The Shape of Sound: a Geometric Morphometrics Approach to Laryngeal Functional Morphology

Diversification of animal vocalizations plays a key role in behavioral evolution and speciation. Vocal organ morphology represents an important source of acoustic variation, yet its small size, complex shape, and absence of homologous landmarks pose major challenges to comparative analyses. Here, we use a geometric morphometric approach based on geometrically homologous landmarks to quantify shape variation of laryngeal cartilages of four rodent genera representing three families. Reconstructed cartilages of the larynx from contrast-enhanced micro-CT images were quantified by variable numbers of three-dimensional landmarks placed on structural margins and major surfaces. Landmark sets were superimposed using generalized Procrustes analysis prior to statistical analysis. Correlations among pairwise Procrustes distances were used to identify the minimum number of landmarks necessary to fully characterize shape variation. We found that the five species occupy distinct positions in morphospace, with variation explained in part by phylogeny, body size, and differences in vocal production mechanisms. Our findings provide a foundation for quantifying the contribution of vocal organ morphology to acoustic diversification.