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Middle-ear function in a monotreme: the Echidna (Tachyglossus aculeatus).

The mechanics of the middle ear bones of echidna (Tachyglossus aculeatus) were studied using the Mossbauer technique. A small radioactive source was placed on the stapes and the alterations in the frequency of the emitted γ-ray, which occurred when the stapes vibrated during sound stimulation, were detected by a proportional counting system and absorber. In this way the velocity of stapes motion could be measured in response to different acoustic frequencies and intensities. 
 
 
 
The ossicular system of echidna consists of a large malleus with an anteromedially directed process, the processus gracilus; an incus which is ankylosed to the malleus, and a columelliform stapes. The tympanic bone supports the eardrum and appears firmly locked to the processus gracilus. The malleus and incus are tightly attached, at their lateralmost corner, to the periotic bone. This latter attachment causes the middle ear chain to be very stiff. 
 
 
 
The velocity of stapes motion at 100 dB SPL is very low (0.0014 mm/sec) at 100 Hz and rises to a peak at 6 kHz (0.1 mm/sec). Velocity decreases at approximately 12 dB/octave as frequency is increased beyond 6 kHz. Amplitude of stapes motion is relatively constant at about 0.002–0.004 μ for 100 dB SPL between 100 Hz and 6 kHz and drops off at 18 dB/octave at higher frequencies. 
 
 
 
Comparison of these results with those for some lizards and placental mammals reveals that the echinda middle ear bones are considerably less efficient in transmitting airborne vibrations than those of the other animals.