Complex rules of vocal sequencing in marmoset monkeys

Vocal sequencing is a key element in human speech. Songbirds have been widely studied as an animal model to investigate neural mechanisms of vocal sequencing, due to the complex syntax of syllable sequences in their songs. However, songbirds are phylogenetically distant from humans. So far, there is little evidence of complex syntactic vocalizations in non-human primates. Here, we analyze phee sounds produced by 160 marmoset monkeys either in isolation or during vocal turn-taking and reveal complex sequencing rules at multiple levels. First, phee syllables exhibited consistent interval patterns among different marmosets, allowing categorization of calls with single and closely spaced 2-4 syllables into 4 grades. Second, the ordering of sequential calls followed distinct probabilistic rules that preferring repetition of the same-grade call and then transition between calls of adjacent grades, but not skip-grade transition. Moreover, inter-call intervals depended on the transition direction. Third, specific ABnA call patterns were discovered to be prominent in long call sequences, and their occurrence exhibited a power-law decrease with increasing “n”, reflecting a long-range sequencing rule in the dependence of later calls on the pattern of earlier calls. Finally, syllable and call intervals as well as call compositions were significantly modified during vocal turn-taking. This complex syntax of vocal sequences in marmosets offers opportunities for understanding the evolutionary origin and neural mechanisms of grammatical complexity in human language. ### Competing Interest Statement The authors have declared no competing interest.