Vocal membranes lower the phonation threshold pressure in rhesus macaques (Macaca mulatta)

The vocal membrane, an extended part of the vocal fold, is present in a broad range of species including non-human primates. Its contribution to animal vocalizations has long been of interest. A theoretical study (Mergell P, Fitch T, Herzel H. 1999 Modeling the role of nonhuman vocal membranes in phonation. J. Acoust. Soc. Am. 105, 2020-2028. (https://doi.org/10.1121/1.424994)) predicted that vocal membranes enhance vocal efficiency by lowering the phonation threshold pressure. To test this, excised larynx experiments were conducted on rhesus macaques (Macaca mulatta). Comparisons before and after surgical removal of the vocal membranes showed that the phonation threshold pressure was indeed lower-and vocal efficiency higher-when the vocal membranes were present. Most experiments exhibited periodic oscillations of the vocal folds and/or membranes, while some showed irregular broadband oscillations potentially indicative of chaos. A computational model representing the vocal membrane as a dynamic, reed-like plate reproduced both periodic and irregular oscillations, depending on parameter settings such as the damping ratio. These simulations suggest that transitions between different regimes can arise from individual anatomical or physiological variation. Although this pilot study is based on two macaque larynges, the results support the idea that vocal membranes may contribute to vocal efficiency and dynamic variability, potentially enabling louder calls with less pulmonary effort.