2. Cetacean Ears
75
important for cetacean middle ears are: (1) Is there an acoustically superior channel? (2) If so, to what does it connect?
There are currently two competing theories. Both are problematic. One
is that bulk motion of the head sets the inner ear in motion. This implies
stimuli are delivered simultaneously to each ear, but this means sound localization verges on impossible. The second theory is that body tissues (predominately bone) conduct sound directly to the ear producing differential
movement but at very small amplitude of the cochlear windows. Support
for one theory comes from experiments by McCormick et al. (1970, 1980)
with anesthetized T. truncatus and Lagenorhynchus obliquidens (Pacific
white-sided dolphin), in which immobilizing the ossicular chain decreased
cochlear potentials by 18 dB, but disrupting the external canal, tympanic
conus, and malleus had little (4dB) or no effect. They concluded sound
entering from the mandible by bone conduction produces a "relative
motion" between the stapes and the cochlear capsule. Fleischer (1978) disagreed. He suggested the surgical procedure damaged the normal ossicular
mechanism. From anatomical studies of preserved material he had concluded that sound from any tissue path is translated through tympanic
vibrations to the ossicles which then pulse the oval window. McCormick et
al.'s theory depends upon tissue conduction and an inertial lag of the
cochlear fluids in a vibrating bulla and assumes fixed or fused tympanoperiotic joints. Fleischer's theory depends upon differential resonance of
the tympanic and periotic bones, a freely mobile stapes, and flexible
tympano-periotic sutures. In fact, neither theory is consistent with the range
of known middle ear variations among cetacean species.
Middle ear air volumes are another topic of debate. The tympanic space
defined by the bullar cavity is relatively large in all cetaceans but bony
walled bullar volume may not be the relevant middle ear space for a whale.
To understand functional middle ear space, soft tissue influences must be
considered.
The middle ear cavity in both odontocetes and mysticetes is lined with a
thick, vascularized mucosa, the corpus cavernosum (Fig. 2.4). This is a distensible tissue, capable of filling the tympanic chamber, but it does not necessarily preclude air in the middle ear cavity. Computerized tomographic
images of live animals show that animals in air at sea level have substantial and equal volumes of air in the middle ear cavities although the corpus
cavernosum is not totally relaxed (Ketten 1998b). What we do not know is
whether there is normally air in the middle ear of any submerged or diving
marine mammal.
Changing middle ear volumes are generally undesirable auditorially. For
diving mammals there are two options: (1) the volumes are somehow maintained, or (2) the volumes are somehow irrelevant. A recent experiment
with D. leucas (the beluga whale) (Ridgway and Carder 1997) found that
although the whale's whistle spectra changed with depth, the hearing
thresholds did not. Their conclusion was that "sound is conducted through
75
important for cetacean middle ears are: (1) Is there an acoustically superior channel? (2) If so, to what does it connect?
There are currently two competing theories. Both are problematic. One
is that bulk motion of the head sets the inner ear in motion. This implies
stimuli are delivered simultaneously to each ear, but this means sound localization verges on impossible. The second theory is that body tissues (predominately bone) conduct sound directly to the ear producing differential
movement but at very small amplitude of the cochlear windows. Support
for one theory comes from experiments by McCormick et al. (1970, 1980)
with anesthetized T. truncatus and Lagenorhynchus obliquidens (Pacific
white-sided dolphin), in which immobilizing the ossicular chain decreased
cochlear potentials by 18 dB, but disrupting the external canal, tympanic
conus, and malleus had little (4dB) or no effect. They concluded sound
entering from the mandible by bone conduction produces a "relative
motion" between the stapes and the cochlear capsule. Fleischer (1978) disagreed. He suggested the surgical procedure damaged the normal ossicular
mechanism. From anatomical studies of preserved material he had concluded that sound from any tissue path is translated through tympanic
vibrations to the ossicles which then pulse the oval window. McCormick et
al.'s theory depends upon tissue conduction and an inertial lag of the
cochlear fluids in a vibrating bulla and assumes fixed or fused tympanoperiotic joints. Fleischer's theory depends upon differential resonance of
the tympanic and periotic bones, a freely mobile stapes, and flexible
tympano-periotic sutures. In fact, neither theory is consistent with the range
of known middle ear variations among cetacean species.
Middle ear air volumes are another topic of debate. The tympanic space
defined by the bullar cavity is relatively large in all cetaceans but bony
walled bullar volume may not be the relevant middle ear space for a whale.
To understand functional middle ear space, soft tissue influences must be
considered.
The middle ear cavity in both odontocetes and mysticetes is lined with a
thick, vascularized mucosa, the corpus cavernosum (Fig. 2.4). This is a distensible tissue, capable of filling the tympanic chamber, but it does not necessarily preclude air in the middle ear cavity. Computerized tomographic
images of live animals show that animals in air at sea level have substantial and equal volumes of air in the middle ear cavities although the corpus
cavernosum is not totally relaxed (Ketten 1998b). What we do not know is
whether there is normally air in the middle ear of any submerged or diving
marine mammal.
Changing middle ear volumes are generally undesirable auditorially. For
diving mammals there are two options: (1) the volumes are somehow maintained, or (2) the volumes are somehow irrelevant. A recent experiment
with D. leucas (the beluga whale) (Ridgway and Carder 1997) found that
although the whale's whistle spectra changed with depth, the hearing
thresholds did not. Their conclusion was that "sound is conducted through