Note orders suggest phrase-inserting structure in male Mueller’s gibbon songs: a case study

Language has a complex and hierarchical structure, which includes context-free grammar and phrase embedding. However, there have been no reports of a phrase-embedding structure in animal vocal communication, although there are several reports of combinational animal sounds that reference different objects and emotions. The songs of male Mueller’s gibbons in our study area consist of two notes, “wa” and “oo,” and combinations of these are flexible. There are various types of note orders in their song phrases. When the animals sing songs by combining acoustic elements flexibly, a complicated syntax may emerge. When phrase “N” is inserted within another phrase “AB,” the generated phrase is shown as “ANB.” We named this structure a phrase-inserting structure. If the phrase “N” itself has an inserting structure elsewhere, “ANB” is shown as “AABB.” This structure is considered a phrase-embedding structure, as defined by Abe and Watanabe (Nat Neurosci 14:1067–1074, 2011). We hypothesized that there is a phrase-inserting structure in male Mueller’s gibbons’ songs. We analyzed 70 songs by a single male and found note orders that suggested a phrase-inserting structure. Among them, there were four examples of phrases with structure within phrases, suggesting phrase embedding. Phrase inserting might be considered a sort of precursory level to recursion. Although our sample size was small, our results show that male Mueller’s gibbons may be able to produce recursive sequences. This provides an important insight into the study of language evolution.