Ear Structures of the Naked Mole-Rat, $\textit{Heterocephalus glaber}$, and Its Relatives (Rodentia: Bathyergidae)

Abstract

Although increasingly popular as a laboratory species, very little is known about the peripheral auditory system of the naked mole-rat, $\textit{Heterocephalus glaber}$. 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 ($\textit{Bathyergus suillus}$, $\textit{Cryptomys hottentotus}$, $\textit{Fukomys micklemi}$, $\textit{Georychus capensis}$ and $\textit{Heliophobius argenteocinereus}$) 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. $\textit{Heterocephalus}$ 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. $\textit{Heterocephalus}$ 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.