Variations of Mammalian Cochlear Shape in Relation to Hearing Frequency and Skull Size

Abstract

The cochlea is the receptive organ of mammalian hearing and variations of its gross morphology have been linked to differences of hearing ability but as yet there has been little quantitative assessment of the proposed link, partly due to the difficulties of defining the complex geometry of the cochlea. The present study aims to quantitatively define the geometry and then determine whether adaptations of cochlear form are linked to the spatial limitations of the skull and hearing among a range of extant mammals. Advanced techniques in micro-CT imaging, 3D image visualization, geometric morphometrics and statistical methods were used to study the bony cochlea across 36 adult eutherian species. Data on hearing frequency were taken from the literature. Results showed that there was a considerable range of variation in form of the mammalian bony cochlea. The cochlear shape was correlated with the number of spiral turns. The low-frequency limit of hearing was negatively correlated with the cochlear length and volume. Also, the ratio of the intermeatal distance to cochlear length showed a correlation with the number of whorls and cochlear shape. It is concluded that the number of spiral turns is a key determinant of the eutherian cochlear shape. An increase in cochlear length and volume is likely to enhance low-frequency sound perception, whilst the cochlear spirals may not be related to hearing frequency. Results suggest that cochlear shape may be linked, at least in part, to the size of the skull either in terms of the wavelengths that can be used for localistion in smaller headed species and/or the demands of maintaining the physiological function of the cochlea in a smaller space.