Rule-based sequences in sooty mangabey vocal communication

Abstract Investigating how non-human animals produce call sequences provides key insights into the evolutionary origins of meaning in vocal communication, including syntax. Many species combine calls into structured sequences, often following specific rules, yet most studies focus on only a few sequences per species. This limits our understanding of their ability to combine calls and convey meaning through sequences. Our study addresses this gap by documenting the vocal sequence repertoire and underlying rules of sooty mangabeys (Cercocebus atys), a West African monkey species. We collected and analysed 1672 recordings from two wild groups in the Taï National Park, Ivory Coast. Sooty mangabeys frequently combine calls but rely on a limited set of sequences. Within these, we identified rules of call ordering and reoccurrence. Notably, they produced non-adjacent dependencies (i.e. the relationship between the first and last call of a sequence, irrespective of intervening calls), which are rarely reported in animal communication. Our findings suggest that sooty mangabeys also use sequences without apparent rules, likely encoding different types of information. While the context of production remains essential for determining meaning, our results highlight how a whole-repertoire approach can reveal rule-based sequences and their potential for meaning expansion beyond the number of individual calls available.