The Ear in Subterranean Rodents Revisited: Cochlear Hair-Cell Populations in African Mole-Rats (Bathyergidae)

Based on von Békésy's premise that "The physical laws served as guidelines for the evolution of the structures and functions of the middle and inner ear," we aimed to understand how the unique subterranean acoustic environment, which promotes the propagation of low-frequency sounds and thereby selects for enhanced low-frequency hearing, influences functional adaptations reflected in the morphological convergence of the cochlea in subterranean African mole-rats (Bathyergidae). We conducted a morphometric analysis of the cochlea in 12 species representing all six genera of African mole-rats, spanning a body mass range of 30-2000 g. Cochlear partitions were examined using light microscopy following the standard surface specimen technique. The mole-rat cochleae has 3-4.3 coils. The length of the basilar membrane (BM) varies from 6.5 to 15.6 mm. Mean densities of inner hair cells (IHC) range from 104 to 122, whereas outer hair cells (OHC) range from 390 to 480 per 1 mm. Hair cell density increased slightly from the base towards the apex in all species studied. The radial width of the cuticular plates of the three rows (triad) of OHC, shown in previous studies to mirror BM width, increased continuously from, on average, 22 ± 3 µm at the base to 35 ± 6 μm at the apex. Length of BM, width of the OHC triad and total number of hair cells (and thus hearing resolution capabilities) are related to body size. When compared to other mammals, the cochleae of bathyergids exhibit quantitative characteristics that closely resemble the apical regions of the cochleae in other species-specifically, those segments tuned to low frequencies. Moreover, the width of OHC triads was strongly correlated with the tonotopic organization of frequencies along the organ of Corti, confirming its value as a structural predictor of auditory capability.