Middle ear structure and bone conduction in Spalax, Eospalax, and Tachyoryctes mole-rats (Rodentia: Spalacidae)

Ontogeny of the malleus in Mesocricetus auratus (Mammalia, Rodentia): Systematic and functional implications for the muroid middle ear

The three mammalian auditory ossicles enhance sound transmission from the tympanic membrane to the inner ear. The anterior anchoring of the malleus is one of the key characters for functional classification of the auditory ossicles. Previous studies revealed a medial outgrowth of the mallear anterior process, the processus internus praearticularis, which serves as an anchor for the auditory ossicle chain but has been often missed due to its delicate nature. Here we describe the development and morphology of the malleus and its processus internus praearticularis in the cricetine rodent Mesocricetus auratus, compared to selected muroid species (Cricetus cricetus, Peromyscus maniculatus, and Mus musculus). Early postnatal stages of Mesocricetus show the formation of the malleus by fusion of the prearticular and mallear main body. The processus internus praearticularis forms an increasing broad lamina fused anteriorly to the ectotympanic in adult stages of all studied species. Peromyscus and Mus show a distinct orbicular apophysis that increases inertia of the malleus and therefore these species represent the microtype of auditory ossicles. In contrast, the center of mass of the malleus in the studied Cricetinae is close to the anatomical axis of rotation and their auditory ossicles represent the transitional type. The microtype belongs to the grundplan of Muroidea and is plesiomorphic for Cricetidae, whereas the transitional type evolved several times within Muroidea and represents an apomorphic feature of Cricetinae.

Alone, in the dark: The extraordinary neuroethology of the solitary blind mole rat

On the social scale, the blind mole rat (BMR; Spalax ehrenbergi) is an extreme. It is exceedingly solitary, territorial, and aggressive. BMRs reside underground, in self-excavated tunnels that they rarely leave. They possess specialized sensory systems for social communication and navigation, which allow them to cope with the harsh environmental conditions underground. This review aims to present the blind mole rat as an ideal, novel neuroethological model for studying aggressive and solitary behaviors. We discuss the BMR's unique behavioral phenotype, particularly in the context of 'anti-social' behaviors, and review the available literature regarding its specialized sensory adaptations to the social and physical habitat. To date, the neurobiology of the blind mole rat remains mostly unknown and holds a promising avenue for scientific discovery. Unraveling the neural basis of the BMR's behavior, in comparison to that of social rodents, can shed important light on the underlying mechanisms of psychiatric disorders in humans, in which similar behaviors are displayed.

Ecological correlates of the morphology of the auditory bulla in rodents: Application to the fossil record

For rodents, hearing is essential to survival. It enables predator evasion, prey detection, and conspecific recognition; it is also likely to be constrained by the physical environment. The resulting hypothetical link between tympanic bulla morphology and ecology has never been investigated across a broad array of rodent species before. Such link may enable the determination of the ecological affinities of many fossil species only known from partial skulls. In this study, we used geometric morphometrics to quantify the shape of the auditory bulla of 197 specimens representing 91 species from 17 families of extant rodents across four different locomotory modes. We used landmarks and semi-landmarks on the ventral and lateral views of the skull to capture morphological characteristics of the bulla and external auditory meatus (EAM). Our results demonstrate an association between bullar morphology and locomotion in rodents. Bullar shape enables the correct classification of 76% of the species in our training set. Fossorial taxa, in particular, show a characteristic morphology including an asymmetric bulla with a dorsally located and laterally expanded EAM that has a small opening diameter. A phylogenetically informed flexible discriminant analysis shows a weak phylogenetic effect on tympanic morphology. There is no evidence for differences in bullar hypertrophy across locomotory categories. The application of this approach to select fossil rodents from the Oligo-Miocene shows broad agreements with prior studies and yields new locomotory inferences for 14 fossil species, including the first proposed locomotion for members of the family Florentiamyidae. Such results call for the timing of burrowing diversification in rodents to be reevaluated.

Ear morphology in two root-rat species (genus Tachyoryctes) differing in the degree of fossoriality

It is supposed that the subterranean lifestyle in mammals is reflected in ear morphology and tuning of hearing to low frequencies. We studied two root-rat species to see if their ear morphology reflects the difference in the amount of their surface activity. Whereas the more subterranean Tachyoryctes splendens possesses shorter pinnae as expected, it has smaller bullae compared to the more epigeic Tachyoryctes macrocephalus. The ratio between the eardrum and the stapedial footplate area and the ratio between the mallear and the incudal lever were lower in T. splendens (19.3 ± 0.3 and 1.9 ± 0.0, respectively) than in T. macrocephalus (21.8 ± 0.6 and 2.1 ± 0.1), probably reflecting the latter's higher surface activity. The cochlea in both species has 3.5 coils, yet the basilar membrane is longer in the smaller T. splendens (13.0 ± 0.5 versus 11.4 ± 0.7 mm), which indicates its wider hearing range and/or higher sensitivity (to some frequencies). In both root-rat species, the highest density of outer hair cells (OHC) was in the apical part of the cochlea, while the highest density of inner hair cells (IHC) was in its middle part. This OHC density pattern corresponds with good low-frequency hearing, whereas the IHC pattern suggests sensitivity to higher frequencies.

Hearing and Vocalizations in the Naked Mole-Rat

Since their discovery, naked mole-rats have been speaking to us. Early field studies noted their extensive vocalizations, and scientists who are fortunate enough to spend time with these creatures in the laboratory setting cannot help but notice their constant peeping, chirruping and grunting (Hill et al., Proc Zool Soc Lond 128:455-514, 1957). Yet, few dwell on the function of these chirps and peeps, being instead drawn to the many other extraordinary aspects of naked mole-rat physiology detailed throughout this book. Still, no biology is complete without a description of how an organism communicates. While the field of naked mole-rat bioacoustics and acoustic communication has been largely silent for many years, we highlight recent progress in understanding how and what Heterocephalus glaber hears and which vocalizations it uses. These efforts are essential for a complete understanding of naked mole-rat cooperation, society and even culture.

The middle ear of the pink fairy armadillo Chlamyphorus truncatus (Xenarthra, Cingulata, Chlamyphoridae): comparison with armadillo relatives using computed tomography

The pink fairy armadillo Chlamyphorus truncatus is the smallest extant armadillo and one of the least-known fossorial mammals. The aim of this study was to establish if its middle ear is specially adapted to the subterranean environment, through comparison with more epigeic relatives of the groups Euphractinae (Chaetophractus villosus, Chaetophractus vellerosus, Zaedyus pichiy) and Dasypodinae (Dasypus hybridus). We examined the middle ears using micro-computed tomography and subsequent three-dimensional reconstructions. D. hybridus has a relatively small middle ear cavity, an incomplete bulla and 'ancestral' ossicular morphology. The other species, including Chlamyphorus, have fully ossified bullae and middle ear ossicles, with a morphology between 'transitional' and 'freely mobile', but in all armadillos the malleus retains a long anterior process. Unusual features of armadillo ears include the lack of a pedicellate lenticular apophysis and the presence, in some species, of an element of Paaw within the stapedius muscle. In common with many subterranean mammals, Chlamyphorus has a relatively flattened malleo-incudal articulation and appears to lack a functional tensor tympani muscle. Its middle ear cavity is not unusually enlarged, and its middle ear ossicles seem less robust than those of the other armadillos studied. In comparison with the euphractines, there is no reason to believe that the middle ear of this species is specially adapted to the subterranean environment; some aspects may even be indicative of degeneration. The screaming hairy armadillo, Chaetophractus vellerosus, has the most voluminous middle ear in both relative and absolute terms. Its hypertrophied middle ear cavity likely represents an adaptation to low-frequency hearing in arid rather than subterranean conditions.

The burrow behavior and influenced factors of a prairie subterranean zokor (Myospalax psilurus)

The Transbaikal zokor (Myospalax psilurus) is a dominant rodent distributed in the meadow steppe of Inner Mongolia in northern China. Due to long history of evolution in subterranean environment, the zokor has an adaptive behavior: sealing burrow entrances. When a burrow is damaged, exposed entrances appear, and within a relatively short time, the zokor would be active in sealing the entrances to reduce risks to its survival. In general, it is thought that zokors avoid light and wind, which is consistent with their behavior of sealing burrow entrances. However, direct evidence from field experimental research has been lacking. This study set up 68 field sampling points in a meadow steppe in Inner Mongolia from August to September, 2014 and used a wind-light isolator to study the effects of wind and light factors on zokor burrow entrance sealing behavior. The results showed that there were no significant correlations between wind or light factors and the frequency of zokor burrow entrance sealing. Therefore, wind and light factors are not direct factors associated with zokors actively sealing burrow entrances.

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.

Structure and function of the mammalian middle ear. I: Large middle ears in small desert mammals

Many species of small desert mammals are known to have expanded auditory bullae. The ears of gerbils and heteromyids have been well described, but much less is known about the middle ear anatomy of other desert mammals. In this study, the middle ears of three gerbils (Meriones, Desmodillus and Gerbillurus), two jerboas (Jaculus) and two sengis (elephant-shrews: Macroscelides and Elephantulus) were examined and compared, using micro-computed tomography and light microscopy. Middle ear cavity expansion has occurred in members of all three groups, apparently in association with an essentially 'freely mobile' ossicular morphology and the development of bony tubes for the middle ear arteries. Cavity expansion can occur in different ways, resulting in different subcavity patterns even between different species of gerbils. Having enlarged middle ear cavities aids low-frequency audition, and several adaptive advantages of low-frequency hearing to small desert mammals have been proposed. However, while Macroscelides was found here to have middle ear cavities so large that together they exceed brain volume, the bullae of Elephantulus are considerably smaller. Why middle ear cavities are enlarged in some desert species but not others remains unclear, but it may relate to microhabitat.

Bone-conduction hearing and seismic sensitivity of the Late Permian anomodont Kawingasaurus fossilis

An investigation of the internal cranial anatomy of the anomodont Kawingasaurus from the Upper Permian Usili Formation in Tanzania by means of neutron tomography revealed an unusual inner and middle ear anatomy such as extraordinarily inflated vestibules, lateroventrally orientated stapes with large footplates, and a small angle between the planes of the anterior and lateral semicircular canals. The vestibule has a volume, which is about 25 times larger than the human vestibule, although Kawingasaurus has only a skull length of approximately 40 mm. Vestibule inflation and enlarged stapes footplates are thought to be functionally correlated with bone-conduction hearing; both morphologies have been observed in fossorial vertebrates using seismic signals for communication. The firmly fused triangular head with spatulate snout was probably used for digging and preadapted to seismic signal detection. The quadrate-quadratojugal complex was able to transmit sound from the articular to the stapes by small vibrations of the quadrate process, which formed a ball and socket joint with the squamosal. Mechanical considerations suggest that the ventrolaterally orientated stapes of Kawingasaurus was mechanically better suited to transmit seismic sound from the ground to the fenestra vestibuli than a horizontal orientated stapes. The low sound pressure level transformer ratio of 2-3 in Kawingasaurus points to a seismic sensitivity of the middle ear and a vestigial or reduced sensitivity to airborne sound. Three hypothetical pathways of bone conduction in Kawingasaurus are discussed: 1) sound transmission via the spatulate snout and skull roof to the otic capsules, 2) relative movements resulting from the inertia of the mandible if sound is percepted with the skull, and 3) bone conduction from the substrate via mandible, jaw articulation, and stapes to the inner ear.