Data from: Meaningful call combinations and compositional processing in the southern pied babbler

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.