The shape of water: adaptations of cochlea morphology in seals and otters

Abstract

Seals (Pinnipedia) and otters (Lutrinae) are two major taxa of the mammalian order Carnivora that independently adapted to a semi-aquatic lifestyle. Their sensory capabilities, including hearing, are still not fully understood. Notably, the functional morphology of amphibious hearing presents an interesting example for studying convergent evolution. We therefore investigated the shape and dimensions of morphological traits of the inner ear and the surrounding structures involved in sound perception using Micro-Computer Tomography (µCT) data of 52 skulls belonging to 38 species of the carnivoran subgroup Caniformia. Principal component analysis (PCA) of cochlea shape variables revealed that 82% of shape variation is explained by PC1 and PC2. A number of traits (e.g. cochlea shape, reduction in cochlea turns, flattening of the cochlea, relative size of tympanum and round window) are significantly different from terrestrial Caniformia. We also found an external cochlea foramen in all Pinnipedia, which is likely related to coping with pressure compensation during diving. Finally, we demonstrate that hearing parameters, such as characteristic hearing frequency, correlate with changes in the morphological traits that separate seals and otters from other Caniformia. Our data suggest that large parts of the morphology of ear region of these semi-aquatic carnivorans have independently adapted to allow hearing in air and water.