Amplitude Increases of Vocalizations are Associated with Body Accelerations in Siamang (Symphalangus syndactylus)

Siamangs (Symphalangus syndactylus), one of the few singing apes, vocalize loudly, often while they move. We hypothesize that movement and vocalization coordinate, possibly due to vigorous thorax-loading movements such as brachiation affecting vocal-respiratory dynamics. To assess this vocal-motor coordination we recorded more than a hundred stereotypical vocalizations combined with movement from two captive Siamang (isolated from 7 hours of singing). We observed that stereotypical calls coincided with a movement display and were performed by juvenile individuals during solo singing (which allowed for isolation of the calls). Investigating these vocal-motor events, we found that body acceleration estimated using computer vision was statistically associated with the nearest peak in the amplitude envelope of the call, and that body acceleration timeseries contained mutual information about the amplitude envelope timeseries during these events. By confirming via quantitative methods that singing and movement are coordinated, the current report invites further mechanistic investigation on vocal-locomotor coupling in siamang.