Analysis of non-linear cochlear mechanics in the marsupial Monodelphis domestica: ancestral and modern mammalian features.

The acoustic distortion product 2f1-f2 was measured in the ear canal of the gray short-tailed opossum. Monodelphis domestica, during stimulation with two pure tone stimuli of the frequencies f1 and f2 (f1 < f2). From 2f1-2f growth functions for different primary frequencies from 4 to 85 kHz, a distortion threshold curve was obtained describing the relative threshold of non-linear mechanical processes in the cochlea. Distortion products could be measured over the entire investigated frequency range and the distortion audiogram proved sensitive (thresholds < 20 dB SPL) over a wide range from 8 to 55 kHz with a broad minimum around 17.5 kHz. Thresholds steeply increased for frequencies below 8 kHz which can be regarded as an original mammalian feature (Heffner and Masterton, 1980). To assess the tuning characteristics of the cochlea, suppression tuning curves of 2f1-2f were measured. The shape and tuning sharpness of these curves resembled that of modern eutherian mammals with a shallower low-frequency and a steeper high-frequency flank and Q10dB values between 2.4 and 7.12. The optimum primary ratio f2/f1 (best-ratio) was on average 1.172. For f2 frequencies between 20 and 30 kHz the ration curves displayed multiple peaks in periodic intervals. The same peaks were also evident for higher-order distortions (3f1-2f2, 4f1-3f2). It is discussed to which extent this finding is consistent with the second filter theory of Brown et al. (1992) and Allen and Fahey (1993) and how in Monodelphis domestica this filter could work differently, possibly representing a less evolved stage, than in eutherian mammals.