2. Cetacean Ears
93
Auditory ganglion cell totals for cetaceans are more than double the
human average. More important, both odontocete and mysticete auditory
innervation denstities are significantly greater than those of other
mammals. Auditory ganglion cell totals range from 68,000 in P phocoena to
over 160,000 in B. physalus. Auditory ganglion cell densities in Type Iodontocetes average 2,900 cells/mm of basilar membrane; 2,500 cells/mm for
Type II odontocetes; and 2,300 cells/mm for mysticetes (Table 2.2). Given
100 inner hair cells/mm and three rows of outer hair cells/inner hair cell
in whales, these data imply a ganglion to hair cell ratio of approximately
7.3: 1 for Type I species, 6.5: 1 for Type II, and 5.7: 1 for Type M. The human
ratio is 2.4: 1; for cats it is 3.7: 1; and for bats, 4: 1 (Firbas 1972; Bruns and
Schmiezek 1980). Since 90% to 95% of all afferent spiral ganglion cells
innervate inner hair cells, the average ganglion cell to inner hair cell ratio
is 27: 1 for cetaceans, or more than twice the average ratio in bats and three
times that of humans.
Wever et al. (1971c) speculated that additional innervation is required in
the odontocete ear to relay greater detail about ultrasonic signals to the
central nervous system in echolocation analyses. Electrophysiological
results are consistent with this speculation. Bullock et al. (1968) found three
distinct categories of response units in the inferior colliculus of dolphins:
those that were signal duration specific, those that responded to changes in
signal rise time, and those that were specialized to short latencies with no
frequency specificity. This division of signal properties among populations
of neurons is consistent with, although not identical to, observations in bats
of multiple categories of facilitation and analysis neurons (Suga 1983; see
also Ridgway, Chapter 6). Clearly, it is reasonable to assume that high
ganglion cell ratios in odontocetes are related to the complexity of information extracted from echolocation signals, but this does not explain
equally dense auditory innervation patterns in mysticetes. Similar odontocete and mysticete ganglion cell densities suggest that baleen whales have
equally complex auditory processing, which raises a new and intriguing
question: What do baleen whales extract acoustically from low to infrasonic
signals?
Comparisons of the ratios of auditory, vestibular, and optic counts in
cetaceans and land mammals underscore the importance of hearing in
whales (Table 2.2). As indicated earlier, vestibular counts in all cetaceans
are exceptionally low. Whale vestibular to auditory ratios are approximately
one-tenth those of land mammals. Optic to auditory ratios in Type II odontocetes and mysticetes are one-half to one-third those of land mammals,
while ratios in Type I odontocetes (0.2 to 0.3) are nearly a magnitude lower.
The most extreme contrast in optic-auditory ratios is the 2oo-fold
difference between the vision top-heavy human value of 38.0 versus the
0.15 ratio for I. geoffrensis, a riverine Type I odontocete that has the lowest
visual acuity of any aquatic mammal (Mass and Supin 1989). Optic to
vestibular ratios for all cetaceans (25 to 45), except I. geoffrensis (6.6), are
midway between those of cats (15.6) and humans (74.3), suggesting that on
93
Auditory ganglion cell totals for cetaceans are more than double the
human average. More important, both odontocete and mysticete auditory
innervation denstities are significantly greater than those of other
mammals. Auditory ganglion cell totals range from 68,000 in P phocoena to
over 160,000 in B. physalus. Auditory ganglion cell densities in Type Iodontocetes average 2,900 cells/mm of basilar membrane; 2,500 cells/mm for
Type II odontocetes; and 2,300 cells/mm for mysticetes (Table 2.2). Given
100 inner hair cells/mm and three rows of outer hair cells/inner hair cell
in whales, these data imply a ganglion to hair cell ratio of approximately
7.3: 1 for Type I species, 6.5: 1 for Type II, and 5.7: 1 for Type M. The human
ratio is 2.4: 1; for cats it is 3.7: 1; and for bats, 4: 1 (Firbas 1972; Bruns and
Schmiezek 1980). Since 90% to 95% of all afferent spiral ganglion cells
innervate inner hair cells, the average ganglion cell to inner hair cell ratio
is 27: 1 for cetaceans, or more than twice the average ratio in bats and three
times that of humans.
Wever et al. (1971c) speculated that additional innervation is required in
the odontocete ear to relay greater detail about ultrasonic signals to the
central nervous system in echolocation analyses. Electrophysiological
results are consistent with this speculation. Bullock et al. (1968) found three
distinct categories of response units in the inferior colliculus of dolphins:
those that were signal duration specific, those that responded to changes in
signal rise time, and those that were specialized to short latencies with no
frequency specificity. This division of signal properties among populations
of neurons is consistent with, although not identical to, observations in bats
of multiple categories of facilitation and analysis neurons (Suga 1983; see
also Ridgway, Chapter 6). Clearly, it is reasonable to assume that high
ganglion cell ratios in odontocetes are related to the complexity of information extracted from echolocation signals, but this does not explain
equally dense auditory innervation patterns in mysticetes. Similar odontocete and mysticete ganglion cell densities suggest that baleen whales have
equally complex auditory processing, which raises a new and intriguing
question: What do baleen whales extract acoustically from low to infrasonic
signals?
Comparisons of the ratios of auditory, vestibular, and optic counts in
cetaceans and land mammals underscore the importance of hearing in
whales (Table 2.2). As indicated earlier, vestibular counts in all cetaceans
are exceptionally low. Whale vestibular to auditory ratios are approximately
one-tenth those of land mammals. Optic to auditory ratios in Type II odontocetes and mysticetes are one-half to one-third those of land mammals,
while ratios in Type I odontocetes (0.2 to 0.3) are nearly a magnitude lower.
The most extreme contrast in optic-auditory ratios is the 2oo-fold
difference between the vision top-heavy human value of 38.0 versus the
0.15 ratio for I. geoffrensis, a riverine Type I odontocete that has the lowest
visual acuity of any aquatic mammal (Mass and Supin 1989). Optic to
vestibular ratios for all cetaceans (25 to 45), except I. geoffrensis (6.6), are
midway between those of cats (15.6) and humans (74.3), suggesting that on