How to hear underground

For communication and orientation in darkness, acoustic signals seem to be predestined, as exemplified by bats and dolphins. However, high sound frequencies (characterized by short wavelength), which can be well localized by small mammals and which are employed in echolocation, are quickly dampened underground. Low frequency sound, on the other hand, is characterized by long waves and cannot be localized easily. Indeed, it was demonstrated that, in a burrow of the blind mole-rat, sounds with a frequency of about 500 Hz were more efficiently transmitted than sounds of low and higher frequencies. Although nothing is known so far about the effect of the tunnel diameter, soil characteristics, temperature, and humidity, it is assumed that acoustic features of all burrows are rather similar. Consistent with the results of these measurements, the vocalization of all nine species (representing six genera) of subterranean rodents studied so far was also tuned to a lower frequency range. Corresponding with acoustics of the environment and with vocalization characteristics, the hearing of five species (of five genera) of subterranean mammals studied exhibited its highest sensitivity in the lower frequency range (0.5-2 kHz). This is quite unusual among small mammals, because hearing and vocalization in related surface dwellers of a comparable body size are usually much higher: about 8-16 kHz, or higher. The hearing range of subterranean mammals is very narrow, and frequencies of about 16 kHz and higher cannot be perceived (similar to humans). Frequency of 500 Hz is characterized by a wavelength of 27 in (68.6 cm). To be able to localize this frequency (to which hearing is tuned), an animal would need to have a head of a corresponding width; this is not possible. However, the transmission of airborne sounds in a tunnel is unidirectional, anyway. Consequently, animals that are confined to their un-

The European badger (Meles meles) builds onto underground tunnels, or setts, inherited from previous generations which results in setts that can be centuries old. (Photo by Roger Wilmshurst/Photo Researchers, Inc. Reproduced by permission.)

derground burrows do not need auricles for directional hearing. Some scholars would tend to label this restricted hearing as degenerated. However, looking at the morphology of the middle and inner ear, many progressive structural specializations enabling tuning of hearing to the given lower frequency range are observed. Indeed, several papers have described diverse morphological features of the middle and inner ear, as well as the expansion of auditory brain centers, which can be found consistently in non-related species of underground mammals and provide an example of convergent evolution. However, while the ear is clearly a low-frequency-tuned receiver, apparently the evolution has not fully utilized all the possibilities, as demonstrated in ears of desert animals, to enhance the sensitivity. On the contrary, some features of the external ear canal, eardrum, and middle ear ossicular chain indicate that sensitivity has been secondarily and actively reduced. Too little is known about the acoustics of burrows, and also the suggestion of Quilliam, 40 years ago, that sound in burrows can be amplified as in an ear-trumpet, has not yet been tested. Should there really be such a stethoscope effect, reduction of sensitivity (in order to avoid deafening) would have to be considered an adaptation in the same way as its increase is in other species.

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