Comparative morphometry of odontocete ears through computerized tomography

Abstract

Describing the auditory periphery of odontocetes is a key conservation issue to further assess the effect of acoustic pollution. Because all odontocetes produce species-specific frequency ranges, differences in echolocation signals should reflect anatomical differences in the auditory pathways. Here, we studied the ears of 15 odontocete species through 3D reconstructions from computerized tomography scans to extract standard measurements (bullae lengths/volumes and cochlear volume) and investigate the discriminatory weight of each of these variables as well as their relation to the species' hearing specificity. Any of the measurements appeared to be a good indicator of the species and could therefore be used to classify them. All the ear lengths and most volumes were strongly linearly correlated (r >0.9) in all species and the proportion between the tympanic and periotic bones appeared to remain constant. This constant ratio could be an indication of a functional relationship between both structures, and might suggest an active role of the odontocete middle ear during target acoustic detection, providing new information on the odontocete sound reception mechanism. Our results are generally consistent with previous studies, although here the coefficients of correlation between animal lengths and the total volume and lengths of the bullae were lower (0.77< r <0.86), indicating that the length of the animals may not be a primary parameter to take into account when defining ear measurements. These results suggest that the measurements described characterize standard ears which could be used as a morphological basis for further species-specific acoustic comparison.