Semicircular canal morphology in Rodentia and its relationship to locomotion

Anatomical structures vary among mammals with different locomotor behaviours, including sensory structures such as the semicircular canals (SCCs) in the inner ear. Recent SCC research has examined various mammalian groups, but there has been a lack of research on rodents, the most speciose and diverse mammalian order. In this study, an extant sample of 98 rodent SCCs from 56 species across seven different locomotor behaviour categories (arboreal, fossorial, gliding, ricochetal, semiaquatic, semifossorial, terrestrial) was used to understand the correlations between SCC morphology and locomotion in rodents. Morphological correlates considered include the radius of curvature (R), overall 3-dimensional shape, and angles between pairs of canals (orthogonality). Our results show that agile arboreal taxa have larger R for their body size, and fossorial taxa have smaller R for their body size. Shape among specialized locomotor behaviours (arboreal, gliding vs. fossorial) can be differentiated, while other "generalist" categories overlap in morphospace. Specialized locomotor categories can be predicted with greater precision and sensitivity, while other generalist categories tend to be miscategorized as terrestrial. Angles between canals are not consistent across locomotor categories, and more agile groups do not have more orthogonal angles, contrary to our predictions. SCC R and overall shape are robust indicators of specialized locomotor behaviours and can be informative in reconstructing the behaviour of fossil rodents.