Macroevolutionary and biomechanical implications of rostral flexion in bat skulls: a major early driver of cranial evolution in bats

Abstract The evolution of unique morphological features can significantly constrain the subsequent phenotypic diversification of a clade. Bats show diverse skull shapes and are salient among mammals in their dramatic reorientation of major cranial features due to changes in the relative position of the rostrum with respect to the braincase (rostral flexion). Previous studies have suggested that rostral flexion evolved early in the history of bats in tandem with divergent echolocation modes. We directly quantified rostral flexion and integrated biomechanical modelling, morphometric, and comparative phylogenetic approaches to explore its functional and evolutionary consequences. We found that strong selection associated with echolocation mode explains the early evolution of rostral flexion in bats, with subsequent diversification at different rates within different echolocation emission types. We find extreme rostral flexion is associated with trade-offs in bite performance and overall evolutionary lability of cranial shape. The divergent evolution of rostral flexion across major bat lineages has thus been a major factor structuring the potential evolutionary pathways for the bat cranium. We further suggest that the early evolution of rostral flexion in bats biased the transitions to and from certain echolocation emission modes, and may have differentially affected the degree of dietary ecological diversification among bat clades.