Peripheral auditory function in the platypus, Ornithorhynchus anatinus

Abstract

Cochlear microphonic (CM) potentials were recorded from the round window of the monotreme Ornithorhynchus anatinus (platypus) in response to pure tones between 500 Hz and 20 kHz. A 1.0-μV threshold curve showed best sensitivity for the platypus at 5 kHz with a high-frequency roll-off of approximately 20 dB/octave and a low-frequency rolloff of approximately 15 dB/octave. Intensity functions obtained at 2, 6, and 10 kHz were essentially parallel over the range of intensities studied. The maximum cochlear microphonic amplitude, even at the most sensitive frequency, did not exceed 100μV. The intensity function obtained at 6 kHz, close to the best frequency of the animal, was linear over a 40-dB range, and passed through a maximum at approximately 90 dB SPL. Click responses showed a cochlear microphonic potential followed by two neural potentials analogous to the N1, N2 frequently reported in other species. This study lends support to the notion that the auditory system of the platypus is intermediate between that of reptiles and mammals.

Evolutionary Paths to Mammalian Cochleae

“…Fortunately, there are studies of hearing in these modern monotreme representatives (Aitkin and Johnstone 1972;Gates et al 1974;Ladhams and Pickles 1996;Mills and Shepherd 2001) and from them, we can learn much more about the soft-tissue evolution of the mammalian cochlea and its function. In one respect, the monotreme hearing organ resembles that of multituberculates, being not coiled and relatively short (4.4 to 7.6 mm in length, Ladhams and Pickles 1996).…”

“…The soft tissues do not conform fully to the shape of the bony canal and are, near the tip, slightly coiled in the opposite direction to the therian (placental and marsupial) cochlea. Modern monotremes have a peculiar middle ear that has been described as being very stiff (Aitkin and Johnstone 1972;Gates et al 1974). Hearing in monotremes is, for mammals of their size, restricted to lower and middle frequencies.…”

“…Hearing in monotremes is, for mammals of their size, restricted to lower and middle frequencies. Audiograms were described as V-shaped, centered at 5 kHz (Gates et al 1974) or Ushaped, with rapid loss of sensitivity below 3 kHz and above about 16 kHz (Mills and Shepherd 2001).…”

“…Indeed, the cochlea of most birds is twice as long as this (5 mm) and their upper frequency limit is from 6 to 10 kHz. Modern monotremes, with a cochlea up to 8 mm long, still have an upper frequency limit below 20 kHz (Gates et al 1974;Mills and Shepherd 2001). The fact that some modern therian mammals with relatively short cochleae (mouse, 6-7 mm) hear very high frequencies (Rosowski 1992) is not very relevant, since their middle-and inner ear systems are the result of a further 200 Ma of evolution.…”

Flexibility within the middle ears of vertebrates

“…The malleus and incus are said to be fused in the echidna and adult platypus too, 56,61 although in the case of the latter this is disputed. 62 There are contradictory reports of malleoincudal fusion in various other species of mammals, which remain to be confirmed; some of these claims The anatomy of the incudo-stapedial articulation, formed between the lenticular apophysis of the incus and the head of the stapes, has been best described in humans and cats. [63][64][65] This articulation is synovial in mammals, with the possible exception of some bat species 50 and the platypus.…”