Underwater Sound Localization using Internally Coupled Ears (ICE)
* Presenting author
Internally coupled ears (ICE), where an interaural cavity acoustically couples the eardrums, are an anatomical trait present in more than half of all terrestrial vertebrates. The superposition of outside and internal pressure on the two eardrums results in internal instead of interaural time and level differences, which are keys to sound localization. Although ICE is primarily a low-frequency terrestrial adaptation, the African clawed frog Xenopus laevis is a fully aquatic species with a distinct air-filled canal between the ears. In water, the speed of sound is four times that in air. Unlike terrestrial animals with ICE, the Xenopus interaural cavity is also medially connected to the lungs. By modeling the inflated lungs as a Helmholtz resonator, we demonstrate their effect in improving hearing in a low-frequency regime, while simultaneously enhancing sound localization in a disjoint high-frequency regime, corresponding to the frequency ranges of male advertisement calls. In conjunction with its unique plate-like eardrums, we show how Xenopus uses its ICE-like interaural coupling to generate considerable internal level differences between eardrum vibrations and thus overcomes the challenges of underwater sound-localization. Taken together, the two arguments of Helmholtz resonator and plate-like eardrums show the potency of ICE and are interpreted accordingly.