Massive Mallei in Moles: Middle Ear Adaptations Subserving Seismic Sensitivity

It has been popularly assumed, at least since the time of Pliny the Elder [1], that the hearing of moles and other subterranean mammals is acute, perhaps to compensate for their poor vision. In recent times, behavioural audiograms of subterranean species (most studies have been of rodents) have shown that audition is biased towards unusually low frequencies for such small mammals, typically being restricted to under about 20 kHz at 60 dB SPL [2, 3, 4, 5]. Low frequencies of a few hundred Hertz have been found to travel further than higher frequencies underground [6], so efficient low-frequency hearing has been regarded as adaptively advantageous [7, 8, 9]. Several features of the middle ear apparatus identified in subterranean mammals, such as enlarged eardrums, enlarged bullar cavities and “freely mobile” ossicles, have accordingly been interpreted as adaptations to improve low-frequency hearing [9, 10, 11]. By contrast, other authors have regarded the hearing of subterranean mammals as “degenerate” or “vestigial”: restricted to low frequencies through loss of high frequency hearing, but not unusually acute at any frequency [2, 3, 4].