Socially learned call sequences reveal gradual development of higher-order structure

Abstract

Syntax was long considered to distinguish human language from other vocal systems, with parallels in non-human animals historically limited to song. However, song lacks discrete meaning, which is a crucial pre-requisite of linguistic syntax. Over the last two decades evidence of combinatoriality in the discrete, semantic calls of an array of taxa has accumulated, providing the opportunity to investigate potentially closer parallels to language. However, most examples remain limited to small repertoires of simple two-call sequences, preventing evidence of complex internal structuring like that seen in human sentences. The recent discovery that several species produce extensive repertoires of much longer call sequences, has provided the opportunity to investigate the full extent of syntactic structure in non-human call systems. Here we demonstrate that Western Australian magpies (Gymnorhina tibicen dorsalis) use multi-level structured ordering rules within their semantic call sequences and that these ordering rules are learned during development. Specifically, we find that calls within sequences up to 15 calls long depend on the two calls given prior and that independently produced segments ( phonemes), calls, and sequences recombine into longer structures, indicating hierarchical organisation. This represents the first evidence of multi-level non-adjacent organisation and learned syntactic structure in a semantic non-human system.