Page 67 - Summer2020

However, their swimming skill isn’t the point of interest here. It is their peculiar style of foraging, a style enabled, presumably, by their conspicuously hypertrophied mallei. Fiercely territorial, individuals scurry about on the sur- face of the erg, intermittently stopping to dip their snout and small, conically shaped head beneath the surface. They are, some have suggested, listening for the low- frequency, soilborne seismic signature that might lead them to their favorite prey, the subterranean dune ter- mite (Fielden et al., 1990).
Biologists have, it turns out, settled on a likely expla- nation of just how the head-dipping behavior of the Namib golden mole might enable its hunting prow- ess. The hypothetical but probable answer is that some golden mole species detect seismic events by tightly coupling their heads to the substrate, taking advantage of inertial bone conduction. The low-frequency seismic waves propagating through the sand of the erg cause the bones of the skull to vibrate in unison. Movement of the loosely coupled ossicles lags behind that of the skull because of inertia, producing relative motion between the stapes footplate and the oval window of the cochlea and transferring energy to the inner ear (Bárány, 1938). We should point out, however, that compression of the bony cochlear wall and/or inner ear fluid inertia also play a role in bone conduction in at least some mammals.
Regardless, the enlarged mallei of some Chrysochlo- rinae species enhance sensitivity to bone conduction, presumably, but almost certainly, permitting the detec- tion of low-amplitude, low-frequency ground vibrations (Narins et al., 1997; Mason 2003). A second ossicular adaptation contributing to and enhancing sensitivity to seismic events is the displacement of the center of mass of the ossicular chain away from its natural rotational axis. The relocation of the center of mass further ampli- fies ossicular motion relative to the skull and, ostensibly, augments the sensitivity of the system to low-frequency seismic signals, unlike that predicted for golden moles with smaller mallei and a center of mass that falls close to the natural rotational axis. In a remarkable tilt to the amazing power of selection-driven adaptation, it can be confidently argued that the head-dipping behavior of the Namib golden mole permits the detection of faint seis- mic signals produced by the wind-driven motion of dune grass mounds scattered about their territories.
Golden Mole Hearing: Can They?
Although we cannot claim to know what Namib golden moles or any other member of the Chrysochlorinae sub- family actually hear, predictions derived from Bárány’s
1938 model of inertial bone conduction suggest that they do. Using morphological measurements of key middle ear structures and calculating relevant middle ear parameters required by this model, Mason (2003) predicted the fre- quency producing peak displacement, the strongest driving force delivered by the stapes to the fluids of the inner ear, at 300 Hz in Grant’s golden mole (Eremitalpa granti granti), a close relative of the Namib golden mole. This frequency corresponds closely to the peak frequency of seismic signals generated by the grassy mounds of the Namib erg (Narins et al., 1997). In addition, the predicted resonant frequency of bone conduction in the Cape golden mole, Chrysochloris asiatica, is 220 Hz (Mason, 2003), a value nearly matching resonant frequencies of 100-200 Hz determined by direct measurements of ossicular velocity in response to vibra- tional stimuli (Willi et al., 2006a,b). Depending on the accuracy of these predictions, these findings point con- fidently to the conclusion that the middle ears of golden moles were almost certainly adapted to detect soilborne seis- mic events, a prediction reinforced by the directed foraging behavior of the Namib golden mole (Lewis et al., 2006).
Further support for the view that golden moles are able to hear can be found in predictions of the high-frequency limit of hearing based on a widely used model of the middle ear (Hemilä et al., 1995). Using this model, Mason (2001) computed upper limits of 5.9 kHz and 13.7 kHz for Grant’s golden mole and the Cape golden mole, respectively. In addition, direct observation of frequency-dependent altera- tions in the mode of ossicular vibration permit, theoretically, uncompromised detection of airborne stimuli in the Cape golden mole (Willi et al., 2006b). The evolutionarily modi- fied middle ear of some golden moles has been described by some as nothing short of ingenious.
We end this section by pointing out the obvious. Although the role of the middle ear of the golden mole
family has been the topic of considerable, highly pro- ductive inquiry, a complete accounting of golden mole hearing will require a comprehensive investigation using behavioral techniques or at the systems level of physiol- ogy before a clear understanding of the auditory capacity of these fascinating mammals is available.
Summer 2020 • Acoustics Today 67