title: Meaningful call combinations and compositional processing in the southern pied babbler.
abstract: Language's expressive power is largely attributable to its compositionality: meaningful words are combined into larger/higher-order structures with derived meaning. Despite its importance, little is known regarding the evolutionary origins and emergence of this syntactic ability. Although previous research has shown a rudimentary capability to combine meaningful calls in primates, because of a scarcity of comparative data, it is unclear to what extent analog forms might also exist outside of primates. Here, we address this ambiguity and provide evidence for rudimentary compositionality in the discrete vocal system of a social passerine, the pied babbler (Turdoides bicolor). Natural observations and predator presentations revealed that babblers produce acoustically distinct alert calls in response to close, low-urgency threats and recruitment calls when recruiting group members during locomotion. On encountering terrestrial predators, both vocalizations are combined into a "mobbing sequence," potentially to recruit group members in a dangerous situation. To investigate whether babblers process the sequence in a compositional way, we conducted systematic experiments, playing back the individual calls in isolation as well as naturally occurring and artificial sequences. Babblers reacted most strongly to mobbing sequence playbacks, showing a greater attentiveness and a quicker approach to the loudspeaker, compared with individual calls or control sequences. We conclude that the sequence constitutes a compositional structure, communicating information on both the context and the requested action. Our work supports previous research suggesting combinatoriality as a viable mechanism to increase communicative output and indicates that the ability to combine and process meaningful vocal structures, a basic syntax, may be more widespread than previously thought.
authors.affiliation: Pied Babbler Research Project, Kuruman River Reserve, Northern Cape 8467, South Africa; Animal Behaviour, Department of Evolutionary Biology and Environmental Studies, University of Zurich, 8057 Zurich, Switzerland; sabrina.engesser@ieu.uzh.ch.
authors.forename: Sabrina
authors.initials: S
authors.lastname: Engesser
authors.affiliation: Pied Babbler Research Project, Kuruman River Reserve, Northern Cape 8467, South Africa; Centre for Evolutionary Biology, School of Animal Biology, The University of Western Australia, Crawley, WA 6009, Australia; Percy FitzPatrick Institute, University of Cape Town, Rondebosch 7701, South Africa;
authors.forename: Amanda R
authors.initials: AR
authors.lastname: Ridley
authors.affiliation: Pied Babbler Research Project, Kuruman River Reserve, Northern Cape 8467, South Africa; Animal Behaviour, Department of Evolutionary Biology and Environmental Studies, University of Zurich, 8057 Zurich, Switzerland; Department of Psychology, University of Warwick, Coventry CV4 7AL, United Kingdom.
authors.forename: Simon W
authors.initials: SW
authors.lastname: Townsend
mesh_terms: D000818:Animals; D046109:Passeriformes / Q000502:physiology*; D014828:Vocalization, Animal / Q000502:physiology*
keywords: call combination; compositionality; language evolution; southern pied babbler; syntax