Chimpanzees combine pant hoots with food calls into larger structures

A growing body of observational and experimental data in nonhuman primates has highlighted the presence of rudimentary call combinations within the vocal communication system of monkeys. Such evidence suggests the ability to combine meaning-bearing units into larger structures, a key feature of language also known as syntax, could have its origins rooted within the primate lineage. However, the evolutionary progression of this trait remains ambiguous as evidence for similar combinations in great apes, our closest-living relatives, is sparse and incomplete. In this study, we aimed to bridge this gap by analysing the combinatorial properties of the pant hoot-food call combination in our closest-living relative, the chimpanzee, Pan troglodytes. To systematically investigate the syntactic-like potential of this structure, we adopted three levels of analysis. First, we applied collocation analyses, methods traditionally used in language sciences, to confirm the combination of pant hoots with food calls was not a random co-occurrence, but instead a consistently produced structure. Second, using acoustic analyses, we confirmed pant hoots and food calls comprising the combination were acoustically indistinguishable from the same calls produced in isolation, indicating the pant hoot-food call combination is composed of individually occurring meaning-bearing units, a key criterion of linguistic syntax. Finally, we investigated the context-specific nature of this structure, demonstrating that the call combination was more likely to be produced when feeding on larger patches and when a high-ranking individual joined the feeding party. Together our results converge to provide support for the systematic combination of calls in chimpanzees. We highlight that playback experiments are vital to robustly disentangle both the function this combination might serve and the similarities with combinations of meaning-bearing units (i.e. syntax) in language. (c) 2021 The Author(s). Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).