ANALYSIS OF NONLINEAR COCHLEAR MECHANICS IN THE MARSUPIAL MONODELPHIS-DOMESTICA - ANCESTRAL AND MODERN MAMMALIAN FEATURES

The acoustic distortion product 2f1-f2 was measured in the ear canal o f the gray short-tailed opossum, Monodelphis domestica, during stimula tion with two pure tone stimuli of the frequencies f1 and f2 (f1<f2). From 2f1-f2 growth functions for different primary frequencies from 4 to 85 kHz, a distortion threshold curve was obtained describing the re lative thresholds of non-linear mechanical processes in the cochlea. D istortion products could be measured over the entire investigated freq uency range and the distortion audiogram proved sensitive (thresholds <20 dB SPL) over a wide range from 8 to 55 kHz with a broad minimum ar ound 17.5 kHz. Thresholds steeply increased for frequencies below 8 kH z 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-f2 were measured. The shape and tuni ng sharpness of these curves resembled that of modern eutherian mammal s with a shallower low-frequency and a steeper high-frequency flank an d 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 3 0 kHz the ratio curves displayed multiple peaks in periodic intervals. The same peaks were also evident for higher-order distortions (3f1-2f 2, 4fl-3f2). It is discussed to which extent this finding is consisten t with the second filter theory of Brown et al. (1992) and Alien and F ahey (1993) and how in Monodelphis domestica this filter could work di fferently, possibly representing a less evolved stage, than in eutheri an mammals.