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

Abstract

There is evidence that spalacine, tachyoryctine, and myospalacine mole-rats all communicate with conspecifics through a form of seismic signaling, but the route for the detection of these signals is disputed. It has been proposed that two unusual anatomical adaptations in Spalax allow jaw vibrations to pass to the inner ear via the incus and stapes: a pseudoglenoid (=postglenoid) fossa which accomodates the condylar process of the mandible, and a bony cup, supported by a periotic lamina, through which the incus articulates with the skull. In this study, a combination of dissection and computed tomography was used to examine the ear region in more detail in both Spalax and its subterranean relatives Tachyoryctes and Eospalax, about which much less is known. Tachyoryctes was found to lack a pseudoglenoid fossa, while Eospalax lacks a periotic lamina and bony cup. This shows that these structures need not simultaneously be present for the detection of ground vibrations in mole-rats. Based on the observed anatomy, three hypothetical modes of bone conduction are argued to represent more likely mechanisms through which mole-rats can detect ground vibrations: ossicular inertial bone conduction, a pathway involving sound radiation into the external auditory meatus, and a newly-described fluid pathway between pseudoglenoid fossa and cranial cavity. The caudolateral extension of the tympanic cavity and the presence of a bony cup might represent synapomorphies uniting Spalax and Tachyoryctes, while the loss of the tensor tympani muscle in Spalax and Eospalax may be convergently derived.

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

“…Not only is the stapedio-vestibular articulation synovial in at least some bathyergids (this could not be assessed in all species) but the gap between stapes footplate and oval window is unusually wide (Fig 4A), which is expected to increase compliance still further. A very wide gap has previously been documented in the spalacid mole-rat Spalax [23] and the gerbil Gerbillurus [42]. …”

“…This suggests that, in these respects at least, it is Heterocephalus which shows the derived condition. It is not clear what the ancestral middle ear cavity morphology would be for the Ctenohystrica because this varies so much among different rodent species, even within the same family [23, 34, 42]. Heterocephalus is the smallest bathyergid: although cochlear coiling is unrelated to body size among mammals in general [41], it is possible that within the Bathyergidae the number of turns is somehow restricted by size, and that size affects middle ear cavity structure.…”

“…A laboratory rat, which is not a low-frequency specialist, has a middle ear cavity volume around 60 mm 3 [59]. The spalacid mole-rat Spalax ehrenbergi has a middle ear cavity which is of moderate size (around 100 mm 3 : [23]). The hearing of Spalax is more acute than that of rats at frequencies below around 1 kHz, but the low-frequency hearing of arid-region rodents such as gerbils, kangaroo rats and chinchillas tends to be better still [9, 13, 70].…”

Vocalizations of the giant mole-rat (Fukomys mechowii), a subterranean rodent with the richest vocal repertoire

“…Hearing adaptation is reflected in morphological specializations of the ear structures (cf. Bruns et al 1988;Burda et al 1989Burda et al , 1992Müller et al , 1992Mason 2001;Schleich and Vassallo 2003;Lange et al 2005;Schleich et al 2006;Mason et al 2010). Frequency characteristics of vocalizations in subterranean rodents match the characteristics of hearing and the tunnel acoustics (Heth et al 1986;Pepper et al 1991;Capranica et al 1974;Credner et al 1997;Francescoli 1999;Veitl et al 2000;Schleich and Busch 2002;Knotková et al 2009).…”

Flexibility within the middle ears of vertebrates

“…63,65 The lenticular apophysis is usually attached to the long process of the incus by means of a thin, bony pedicle, which in many species appears to be flexible. 51,63,66 A finiteelement model of the cat incudo-stapedial articulation prompted the conclusion that the pedicle may in fact be more flexible than the articulation between the lenticular apophysis and the stapes itself. 64 Experimental evidence for flexibility within the mammalian middle ear From experiments using a capacitive probe, Møller 67 concluded that the malleus and incus of the cat are rigidly coupled at frequencies up to 2.5 kHz, at 'reasonably low' sound pressure levels.…”