Morphometry of the vestibular organ in neonate and adult African mole-rats Cryptomys species

Ear Structures of the Naked Mole-Rat, Heterocephalus glaber, and Its Relatives (Rodentia: Bathyergidae)

Although increasingly popular as a laboratory species, very little is known about the peripheral auditory system of the naked mole-rat, Heterocephalus glaber. In this study, middle and inner ears of naked mole-rats of a range of ages were examined using micro-computed tomography and dissection. The ears of five other bathyergid species (Bathyergus suillus, Cryptomys hottentotus, Fukomys micklemi, Georychus capensis and Heliophobius argenteocinereus) were examined for comparative purposes. The middle ears of bathyergids show features commonly found in other members of the Ctenohystrica rodent clade, including a fused malleus and incus, a synovial stapedio-vestibular articulation and the loss of the stapedius muscle. Heterocephalus deviates morphologically from the other bathyergids examined in that it has a more complex mastoid cavity structure, poorly-ossified processes of the malleus and incus, a ‘columelliform’ stapes and fewer cochlear turns. Bathyergids have semicircular canals with unusually wide diameters relative to their radii of curvature. How the lateral semicircular canal reaches the vestibule differs between species. Heterocephalus has much more limited high-frequency hearing than would be predicted from its small ear structures. The spongy bone forming its ossicular processes, the weak incudo-stapedial articulation, the columelliform stapes and (compared to other bathyergids) reduced cochlear coiling are all potentially degenerate features which might reflect a lack of selective pressure on its peripheral auditory system. Substantial intraspecific differences were found in certain middle and inner ear structures, which might also result from relaxed selective pressures. However, such interpretations must be treated with caution in the absence of experimental evidence.

High morphological variation of vestibular system accompanies slow and infrequent locomotion in three-toed sloths

The semicircular canals (SCs), part of the vestibular apparatus of the inner ear, are directly involved in the detection of angular motion of the head for maintaining balance, and exhibit adaptive patterns for locomotor behaviour. Consequently, they are generally believed to show low levels of intraspecific morphological variation, but few studies have investigated this assumption. On the basis of high-resolution computed tomography, we present here, to our knowledge, the first comprehensive study of the pattern of variation of the inner ear with a focus on Xenarthra. Our study demonstrates that extant three-toed sloths show a high level of morphological variation of the bony labyrinth of the inner ear. Especially, the variation in shape, relative size and angles of their SCs greatly differ from those of other, faster-moving taxa within Xenarthra and Placentalia in general. The unique pattern of variation in three-toed sloths suggests that a release of selection and/or constraints on their organ of balance is associated with the observed wide range of phenotypes. This release is coincident with their slow and infrequent locomotion and may be related, among other possible factors, to a reduced functional demand for a precise sensitivity to movement.

Morphological, morphometric, and functional differences in the vestibular organ of different breeds of the rat (Rattus norvegicus)

In the laboratory rat, differences in shape, dimension and function of the cochlea have been reported for various breeds. In contrast, no comparable investigations to date have been undertaken for the vestibular organ in different breeds of the rat. Vestibular organs of two breeds of rat (Wistar, Sprague-Dawley) were analyzed morphologically and morphometrically by means of microdissection techniques in order to determine the mechanical sensitivity of the cupula according to Oman et al; (Acta Otolaryngol., 1987;103:1-13, 1987). Differences in shape of the lateral semicircular duct exist between the two breeds and the cupular mechanical sensitivity is significantly higher in Wistar than in Sprague-Dawley rats. With respect to the other semicircular ducts, no differences in shape were found between the two strains. The cupular mechanical sensitivity of the anterior semicircular duct, however, is higher in Wistar than in Sprague-Dawley rats. The breeds also differ in the shape of their utriculus; obviously a correlation exists between the latter and the cupular mechanical sensitivity of the semicircular ducts. There are differences in the vestibular organs between the two breeds of the laboratory rat investigated. The cupular mechanical sensitivity of the semicircular duct does not seem to be correlated to body mass. The size and morphology of the utriculus influence the mechanical sensitivity of a single duct, but differences only become significant if other parameters also differ.

Convergent evolution of the vestibular organ in the subterranean mole-rats, Cryptomys and Spalax, as compared with the aboveground rat, Rattus

The membranous labyrinth of the vestibular organ (examined in toto) in two unrelated species of subterranean rodents, Cryptomys sp. from Zambia and Spalax ehrenbergi from Israel, was in many parameters (streamline length, curvature radius, and cross-sectional area of the lumen) relatively or even absolutely (especially the cross-sectional area) larger than in the laboratory Norway rat. The mechanical sensitivity of the vestibular organ (estimated according to the mathematical model of Oman et al., [1987] Acta. Otolaryngol (Stockh.) 103:1-13) was similar in both subterranean rodent species and significantly higher than that in the laboratory rat. The most pronounced differences in morphometry and the resulting mechanical sensitivity between the subterranean forms and the rat occurred in the lateral (i.e., phylogenetically and ontogenetically most recent and presumably most plastic) semicircular duct. The area of the sensory epithelia, and density and total numbers of vestibular receptors, were estimated on surface specimens for both maculae and for all three cristae for all three species. While the density of hair cells in comparable sensory epithelia was similar in all three species, the sensory area and thus, also, the total receptor counts were significantly larger in both subterranean forms. The peripheral vestibular organ in subterranean rodents is, in comparison to a generalized aboveground dwelling form, i.e., the rat, progressively specialized, and in any case cannot be denoted as degenerate.

Functional allometry of the semicircular ducts in subterranean mole-rats Cryptomys (Bathyergidae, Rodentia)

BACKGROUND

Since the spatial mobility of subterranean mole-rats is restricted and their skull strongly modified, morpho-functional regression of their vestibular organ may be expected. On the other hand, vestibular sense may be indispensable for orientation in a world deprived of most external sensory cues. Since vestibular sensitivity is determined by the size of the labyrinth, morphometrical analysis may be employed to assess the sensitivity and to test whether the organ is degenerate or progressively specialized.

METHODS

We analyzed the shape and size of the membranous labyrinth of vestibular organs in toto in three species of African subterranean blind mole-rats of the genus Cryptomys (Bathyergidae) differing in body size yet not in habitats, ways of life, or employed orientation and locomotory strategies. The mechanical sensitivity of the cupula was estimated according to Oman et al. (1987. Acta Otolaryngol. (Stockh.), 103:1-13) for each of the three semicircular ducts.

RESULTS

The size and shape of the vestibular organ changed predictably depending on species-specific (yet not individual) body mass. In some aspects, particularly the cross-sectional area, the ducts were larger than expected for a mammal of a comparable body size. In all species examined, the anterior semicircular ducts exhibited the highest sensitivity.

CONCLUSIONS

The vestibular organs of blind subterranean mole-rats are not regressed morpho-functionally.