Middle ear and cochlear receptors in the subterranean mole-rat, Spalax ehrenbergi

The eardrum and the auditory ossicles of the middle ear, and the population of cochlear receptors in subterranean mole-rats of Spalax ehrenbergi complex were qualitatively and quantitatively analyzed. The area of the eardrum was 10.2 (S.D. = 0.9) mm², that of the stapedial footplate 0.6 (S.D. = 0.07) mm², the malleus: incus lever ratio was 1.25:1. The malleus is simply built, not attached to the tympanic ring. The incus is large; its long process is almost parallel to the manubrium of the malleus. The short process of the incus is provided with a peculiar bony cap, a structure thus far undescribed in mammalian auditory ossicles. Firmness of mutual connection of the ossicles may vary among different forms of Spalax. The size and form of the stapes are characterized by a large variability as well. The length of the basilar membrane was 12.6 (S.D. = 0.7) mm. The mean density of inner hair cells amounted to 93.3 that of outer hair cells to 386.4 cells per mm of the basilar membrane length. In general, the density increased from the base towards the apex. On average there were 415 outer, 100 inner hair cells in the organ of Corti; the ratio being constant along the basal 65% of the length and decreasing in the apical part of the organ of Corti. The radial width of the triad of outer hair cells increased and remained constant along the apical half of the Corti organ. Based on available findings of comparative and functional morphology, most of the studied features may be considered correlates of low-frequency tuning of the hearing organ in mole-rats.