Auditory systems of heteromyidae: Cochlear diversity

Abstract Cochleae (125) from 26 species of the rodent family Heteromyidae (genera Dipodomys. Microdipodops, Perognathus , and Liomys ) were compared. In Perognathus and Liomys the scala tympani in the apical portion is extremely narrow with a correspondingly minute helicotrema. In Liomys there is no bone separating scala tympani from spiral ganglion in the upper second and entire third turn. In all species studied the zona pectinata of the basilar membrane is enlarged, with a hyaline mass between upper and lower basilar membrane fibers. This zona pectinata hypertrophy is least at the base of the cochlea and greatest in the upper second turn, decreasing again toward the apex. Basilar membrane width increases rapidly in the first turn and then changes only slightly. Except for Liomys , all the heteromyids studied have hypertrophied Hensen's cells with long apical processes supporting and forming an elevated reticular lamina. These Hensen's cells reach their maximum size in the upper second and lower third turns; throughout they rest on inner Claudius' cells rather than the basilar membrane. Relative to naso‐occipital length the cochlear specializations are greatest in Microdipodops and least in Liomys just as is the case for middle ear modifications. The morphological data are consistent with the concept that standing wave phenomena may be important in heteromyid cochlear biomechanics. Single unit data of other workers are also consistent with this interpretation. Like middle ear morphology, inner ear morphology appears adapted to low‐frequency sensitivity–especially in Dipodomys and Microdipodops .