Pathways for Sound Transmission to the Inner Ear in Amphibians

Abstract

The role of the tympanic ear in tetrapods is fairly well understood: it has to cope with demands including impedance matching, sound localization, and protection from high sound levels and static pressures. Adaptations in frogs and other tetrapods might well be analogous rather than homologous (Lombard and Bolt 1979), allowing one to examine critically assumptions about what is required of an ear. Indeed, many amphibians lack a tympanic ear altogether yet are still capable of hearing in air. Hetherington and Lindquist (1999) suggest that audition in early tetrapods might have involved the lung. Acoustic communication in frogs commonly relies on airborne sound, but it can also involve aquatic (Yager 1996) or seismic (Lewis and Narins 1985) channels. The mechanisms for sound transfer to the inner ear in these cases are far less clear, possibly involving structures unique to amphibians such as the bronchial columella or the opercularis system. The functions of these structures have still not been conclusively resolved: although the relevant anatomy is in many cases well described and there is a wealth of ingenious proposals for how sound transfer might be achieved, rigorous experimental work that might distinguish between the competing hypotheses is generally lacking. The need for further study is especially acute in the case of caecilians and urodeles. Their small size, poikilothermy, and ability to breathe cutaneously make amphibians ideal subjects for physiological experiments. Many of the unanswered questions about amphibian hearing are therefore eminently tractable, and are, it is hoped, to be resolved in the near future.