Table 1.
Details of specimens examined in this project, including CT scan information.
Fig 1.
The external ear of Heterocephalus.
A: Lateral view of a live, adult animal. The position of the external ear, which lacks a pinna, is indicated by an arrow. B: Photomicrographic close-up of the left external ear opening of a different adult specimen, post-mortem. Note that the lumen is semi-occluded with hairs and cerumen. Scale bar for B is 1 mm.
Fig 2.
MicroView reconstruction of the skull of an adult Heterocephalus (65 months).
A: Ventral view of skull. B: Lateral view, the left mandible having been digitally removed. The approximate extent of the left middle ear cavity is indicated in both cases by red shading. Scale bar 10 mm.
Fig 3.
Middle ear structures of Heterocephalus.
A: Photomicrograph of a dissected left ear of an adult Heterocephalus (54 months), medial view. The malleus, incus and tympanic membrane are visible; the stapes and tensor tympani muscle have been removed. Scale bar 1 mm. B: Tomogram section through the left malleus of an adult mole-rat (65 months), posterior view. Note the difference between the compact bone which forms the head and the spongy bone which forms the manubrium. The tensor tympani tendon is also visible. Scale bar 0.5 mm. APSC = ampulla for posterior semicircular canal; ASC = anterior semicircular canal; C = cochlea; DMC = dorsal mastoid cavity; ED = bony tube for endolymphatic duct; ER = epitympanic recess; FP = footplate of stapes; HM = head of malleus; LA = lenticular apophysis of incus; LPI = long process of incus; LSC = lateral semicircular canal; MI = malleoincus; MM = manubrium of malleus; OW = oval window; PC = posterior crus of stapes; PD = bony tube for perilymphatic duct (canaliculus cochleae); PMC = posteromedial mastoid cavity; PSC = posterior semicircular canal; RW = round window; SPI = short process of incus; TC = tympanic cavity; TM = tympanic membrane; TT = tensor tympani tendon; V = vestibule of inner ear; VMC = ventral mastoid cavity.
Fig 4.
The stapes of Heterocephalus, in situ.
A: MicroView reconstruction of the left stapes region in an adult mole-rat (65 months), ventrolateral view. The malleoincudal complex has been coloured blue, the stapes yellow. Note the very wide gap between stapes footplate and the rim of the oval window. This gap and the opening of the round window (both shaded grey) are covered over by membranes, but these do not show up on the CT reconstruction. Scale bar 0.5 mm. B: Histological section of the stapes of a neonatal Heterocephalus, showing the synovial structure of the stapedio-vestibular articulation. The red arrows indicate the synovial joint capsule, which lies between the stapes footplate and the rim of the oval window. Scale bar 0.1 mm. APSC = ampulla for posterior semicircular canal; ASC = anterior semicircular canal; C = cochlea; DMC = dorsal mastoid cavity; ED = bony tube for endolymphatic duct; ER = epitympanic recess; FP = footplate of stapes; HM = head of malleus; LA = lenticular apophysis of incus; LPI = long process of incus; LSC = lateral semicircular canal; MI = malleoincus; MM = manubrium of malleus; OW = oval window; PC = posterior crus of stapes; PD = bony tube for perilymphatic duct (canaliculus cochleae); PMC = posteromedial mastoid cavity; PSC = posterior semicircular canal; RW = round window; SPI = short process of incus; TC = tympanic cavity; TM = tympanic membrane; TT = tensor tympani tendon; V = vestibule of inner ear; VMC = ventral mastoid cavity.
Table 2.
Measurements of middle ear subcavity and bony labyrinth volumes.
Fig 5.
Middle ear cavities of bathyergids.
WinSurf reconstructions of the left middle ear cavities and associated structures in four species of bathyergids (Heterocephalus glaber, Fukomys micklemi, Cryptomys hottentotus and Georychus capensis), from lateral and slightly anterior views. The walls of the middle ear cavities are shown semitranslucent. Positions of the tympanic membranes are indicated by brown shading. The epitympanic recess is colour-coded red, the dorsal mastoid cavity blue and the ventral mastoid cavity green. The ossicles are yellow. The posteromedial mastoid cavity, present only in Heterocephalus, is not visible in this view. The posterior parts of the middle ear cavities of the other species extend into the mastoid region and are equivalent to the ventral mastoid cavity of Heterocephalus, but they lack constricted entrances and so have not been separately coloured. Not to scale. APSC = ampulla for posterior semicircular canal; ASC = anterior semicircular canal; C = cochlea; DMC = dorsal mastoid cavity; ED = bony tube for endolymphatic duct; ER = epitympanic recess; FP = footplate of stapes; HM = head of malleus; LA = lenticular apophysis of incus; LPI = long process of incus; LSC = lateral semicircular canal; MI = malleoincus; MM = manubrium of malleus; OW = oval window; PC = posterior crus of stapes; PD = bony tube for perilymphatic duct (canaliculus cochleae); PMC = posteromedial mastoid cavity; PSC = posterior semicircular canal; RW = round window; SPI = short process of incus; TC = tympanic cavity; TM = tympanic membrane; TT = tensor tympani tendon; V = vestibule of inner ear; VMC = ventral mastoid cavity.
Fig 6.
Bony labyrinths and mastoid cavities of Heterocephalus.
WinSurf reconstructions of the left bony labyrinths and mastoid cavities of three Heterocephalus specimens of different ages, seen from approximately posterior views. In the youngest specimen on the left, coloured arrows indicate the future directions of expansion of the middle ear cavity into the mastoid region. Blue = dorsal mastoid cavity; green = ventral mastoid cavity; orange = posteromedial mastoid cavity. The 38-month-old specimen had smaller mastoid cavities than many younger specimens, so this diagram should not be taken to indicate a strict temporal sequence. APSC = ampulla for posterior semicircular canal; ASC = anterior semicircular canal; C = cochlea; DMC = dorsal mastoid cavity; ED = bony tube for endolymphatic duct; ER = epitympanic recess; FP = footplate of stapes; HM = head of malleus; LA = lenticular apophysis of incus; LPI = long process of incus; LSC = lateral semicircular canal; MI = malleoincus; MM = manubrium of malleus; OW = oval window; PC = posterior crus of stapes; PD = bony tube for perilymphatic duct (canaliculus cochleae); PMC = posteromedial mastoid cavity; PSC = posterior semicircular canal; RW = round window; SPI = short process of incus; TC = tympanic cavity; TM = tympanic membrane; TT = tensor tympani tendon; V = vestibule of inner ear; VMC = ventral mastoid cavity.
Fig 7.
Auditory ossicles of bathyergids.
Left auditory ossicles of bathyergid mole-rats, drawn to scale. The mallei and incudes, which are fused in these animals, are shown from an internal view in each case, the stapedes from a dorsal view. A: Heterocephalus glaber; B: Fukomys micklemi; C: Heliophobius argenteocinereus; D: Bathyergus suillus; E: Georychus capensis; F: Cryptomys hottentotus. Two stapedes are shown for Heterocephalus: the lower one comes from a neonate and has a small intercrural foramen and a cartilaginous head and neck (indicated by a dashed outline). Two stapedes are also shown for Cryptomys, in this case illustrating individual variation in the adult ossicle. Scale bar 2 mm. APSC = ampulla for posterior semicircular canal; ASC = anterior semicircular canal; C = cochlea; DMC = dorsal mastoid cavity; ED = bony tube for endolymphatic duct; ER = epitympanic recess; FP = footplate of stapes; HM = head of malleus; LA = lenticular apophysis of incus; LPI = long process of incus; LSC = lateral semicircular canal; MI = malleoincus; MM = manubrium of malleus; OW = oval window; PC = posterior crus of stapes; PD = bony tube for perilymphatic duct (canaliculus cochleae); PMC = posteromedial mastoid cavity; PSC = posterior semicircular canal; RW = round window; SPI = short process of incus; TC = tympanic cavity; TM = tympanic membrane; TT = tensor tympani tendon; V = vestibule of inner ear; VMC = ventral mastoid cavity.
Fig 8.
Bony labyrinths of three bathyergid species.
WinSurf reconstructions of the left bony labyrinths of Heterocephalus glaber, Cryptomys hottentotus and Fukomys micklemi, seen from approximately lateral (left column), posterior (middle column) and medial (right column) views. Grey shading has been added to indicate the positions of the openings into the labyrinth. The stapedes are shown in yellow. Note that Heterocephalus has fewer cochlear turns than Cryptomys and Fukomys. The lateral semicircular canal of Heterocephalus reaches the vestibule directly, whereas in Fukomys it merges with the ampulla of the posterior canal; the condition in Cryptomys is intermediate. Not to scale. APSC = ampulla for posterior semicircular canal; ASC = anterior semicircular canal; C = cochlea; DMC = dorsal mastoid cavity; ED = bony tube for endolymphatic duct; ER = epitympanic recess; FP = footplate of stapes; HM = head of malleus; LA = lenticular apophysis of incus; LPI = long process of incus; LSC = lateral semicircular canal; MI = malleoincus; MM = manubrium of malleus; OW = oval window; PC = posterior crus of stapes; PD = bony tube for perilymphatic duct (canaliculus cochleae); PMC = posteromedial mastoid cavity; PSC = posterior semicircular canal; RW = round window; SPI = short process of incus; TC = tympanic cavity; TM = tympanic membrane; TT = tensor tympani tendon; V = vestibule of inner ear; VMC = ventral mastoid cavity.
Table 3.
Inner ear measurements, made from CT scan data.
Fig 9.
Relationships between inner ear measurements and body mass.
Relationships between the natural logarithms of inner ear measurements and body mass. Plot A considers the total volume of the bony labyrinth. Plot B considers the length of the cochlear canal. Plots C, D and E consider the radii of curvature of the anterior semicircular canal (SCC), lateral semicircular canal and posterior semicircular canal respectively. Plot F shows the mean cross-sectional diameter of all three semicircular canals. In each plot, data for 28 extant mammalian species (black crosses) were taken from Ekdale (2013), and a least squares regression line fitted as in that paper. Data-points representing mean values for bathyergids were superimposed as red symbols: Heterocephalus individuals over 100 days old (square), Bathyergus (open triangle), Cryptomys (solid triangle), Fukomys (circle), Georychus (diamond) and Heliophobius (plus). Body mass data, where available, were taken from Table 1 for the individual animals concerned. Unrecorded body masses were estimated as follows: Fukomys 89 g, a mean value for animals from the same laboratory colony [78]; Cryptomys 82 g, Georychus 211 g [79]. Inner ear data were taken from Tables 2 and 3.
Table 4.
Predictions of hearing limits in bathyergid mole-rats.