Controlling vocal membranes in nonhuman primate vocalizations

The vocal membrane (VM), i.e., an appendage extending from a supero-medial portion of the vocal fold, is present in a range of species, including bats and nonhuman primates. Notably, the vocal membrane is absent in humans, i.e., medio-superior surface of the human vocal fold is known to be rounded. It has been a matter of great interest how this small tissue contributes to animal vocalizations and why this feature was lost in the human lineage. Acoustic functions of the VM are, however, yet to be fully understood. Recent studies based on excised larynx experiments and in vivo measurements of rhesus macaques and chimpanzees showed that the VMs do vibrate during the vocalizations of these animals. Depending upon the glottal configurations, various oscillation patterns have been observed: (i) only VMs vibrate, (ii) VMs vibrate in-phase/out-of-phase with the vocal folds. In the present study, physical model as well as mathematical model of the VMs are developed to clarify how these oscillation patterns can be controlled and how the VMs induce bifurcations and chaos. Application of the controlled oscillation patterns of the VMs to yodeling, i.e., frequency jumps produced in New World monkeys, is further discussed.