The Auditory Morphology of the Tungara Frog

Frogs have evolved thin eardrums and auditory ossicles to match the low impedance of air with the high impedance of body tissues and allow communication through airborne sound. Certain species of frogs live their adult lives and communicate underwater. Studies have shown that these species possess relatively thick eardrums with a cartilaginous disc immediately deep to them, which could potentially be adaptations for underwater hearing. When male tungara frogs call to attract females for mating, female tungara frogs approach the call initially in air and then in water. Such behavior led to the hypothesis that the auditory morphology of tungara frogs also contain specializations. To test this hypothesis, we used photographic dissections, resin histology and microCT images to screen the structure of the middle ear of the tungara frog. We observed that the eardrum is visually indistinguishable from the skin of the rest of the body, that the eardrum is relatively thick, and that the extrastapes is very broad at its attachment to the eardrum, forming a cartilaginous disc. These features more closely resemble the ear structures of underwater specialists than the ear structures of terrestrial specialists. The observations are evidence that underwater hearing is of high importance to tungara frogs and strengthen the evidence that a cartilaginous disc in the middle ear is an adaptation for underwater hearing. Since the tungara frog can hear both in air and in water, its auditory morphology can help reveal the design features that are key for effective hearing in each medium