Combination order and iteration may shift meaning in sooty mangabey vocal sequences

Human language expands meaning through the structured combination of sounds, but such combinatorial mechanisms remain rare in nonhuman animals, raising questions about the evolution of this capacity. Shifts in meaning from single calls to combinations (i.e., compositional-like) appear across a range of combinations and contexts in apes, but evidence from other species is mainly restricted to alarm contexts, possibly reflecting limited research effort. To address this gap, we applied a quantitative, whole-repertoire approach to assess meaningful combinatorial capacities in sooty mangabeys (Cercocebus atys), a forest-dwelling monkey. We recorded 1,751 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 revealed that, unlike its components or the reversed bigram frequently used during feeding, ‘twitter_grunt’ occurred predominantly during infant-directed affiliations, suggesting order-sensitive compositionality. Iterative sequences of ‘grunt’ and ‘twitter’ calls also showed context-specific shifts, indicating a possible role for iteration in meaning generation. These findings suggest that meaningful combinatorial capacities extend beyond great apes and alarm contexts, pointing to deeper evolutionary roots. However, repertoire-wide use of meaning-shifting bigrams remains unconfirmed outside hominids.