A proposal for epithelial dominance in extremely high fundamental frequency vocalizations.

In this hypothesis article, we explore the upper limit of the fundamental frequency in vocalization. Most mammalian vocalizations are produced by airflow-induced, self-sustained vibration of vocal folds, with fundamental frequency being determined by multiple tissue layers in the folds, including muscle, ligament, and epithelial tissues. These layers contribute to vocal fold length, depth of vibration, and viscoelasticity needed for oscillation. While current vocal fold models explain a large range of frequencies, some extremely high-frequency vocalizations (e.g., whistle voice in humans) remain unexplained based on known tissue properties. We hypothesize that the thin layers near the epithelial surface become primary contributors to elasticity at high frequencies. Anatomical studies indicate weak allometric scaling in the epithelium, i.e., number of epithelial cell layers and thickness of the epithelium scale weakly with body size. This could allow species to produce frequencies outside the typical size-dependent spectral range if this layer dominates. Computational simulations using tissue property data support this hypothesis. We propose a model in which epithelial cells combined with collagen fibers in the lamina densa form structures capable of generating fundamental frequencies in the kilohertz range with minimal depths of vibration.