This beautiful little frog (Sechellophryne gardineri or Gardiner's Seychelle frog), found only on two small islands in the Seychelles, illustrates a couple of principles of evolution. Being so small - it is the smallest known vertebrate - the normal amphibian ear wouldn't work.
Unlike mammals and birds, most amphibians have their eardrum on the outside instead of down a tunnel. They still have a middle ear and a cochlea as the sensory organ however. S. gardineri has no eardrum and even lacks the small bone which transmits sound to the cochlea. It does have a cochlea though, and the males are especially vocal. Tests have confirmed that they have no problem hearing sounds.
Now Renaud Boistel of the University of Poitiers in France has discovered that S. gardineri hears through its mouth. This is the first vertebrate known to hear with its mouth, but Boistel thinks it could explain how about six percent of frogs manage without a middle ear as do all salamanders and caecilians.
This shouldn't really surprise us because we actually hear our own voice mostly through the bones of our skull, which is why we can hear ourselves speak when we have headphones on or ear-plugs in, although we tend to speak louder to compensate for the slight loss of hearing via the 'normal' route. It also explains why our voice seems different to us when we have a cold and our sinuses are blocked. With S. gardineri this effect has been enhanced by its mouth acting as a resonance chamber.
What S. gardineri illustrates is a couple of important principles:
- An organism's potential for evolution can be constrained by loss of function being detrimental. We can assume that S. gardineri once had normal hearing because so many frogs do compared to the few which don't, that it was probably present in their common ancestor and has since been lost in the six percent without it. It obviously conveyed an advantage on S. gardineri's ancestor to become very small - the reasons for this aren't important and can only be guessed at. However, there was a lower limit to this miniaturization due to loss of hearing - until the mouth was co-opted. This rendered the normal auditory mechanism redundant and allowed it to atrophy whilst the frog was now able to become even smaller with no loss of function. This also illustrated the pragmatic nature of evolution and how, once a barrier to evolution has been overcome, a new evolutionary landscape opens up which evolution can explore. By the simple, unguided process of trial and error, evolution can often find a way round barriers, sometimes in surprising ways.
- Pre-existing structures can be co-opted to new functions (a process known as exaptation). In this case the mouth was exapted to become part of the auditory mechanism. This explains many examples of what creationist pseudo-scientists claim to be examples of irreducible complexity but which are, in reality, examples of exaptation of pre-existing structures and processes to a new function.
Listening through your mouth may seem unnatural, but Gardiner's Seychelles frog is one of many species that use familiar organs for unusual purposes. Organs often evolve to become better at doing one thing, then wind up doing something else entirely. Biologists call this re-purposing exaptation. Other examples are equally bizarre: the giant California sea cucumber can eat through its anus, while the Chinese soft-shelled turtle regularly urinates through its mouth.So a beautiful little gem of a frog from two tiny Indian Ocean islands gives the lie to creationist frauds.
Michael Marshall; Miniature frog can hear with its mouth; New Scientist 02 September 2013
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