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Van Dijk, P., Mason, M.J. & Narins, P.M. (2002) Distortion product otoacoustic emissions in frogs: correlation with middle and inner ear properties. Hearing Research 173: 100-108.
When you present vertebrate inner ears simultaneously with two pure tones at certain frequencies, and place a sensitive microphone outside the ear, you can often record additional tones at new frequencies which appear to be generated by the ear itself. These are known as "distortion product otoacoustic emissions", or DPOAEs. In the present study, DPOAEs were used to see which frog species possess an emission-generating mechanism, and, by comparing species with diverse ear structures, whether the presence or absence of emissions is related more to middle ear or to inner ear mechanisms. Laser interferometric recordings of tympanic membrane velocity, or the velocity of the skin in the otic region (for those species lacking tympanic middle ears), were used to assess relative sensitivity of the middle ears of these frogs to airborne sound.
We recorded DPOAEs from Rana pipiens and Xenopus laevis, but not from Scaphiopus couchii or Bombina orientalis. This shows that a tympanic ear is neither necessary nor sufficient for DPOAE recordings (because Xenopus lacks a tympanic ear and has emissions, whereas Scaphiopus has a tympanic ear but lacks emissions), and that a well-developed caudal extension to the amphibian papilla of the inner ear, as found in Rana, is not necessary either (because Xenopus lacks such an extension). These findings result in the rejection of both of the previous hypotheses from the literature, which had been ventured to explain the presence or absence of OAEs in frogs! There may instead be a correlation with inner ear sensitivity.
The nature of DPOAEs in frogs was investigated further by Van Dijk et al. (2003).
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