Mostly Malleus: Ground Sound Detection by the Golden Mole

Golden moles are nocturnal, surface-foraging mammals with rudimentary vision, several of which possess massively hypertrophied mallei that presumably confer low-frequency, substrate-vibration sensitivity through inertial bone conduction.  Based on middle ear anatomy, the ossicular mass distribution and the anchorage points, we have hypothesized that there are several degrees of freedom of the middle ear apparatus of C. asiatica. A horizontal vibration, which drives the head sideways will excite the rotational mode wy, whereas a vertical vibration excites mode wz. We suggest that these two modes play the main role in inertial bone conduction in response to seismic stimuli, since they depend on both the increased mallear mass and the displacement of its center of mass from the rotational axis.  In addition to these two modes, we postulate that there is a third vibrational mode, namely the rotation of the ossicular chain about the long axis of the malleus (wx) in response to airborne stimuli. Laser Doppler vibrometric measurements of the malleus head in response to seismic stimuli in Chrysochloris reveals peak sensitivity to frequencies below 300 Hz.  Functionally, they appear to be low-frequency specialists, and it is likely that golden moles hear principally through substrate conduction. The possibility of using these animals to detect precursors of earthquakes will be discussed.  Supported by NIH grant no. DC00222 to PMN.