Understanding whether the long and elaborate songs of male gibbons have syntax and hierarchical structures is an interesting question in the evolution of language, because gibbons are near humans in the phylogenetic tree and a hierarchically organized syntax is considered to be a basic component of human language. We conducted field research at Danum Valley Conservation Area in northern Borneo to test the hypothesis that gibbon songs have syntax and chunks. We followed one Mueller’s gibbon group for 1 week in (...) the dry and rainy seasons every year from 2001 to 2009, collecting vocal and behavioral data. Results show that songs emitted by the studied male gibbon were governed by combinatory rules. Some context-dependent songs had different combinatory rules, although they overlapped with the songs whose contexts were uncertain. The male Mueller’s songs had characteristics that suggest existence of chunk structure. These results provided an important perspective in the study of language origin. (shrink)