important, but neglected measure of social complexity is dominance style (the strictness with which the dominance hierarchy is enforced: tolerant to despotic). As the outcomes of social interactions in more tolerant societies are more uncertain (Dobson, 2012), we predicted that more tolerant individuals and species would have a greater need for more frequent and more diverse vocal signals to negotiate their social interactions. Using Bayesian analyses on observational data from wild groups of 25 species, we show that more tolerant individuals vocalise at a higher rate, but more despotic species have a wider range of hierarchy-related vocalisations in their repertoires. Our findings indicate that dominance style is a valuable social variable for understanding vocal usage and evolution in primates.
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