Contextual encoding in titi monkey alarm call sequences

Many primates produce one type of alarm call to a broad range of events, usually terrestrial predators and non-predatory situations, which raises questions about whether primate alarm calls should be considered ‘functionally referential’. A recent example is black-fronted titi monkeys, Callicebus nigrifrons , which emit sequences of B-calls to terrestrial predators or when moving towards or near the ground. In this study, we reassess the context specificity of these utterances, focussing both on their acoustic and sequential structure. We found that B-calls could be differentiated into context-specific acoustic variants (terrestrial predators vs. ground-related movements) and that call sequences to predators had a more regular sequential structure than ground-related sequences. Overall, these findings suggest that the acoustic and temporal structure of titi monkey call sequences discriminate between predator and non-predatory events, fulfilling the production criterion of functional reference. Significance statement Primate terrestrial alarm calls are at the centre of an ongoing debate about meaning in animal signals. Primates regularly emit one alarm call type to ground predators but often also to various non-predatory events, raising questions about the referential nature of these signals. In this study, we report observational and experimental data from wild titi monkeys and show that terrestrial alarm calls are usually given in sequences of acoustically distinct variants composed in structurally distinct ways depending on the external event. These differences are salient and could help recipients to distinguish the nature of the call eliciting event. Since most previous studies on animal alarm calls have not checked for acoustic variants within different call classes, it may be premature to conclude that primate terrestrial calls do not meet the criteria of functional reference.