Many non-human primates have large and acoustically diverse vocal repertoires. However, our understanding of the phonatory mechanisms underlying such acoustic variation is poor. Representative exemplars of some of the articulatory gestures used by free-ranging rhesus monkeys while uttering a range of vocalizations are provided. Results reveal that different call types appear to be associated with characteristic lip configurations and mandibular positions. Quantitative analyses of the 'coo' vocalization indicate that changes in the position of the mandible are reliably associated with changes in dominant frequency (i.e. resonance frequency), but not with changes in fundamental frequency. This finding suggests that rhesus monkeys can modify the spectral properties of the signal, independent of the glottal source (i.e. fundamental frequency). Such articulatory manoeuvres contribute to the animal's potential acoustic space, thereby potentially increasing the array of meaningful vocalizations within the repertoire.
Hauser, M. D., Evans, C. S., & Marler, P. (1993). The role of articulation in the production of rhesus monkey, Macaca mulatta, vocalizations. Animal Behaviour, 45(3), 423-433. https://doi.org/10.1006/anbe.1993.1054