Tracheal chambers as a key innovation for high-frequency emission in bat echolocation.

Key innovations are pivotal for biodiversity and facilitating evolutionary success, enabling organisms' adaptation to various ecological niches through the diversification of phenotypic traits. In mammals, notable adaptations include evolving hypsodonty for grazing on grasses and, for bats, evolving echolocation and wing acquisition. Recent studies have highlighted the overlooked morphological diversity of the bat larynx, a key organ for echolocation capabilities. Tracheal chambers, found on the trachea's first rings, are enigmatic components within the laryngeal complex whose origins and functions remain incompletely understood. We hypothesized that these structures demonstrate evolutionary convergence and represent a key innovation responsible for the diversification of laryngeal echolocation behaviors. By analyzing 51 bat species, their laryngeal cartilages, and tracheal chambers, we explored relationships between body mass, sound frequencies, and chamber volumes. We hypothesized that tracheal chambers likely facilitated laryngeal echolocation capabilities in bats. Ancestral state reconstructions were conducted to understand the evolution of tracheal chambers and echolocation behaviors in bats. We conclude that tracheal chambers might support higher frequency sound production and could have been pivotal for high-duty cycle echolocation specialization during the development of nasally emitted calls, which allowed bats to thrive in diverse environments. We suggest that tracheal chambers are specialized phenotypic adaptations that might have helped in diversifying laryngeal echolocation behaviors and the evolutionary success of bats.