Additional row of outer hair cells — The unique pattern of the Corti organ in a subterranean rodent, the Gansu zokor (Eospalax cansus)

Acoustic conditions in burrows are different from those aboveground and restrict hearing of subterranean mammals to low frequencies, which is reflected in the ear morphology. While low-frequency adaptations of the middle ear attracted more attention of researches, the inner ear remained rather understudied. Here, we examined the cochlea of the inner ear of the Gansu zokor (Eospalax cansus), a subterranean rodent from the Tibetan Plateau. We focused on the quantitative parameters of the organ of Corti, which are assumed to determine hearing sensitivity and frequency tuning. Apart from the morphological traits common to the ear of subterranean rodents studied thus far, the Gansu zokor shows two unique features: the presence of a fourth row of outer hair cells along 20% to 50% of the basilar membrane length and almost constant width of the organ of Corti over more than 10% of its spiral length. Both these anomalies occur in the middle of the cochlear spiral. These features are unusual in comparative morphology of the organ of Corti and presumably are reflected in the functional specialization. They are expected to affect sensitivity and/or resolution of hearing in the frequency range registered in the given cochlear segment. The Gansu zokor thus profiles to an interesting candidate for hearing research which might provide further insight not only into morpho-functional adaptations in subterranean mammals in particular but also in the function of outer hair cells in general.