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No abstract received.

No abstract received.

Language is considered as the great divide between the cognitive and social ability of humans and those of other animals. How did language emerge and evolve into such a complex system? A promising approach to this is to compare and contrast language with animal signal systems, which involve key substrates for language. We focus on birdsong development from the aspect that language is a learned vocal behavior. Although birdsong is different from language in many ways, they share biological foundations for vocal learning. So far birdsong study has yielded significant implications for language evolution; for example, a cultural evolution of birdsong (Fehér, Wang, Saar, Mitra, & Tchernichovski, 2009).

Here we study the development of phonology and syntax in Bengalese finch song. Adults sing complex song that consists of a number of chunks, which in turn consist of a few patterned notes (Okanoya, 2004). Juveniles learn individually distinct song by imitating adult males. To track the entire song development, 24-hour recording was conducted for 16 juveniles every 4 to 5 days after hatching. When recording, each juvenile was kept in a soundproof box with a microphone, and all singing activities were recorded. From all the recordings, we computed six acoustic features of notes, such as note duration, mean pitch, and mean Wiener entropy. Figure 1 shows (a) a phonological development in the acoustic feature space, and (b) a syntactic development. At day 50, every note was acoustically similar to each other. At day 60, notes with longer duration emerged abruptly, and then ones with harmonics diverged from the residuals, which gradually differentiated with development. Finally, eight types of notes emerged from a single acoustic stem-cluster. The recorded songs were converted to texts by annotating letters to identical note types, and then a grammatical inference method (Kakishita, Sasahara, Nishino, Takahasi, & Okanoya, 2009) was applied. Before day 70, it was not able to extract a syntax due to the transitional instability of notes. Song notes stabilized with development, becoming some patterned chunks, and the transitions also gradually stabilized. After day 100, the song syntax was crystallized.

The results demonstrated a co-developmental process of phonology and syntax in birdsong; exposed in a social environment, juveniles developed note types and sequential and sub-sequential structures as well. While the striking parallels between birdsong and language in development, there is a significant difference. It is known that human infants develop words with the aid of contextual semantic cues. Thus words develop based not only on phonological rules but also on semantic constraints. Song chunks look similar to words in form, but they develop without any atomic meanings. Birdsong therefore lacks 'double articulation,' by which small meaningless sound units combine into large meaningful units. These suggest that a precursor of syntax, like song syntax, could emerge from a learned vocal behavior, evolving relatively independent of semantics; however, to become syntax with double articulation, semantic constraints are indispensable in development. What adaptation mechanism is required for the syntax-semantics entanglement to become exist? Our findings raise further questions to be solved.

Note from Publisher: This article contains the abstract and references.

Birdsongs have been studied with increasing interest for language evolution; in particular, birds with relatively simple songs and several tens of song elements, such as Zebra finch and Bengalese finch, have been well-studied (Berwick, Okanoya, Beckers, & Bolhuis, 2011). It is, however, still unclear how birds with complex songs and a large repertoire learn and organize their songs, although there have been a number of qualitative studies (Todt & Hultsh, 1998)…