On the frog amphibian papilla.
Comparative morphological studies in living species of frogs and toads (anurans) indicate that the amphibian papilla (a specialized auditory sensory organ of the amphibian inner ear) has undergone considerable elongation over the course of evolution. Physiological studies on the amphibian papilla of one recently-derived species (Rana catesbeiana) provide strong evidence that frequency selectivity (tuning) in that organ is accomplished by one or more traveling wave structures, in many ways analogous to that of the mammalian cochlea. I suggest that the process of elongation inferred from the comparative morphology is a direct reflection of the evolution of a single traveling wave structure (i.e., convergence on an auditory mechanism analogous to that in the mammal). One selective advantage of such a mechanism is the combination of high rejection ratio in frequency filtering and good temporal resolution of acoustical stimuli.