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.

语言的表达力在很大程度上源于其组合性（compositionality）：具有独立意义的词汇可被组合为具备衍生意义的更大或高阶结构。尽管组合性对语言至关重要，但学界对该句法能力的演化起源与出现过程仍所知有限。尽管既往研究已证实灵长类具备组合有意义叫声的初步能力，但由于比较研究数据匮乏，尚不清楚灵长类之外的类群是否也存在类似的句法表达形式。本研究旨在厘清这一认知模糊之处，并为社会性雀形目鸟类斑鸫鹛（Turdoides bicolor）的离散发声系统中存在初步组合性提供实验证据。自然观察与天敌暴露实验显示，斑鸫鹛会发出声学特征迥异的警戒叫声（alert calls）以应对近距离低紧急度威胁，在移动过程中招募群体成员时则发出招募叫声（recruitment calls）。当遭遇陆生捕食者时，这两种叫声会被组合为“围攻呼叫序列（mobbing sequence）”，以在危险情境中完成群体成员招募。为探究斑鸫鹛是否以组合方式处理该呼叫序列，我们开展了系统性实验，分别回放单个叫声、自然产生的呼叫序列与人工构造的序列。相较于单个叫声或对照序列，斑鸫鹛对围攻呼叫序列回放的反应最为强烈，表现出更高的警觉性与更快的扬声器趋近速度。我们据此认为，该呼叫序列属于组合性结构，可同时传达情境与所需行动的相关信息。本研究支持既往提出的观点，即组合性是提升沟通输出量的可行机制；同时表明，组合与处理有意义发声结构（即基础句法）的能力，可能比此前认知中更为广泛存在。