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| Science Forum Index » Bio Evolution Forum » The Middle Ear Bones of Cetaceans (Dolphins and... |
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 Posted: Sun Oct 18, 2009 5:30 pm |
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1) Why should evolution favor the preservation of middle ear bones in
cetaceans (i.e., dolphins, whales, propoises)?
2) How has the middle ear bones of cetaceans evolved relative to
land mammals.
3) Are the middle ear bones of cetaceans (dolphins, porpoises and
whales) necessary for hearing underwater?
All living mammals have three bones of the middle air (anvil,
mallet and stirrups). These three bones are homologous to certain
bones in the jaw of fish. Reptiles and birds have one of these three
bones to help with hearing. Fish do not have these middle ear bones
for hearing.
However, fish don't need these middle ear bones. The water that
surrounds them allows sound to pass unimpeded to their otic capsules.
Otic capsules are homologous to the cochlea of mammals, and have not
changed signficantly since the Ordivician period. Since the skin
itself matches the impedance of the otic fluid, no special bones are
needed to get the sound into the otic capsule.
The middle ear bones for mammals greatly enhance the For land
mammals, the interface between air and blood vessels reflects most
sound. So the middle ear bones serve to match the impedance of the
cochlear fluid, the middle ear bones greatly increase the transmission
of sound.
It would seem to me that natural selection for middle ear bones
is greatly decreased for aquatic mammals. Cetaceans live in the same
environment as fish. So sound should propagate into their cochlea much
better even without middle ear bones. It would seem to me that the
middle ear bones of cetaceans would be degenerate, or have some other
modification, relative to the middle ear bones of land animals.
However, cetaceans have very prominent middle ear bones. So it seems
to me that my initial assumptions are wrong, somewhere. But how? |
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