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

Nicolas Louis Michel Brualla , Laura AB Wilson, Khizar Hayat, Michael Doube, Tu Vuong Tan, Thongchai Ngamprasertwong, Thanakul Wannaprasert, Daisuke Koyabu

Key innovations play a crucial role in driving biodiversity and facilitating evolutionary success by enabling organisms to adapt to various ecological niches through the diversification of phenotypic traits. These innovations have been observed in different vertebrate clades, such as mammals evolving hypsodonty to graze on contemporary grasses and bats with the evolution of echolocation, alongside wing acquisition. Recent studies have shed light on the overlooked morphological diversity of the larynx in bats, a key organ involved in echolocation capabilities. Tracheal chambers, found on the first rings of the trachea, are enigmatic components of the laryngeal complex whose origins and functions have yet to be fully elucidated. We hypothesised that these structures may show evolutionary convergence and represent a key innovation associated with laryngeal echolocation. The present study examines 50 bat species, their laryngeal cartilages and tracheal chambers. We explored relationships between body mass, sound frequencies, and chamber volumes, as we hypothesise that tracheal chambers may have facilitated laryngeal echolocation capabilities in bats. Ancestral state reconstructions were conducted to understand the evolution of tracheal chambers and laryngeal echolocation behaviours in bats. We conclude that tracheal chambers allow higher frequency sound production and were pivotal for the specialization of high-duty cycle echolocation during the evolution of bats emitting calls nasally, contributing to their ability to thrive in diverse environments. We suggest that tracheal chambers are key innovations that enhance laryngeal echolocation behaviours and the evolutionary success of bats.