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Mason, M.J., Lai, F.W.S., Li, J.-G. & Nevo, E. (2010) Middle ear structure and bone conduction in Spalax, Eospalax and Tachyoryctes mole-rats (Rodentia: Spalacidae). Journal of Morphology 271: 462-472.
In this paper, we use a combination of micro-CT scanning and traditional dissection techniques to investigate the structure and infer the function of the middle ear apparatus in these related mole-rat species. The ears of these animals proved to be quite similar in appearance, although the zokor Eospalax fontanierii (= Myospalax cansus) had a simpler middle ear cavity structure than that of Tachyoryctes splendens, or any of the four Spalax ehrenbergi allospecies examined. We found that, among other unusual features, both Spalax and Eospalax lack a tensor tympani muscle within their middle ear: they may represent the only rodents in which this is the case.
Contrary to widely-cited reports regarding the function of the Spalax middle ear, we found no clear anatomical evidence to support a bone-conduction pathway from pseudoglenoid fossa via periotic lamina, incus and stapes to the inner ear. Instead, we propose three alternative pathways which might subserve vibrational hearing:
(1) A form of ossicular inertial bone conduction, based on a pivot-point at the short process of the incus and taking advantage of the loose articulations of the ossicles;
(2) Radiation of sound from the head into the external ear canal and transmission via the tympanic membrane and ossicles, a mode of hearing which might be facilitated by the behaviour of pressing the side of the head to the burrow wall ("jaw-listening"), if this occluded the external meatus;
(3) A fluid-based transmission pathway, whereby mandibular vibration sets the cerebrospinal fluid into motion via the postglenoid foramen (found within the pseudoglenoid fossa in Spalax and Eospalax).
These hypotheses remain to be tested, but offer more likely pathways by which these animals might detect the ground-borne vibrations that all three appear to use for intraspecific communication.
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