Interplay between evolutionary history, morphological constraints and functional adaptations in the primate cochlea

How the intricate mammalian cochlea evolved, and its functional implications, remain only partly understood. Here, we explore cochlear morphology across 101 extant and fossil species of the mammalian grand order Euarchonta using micro-computed tomography, three-dimensional geometric morphometrics and phylogenetic comparative analyses. We find substantial shape variation across taxa, likely driven by an interplay between evolutionary history, morphological constraints and potentially functional demands, although these remain difficult to interpret. Evolutionary models suggest the rate of cochlear shape evolution was heterogeneous, with some lineages showing particularly high rates, likely linked with adaptive selection pressures (e.g. tarsiers, Cercopithecus). Ancestral state reconstructions reveal lemuriforms retain the ancestral strepsirrhine cochlear shape-conical with around 2½ turns-while lorisiforms exhibit a derived cylindrical cochlea with increased coiling. The highly coiled cochlea of tarsiers reflects cranial constraints and functional demands, particularly for high-frequency hearing. In anthropoids, platyrrhines nest within catarrhine variation. Among the latter, cercopithecins trend towards increased coiling, whereas colobines and hominoids retain less coiled shapes. Finally, while body size has little effect on cochlear shape, its interaction with cochlear length predicts the number of turns, supporting the theory that cochlear coiling enabled the development of a longer basilar membrane within a small petrous space.