2. Cetacean Ears
101
turns range 1.5 to 2.5 and are independent of animal size. Odontocete
cochlear duct structures are hypercellular. Stria vascularis and spiral ligament in particular are densely packed with duplicate cell populations, which
suggest relatively rapid metabolic processes that are consistent with the
importance of hearing to cetaceans and with moderately high background
noise in ocean environments. Auxiliary outer osseous laminae support 20%
to 60% of the basilar membrane length in odontocetes, adding stiffness.
In mysticetes, the spiral ligament is less well developed and outer osseous
laminae are absent or reduced. The cochlear duct cytoarchitecture of mysticetes is unremarkable. Mysticete basilar membranes scale consistently
with land mammal generalist ears.
Spiral ganglion cell densities are significantly greater in whales than in
land mammals, averaging 2,000 to 4,000 cells/mm. Greatest densities are
found in the highest frequency odontocetes, but all whales have densities
and fiber diameters that are significantly greater than those of land
mammals. Vestibular elements are disproportionately small in all whales,
possibly reflecting reduced azimuthal cues as a result of cervical fusion and
limited head motion.
Modern Cetacea have three inner ear structural formats that coincide
with acoustic groups: low to infrasonic Type M mysticetes, upper range
ultrasonic Type I odontocetes, and lower range ultrasonic Type II odontocetes. Type I and Type II cochleae are adapted for ultrasonic ranges with
exceptionally stiff basilar membranes and extensive bony membrane buttressing. Basilar membrane thickness to width ratios are higher for the basal
turn of Type I odontocetes than for any other mammal. Mysticete (Type M)
cochlea have exceptionally wide, thin basilar membranes and no stiffening
agents, implying they are adapted to low to infrasonic frequencies.
The debate on middle ear function desperately needs to be invigorated
with measures from more than one species, particularly if a general solution is to be obtained for odontocetes and mysticetes. Middle ear anatomies
are sufficiently different between odontocetes and mysticetes, particularly
with respect to couplings to other head tissues that it seems unlikely that
a common mechanism is at work. Therefore, new data must come from
both groups.
Data on mysticete ears continue to be relatively scarce, but what is available suggests they are adapted for sonic to infrasonic frequencies, which is
consistent with mysticete vocalization data.
Psychophysical research on odontocetes ranks among the best available
in the world, but at the moment, cetaceans do not afford the same controlled research opportunities, particularly direct physiologic measures, that
are possible in other species. In that sense, cetacean auditory research is not
physiologically competitive. However, as techniques improve and become
more accessible for high-speed, high-resolution, noninvasive measurements
of neural activity, such as functional magnetic resonance imaging (fMRI)
and evoked potentials, it will be possible to dramatically broaden our
cetacean physiologic database. Comparative anatomy has a role in these
101
turns range 1.5 to 2.5 and are independent of animal size. Odontocete
cochlear duct structures are hypercellular. Stria vascularis and spiral ligament in particular are densely packed with duplicate cell populations, which
suggest relatively rapid metabolic processes that are consistent with the
importance of hearing to cetaceans and with moderately high background
noise in ocean environments. Auxiliary outer osseous laminae support 20%
to 60% of the basilar membrane length in odontocetes, adding stiffness.
In mysticetes, the spiral ligament is less well developed and outer osseous
laminae are absent or reduced. The cochlear duct cytoarchitecture of mysticetes is unremarkable. Mysticete basilar membranes scale consistently
with land mammal generalist ears.
Spiral ganglion cell densities are significantly greater in whales than in
land mammals, averaging 2,000 to 4,000 cells/mm. Greatest densities are
found in the highest frequency odontocetes, but all whales have densities
and fiber diameters that are significantly greater than those of land
mammals. Vestibular elements are disproportionately small in all whales,
possibly reflecting reduced azimuthal cues as a result of cervical fusion and
limited head motion.
Modern Cetacea have three inner ear structural formats that coincide
with acoustic groups: low to infrasonic Type M mysticetes, upper range
ultrasonic Type I odontocetes, and lower range ultrasonic Type II odontocetes. Type I and Type II cochleae are adapted for ultrasonic ranges with
exceptionally stiff basilar membranes and extensive bony membrane buttressing. Basilar membrane thickness to width ratios are higher for the basal
turn of Type I odontocetes than for any other mammal. Mysticete (Type M)
cochlea have exceptionally wide, thin basilar membranes and no stiffening
agents, implying they are adapted to low to infrasonic frequencies.
The debate on middle ear function desperately needs to be invigorated
with measures from more than one species, particularly if a general solution is to be obtained for odontocetes and mysticetes. Middle ear anatomies
are sufficiently different between odontocetes and mysticetes, particularly
with respect to couplings to other head tissues that it seems unlikely that
a common mechanism is at work. Therefore, new data must come from
both groups.
Data on mysticete ears continue to be relatively scarce, but what is available suggests they are adapted for sonic to infrasonic frequencies, which is
consistent with mysticete vocalization data.
Psychophysical research on odontocetes ranks among the best available
in the world, but at the moment, cetaceans do not afford the same controlled research opportunities, particularly direct physiologic measures, that
are possible in other species. In that sense, cetacean auditory research is not
physiologically competitive. However, as techniques improve and become
more accessible for high-speed, high-resolution, noninvasive measurements
of neural activity, such as functional magnetic resonance imaging (fMRI)
and evoked potentials, it will be possible to dramatically broaden our
cetacean physiologic database. Comparative anatomy has a role in these