Dual Sound Sources in Siamangs Generate Individually Rhythmic and Temporally Coordinated Vocal Emissions

Temporal organization is crucial for efficient communication in many animal species, particularly those with complex vocal output. Singing primates structure their songs into rhythmic categories based on small-integer ratios between adjacent intervals. However, rhythmic organization usually emerges from a single sound source, and it remains unknown whether any species can produce rhythms through the coordination of multiple vocal production mechanisms. This study investigates rhythmicity in the song of siamangs (Symphalangus syndactylus). This singing primate uses two distinct vocal mechanisms: modulation of the oral tract and radiation through the laryngeal sac. We analyzed the temporal structure of vocal units resonated in the oral tract (mouth units) and those by the laryngeal sac (sac units), as well as the combined output of both. Both mouth and sac units displayed an isochronous pattern (1:1), corresponding to adjacent intervals of the same duration. The combined output showed significant isochrony (1:1) and two additional nonsignificant clusters at ratios of 1:9 and 9:1. These findings suggest that the interaction between the two vocal mechanisms yields a more complex rhythmic structure, representing a unique case of vocal coordination in nonhuman primates.