2. Cetacean Ears
89
sistent with broad, flaccid membranes that encode infrasonics well below
human lower functional limits of hearing.
Obviously these are very gross approximations. They are presented primarily to illustrate how structure underlies, and implies, exceptional hearing
abilities in whales, but they also underscore how functional features may
interact and how a single metric can mislead. Odontocetes, on average, have
basilar membranes two to five times as long as those of microchiropteran
bats, yet they evolved similar hearing capacities. For odontocetes and bats,
basilar membrane stiffness distributions are the overarching feature related
to the membrane response. Length in both cases is irrelevant.
The most extreme example of this is found among CF/FM Rhinolophid
and Pteronotid bats. These bats have basilar membranes with remarkable
tuning characteristics. A disproportionate amount (4 to 5mm) of the total
available membrane (14 to 16mm) encodes a frequency difference of less
than 10 kHz. As much as 30% of the basilar membrane, starting from the
basal end, has a relatively constant thickness and width. This segment terminates in a cliff where the membrane thickness drops from 30 to 51lm
within 1mm. The region of rapid change and low neural density is commonly called the acoustic fovea (Bruns and Schmieszik 1980).
There is preliminary evidence for an acoustic foveal region in P. phocoena, a Type I odontocete (Ketten 1998a) but it is unclear whether the
membrane shapes serve the same acoustic purpose as in bats. P. phocoena
has a membrane segment that has excessive thickness, stable contours, and
bidirectional fibers, all of which have been mentioned as features of CF/FM
bat foveal areas (Kossl and Vater 1995; Ketten 1998a). One function
proposed for the basilar membrane foveal region is that it provides a
reflection zone that engenders standing waves (see Kossl and Vater 1995
for review). In bats, the frequencies represented in this nearly constant
cross-sectional area correspond to CF 2 , the second harmonic of their
echolocation signal. The specialized regions of the basilar membrane
optimize detection and analysis of Doppler-shifted echoes by providing a
mechanism to enhance the CF 2 signal in comparison to an overlapping
call and to detect subtle features in the echo related to prey wing beat patterns (Grinnell 1995). One difficulty in extrapolating this function to an
odontocete is that there is no evidence that dolphins or porpoises use
Doppler, particularly since the faster sound speed in water implies dolphins
can obtain multiple echoes in a short time, negating the advantage that
Doppler affords bats in air of being able to resolve prey velocity from one
echo (Au 1993).
Aside from inherent stiffness, the next most significant cochlear feature
related to basilar membrane resonance is the structure and extent of basilar
membrane support. Bony spiral paired laminae are a striking and archetypal feature of high-frequency cochlea. As with other cochlear structures,
odontocetes take them to extremes. There are inner and outer bony laminae
in all odontocete cochlea (Figs. 2.6, 2.7B, 2.9). The internal laminae form a
wedge that runs the full length of the basilar membrane. The thickness of
89
sistent with broad, flaccid membranes that encode infrasonics well below
human lower functional limits of hearing.
Obviously these are very gross approximations. They are presented primarily to illustrate how structure underlies, and implies, exceptional hearing
abilities in whales, but they also underscore how functional features may
interact and how a single metric can mislead. Odontocetes, on average, have
basilar membranes two to five times as long as those of microchiropteran
bats, yet they evolved similar hearing capacities. For odontocetes and bats,
basilar membrane stiffness distributions are the overarching feature related
to the membrane response. Length in both cases is irrelevant.
The most extreme example of this is found among CF/FM Rhinolophid
and Pteronotid bats. These bats have basilar membranes with remarkable
tuning characteristics. A disproportionate amount (4 to 5mm) of the total
available membrane (14 to 16mm) encodes a frequency difference of less
than 10 kHz. As much as 30% of the basilar membrane, starting from the
basal end, has a relatively constant thickness and width. This segment terminates in a cliff where the membrane thickness drops from 30 to 51lm
within 1mm. The region of rapid change and low neural density is commonly called the acoustic fovea (Bruns and Schmieszik 1980).
There is preliminary evidence for an acoustic foveal region in P. phocoena, a Type I odontocete (Ketten 1998a) but it is unclear whether the
membrane shapes serve the same acoustic purpose as in bats. P. phocoena
has a membrane segment that has excessive thickness, stable contours, and
bidirectional fibers, all of which have been mentioned as features of CF/FM
bat foveal areas (Kossl and Vater 1995; Ketten 1998a). One function
proposed for the basilar membrane foveal region is that it provides a
reflection zone that engenders standing waves (see Kossl and Vater 1995
for review). In bats, the frequencies represented in this nearly constant
cross-sectional area correspond to CF 2 , the second harmonic of their
echolocation signal. The specialized regions of the basilar membrane
optimize detection and analysis of Doppler-shifted echoes by providing a
mechanism to enhance the CF 2 signal in comparison to an overlapping
call and to detect subtle features in the echo related to prey wing beat patterns (Grinnell 1995). One difficulty in extrapolating this function to an
odontocete is that there is no evidence that dolphins or porpoises use
Doppler, particularly since the faster sound speed in water implies dolphins
can obtain multiple echoes in a short time, negating the advantage that
Doppler affords bats in air of being able to resolve prey velocity from one
echo (Au 1993).
Aside from inherent stiffness, the next most significant cochlear feature
related to basilar membrane resonance is the structure and extent of basilar
membrane support. Bony spiral paired laminae are a striking and archetypal feature of high-frequency cochlea. As with other cochlear structures,
odontocetes take them to extremes. There are inner and outer bony laminae
in all odontocete cochlea (Figs. 2.6, 2.7B, 2.9). The internal laminae form a
wedge that runs the full length of the basilar membrane. The thickness of