Time without sounds for tadpoles
Tadpoles undergo a complete metamorphosis on their way to becoming an adult frog. This does not only apply to physical development. The sensory organs and the possibilities of sensory stimulus reception also change. There is even a phase during metamorphosis when a tadpole is no longer able to perceive acoustic stimuli because of these changes. During the metamorphosis from tadpole to frog, extreme transformations take place overall, but these happen step by step in order to keep the changes as unrisky as possible. For example, even as the tadpole grows legs, it is still able to swim with its caudal fin. The gills are only lost when the lungs are fully functional.
But the sensory organs and their receptivity also have to be adapted to changed living conditions. In the course of metamorphosis, the tadpole loses the ability to perceive vibrations in the water. Such a sense is not needed on land. The angle between the eyes is reduced to allow a better view ahead. And hearing is also changing, since there is a great difference between sound being transmitted through water and air.
Seth Boatright-Horowitz and Andrea Megela Simmons of Brown University in Providence describe the evolution of the frog's auditory system in the Proceedings of the National Academy of Sciences (December 23, 1997 issue). Before the outer eardrum forms, the tadpole can hear fully. The sounds seem to penetrate the head and are likely picked up directly from the inner parts of the ear. At the end of their metamorphosis into country dwellers, they hear just as well.
They develop an external tympanic membrane around the time the forelimbs are also developing. But just during this period - while the legs are growing - the tadpoles seem to be completely deaf. The scientists did not find any activity in the part of the brain related to sound processing. The auditory system undergoes a complete reorganization, and the frogs must relearn how to locate and recognize sounds. The researchers suggest that this may avoid confusion that would otherwise result from sounds behaving so completely differently in the air.
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