Call combination order and iterations may shift meaning in sooty mangabey vocal sequences

BACKGROUND: Human language expands meaning through the structured combination of sounds, but such mechanisms remain rare in nonhuman animals, raising questions about their evolution. Shifts in meaning from single calls to combinations appear across a range of combinations and contexts in apes, but current evidence from other species is mainly restricted to alarm contexts. To address this, we applied a quantitative, whole-repertoire approach to assess meaningful combinatorial capacities in sooty mangabeys (Cercocebus atys), a forest-dwelling monkey. RESULTS: We recorded 1751 vocal utterances from two groups in the Taï National Park, Ivory Coast. Using context of production as a proxy for potential meaning, we focused on two mechanisms: (1) bigrams-sequences of two different calls, and (2) iteration-reoccurrence of the same call type interspersed with others. Euclidean distance analyses suggested order-sensitive meaning in female bigrams combining 'grunt' and 'twitter' calls. Whereas single 'grunts', 'twitters' and the bigram 'grunt_twitter' occurred frequently during feeding, 'twitter_grunt' occurred predominantly during infant-directed affiliations. Female iterative sequences in which 'grunt' and 'twitter' calls recurred (e.g. 'twitter_grunt_twitter') also showed context-specific shifts, indicating a possible role for iteration in meaning generation. CONCLUSIONS: These findings suggest that meaningful combinatorial capacities can extend beyond great apes and alarm contexts to socially benign interactions in monkeys. However, repertoire-wide use of meaning-shifting bigrams remains unconfirmed outside hominids.