2. Cetacean Ears
91
The actual length of the outer lamina in odontocetes is a species-specific
characteristic, but when expressed as a percentage of membrane or cochlear
duct length, the laminae divide into two distinct groups that coincide with
ear and echolocation signal types (Fig. 2.9). In Type II delphinids (peak frequency 40 to 80kHz), the outer bony lamina is present for less than 30%
of the cochlear duct (Table 2.1). In Type I phocoenids (peak frequencies
>100kHz), the outer lamina is present for more than 60% of the cochlear
duct. The basilar membrane therefore has substantial buttressing at both
edges over twice as much of its length, proportionally, in Type I versus Type
II odontocetes. Type I species use, and presumably hear, higher ultrasonic
signals. A longer outer lamina in Type I cochleae presumably increases
membrane stiffness, which increases the resonant frequency of that portion
of the membrane compared to an equivalently shaped membrane in a Type
II animal without bony outer membrane support. When combined with the
differences observed in membrane ratios, differences in the percentage of
membrane buttressed by outer bony laminae provide a simple but important mechanistic link for species-specific ultrasonic ranges in Odontoceti.
Fleischer (1976a) suggests that because dolphin basal inner osseous
laminae are constructed of compact bony fibers interwoven to form a dense
meshwork, dolphin inner laminae have virtually the same rigidity as solid
bone but with less mass. He concluded, based on changes in the solidity and
cross-sectional area of the inner laminar plates in the typical T. truncatus
inner ear, that the stability gradient of the inner osseous lamina changes
one hundred-fold from base to apex in dolphins. The outer osseous spiral
lamina, by comparison, is largely solid compact bone at the basal end with
noticeable fibrous inclusions only as it begins to disappear apically. Fleischer therefore estimated a magnitude greater; i.e., a thousand-fold baseapex stiffness gradient, for dolphin outer laminae. If these observations and
gradient estimates are even vaguely correct, they suggest that differences
amongst species in both the mass and stiffness of the outer versus inner suspension of the basilar membrane are highly significant elements affecting
membrane motion that are generally overlooked in basilar membrane
models (see de Boer 1996).
In low-frequency mammals, the inner laminae are poorly developed and
outer laminae are reduced or absent. Mysticetes are no exception to this
pattern. The cross-sectional separation of the tympanal and vestibular
plates is large in mysticetes (300 11m, 60 11m at the apex), but the struts are
so thin that the two lightweight laminar plates appear to be disjunct in many
places (Fig. 2.6C,D) (Norris and Leatherwood 1981). Towards the apex the
struts disappear, leaving only parallel, uncoupled laminae or, in some cases,
a single plate for support in the upper apical turn. The outer lamina is absent
or reduced to a disjointed thread. It is assumed to be dysfunctional and is
probably vestigial in mysticetes.
One other point on laminar construction should be made. In part because
of their rarity and post-mortem condition, whale ears, like fossil material,
91
The actual length of the outer lamina in odontocetes is a species-specific
characteristic, but when expressed as a percentage of membrane or cochlear
duct length, the laminae divide into two distinct groups that coincide with
ear and echolocation signal types (Fig. 2.9). In Type II delphinids (peak frequency 40 to 80kHz), the outer bony lamina is present for less than 30%
of the cochlear duct (Table 2.1). In Type I phocoenids (peak frequencies
>100kHz), the outer lamina is present for more than 60% of the cochlear
duct. The basilar membrane therefore has substantial buttressing at both
edges over twice as much of its length, proportionally, in Type I versus Type
II odontocetes. Type I species use, and presumably hear, higher ultrasonic
signals. A longer outer lamina in Type I cochleae presumably increases
membrane stiffness, which increases the resonant frequency of that portion
of the membrane compared to an equivalently shaped membrane in a Type
II animal without bony outer membrane support. When combined with the
differences observed in membrane ratios, differences in the percentage of
membrane buttressed by outer bony laminae provide a simple but important mechanistic link for species-specific ultrasonic ranges in Odontoceti.
Fleischer (1976a) suggests that because dolphin basal inner osseous
laminae are constructed of compact bony fibers interwoven to form a dense
meshwork, dolphin inner laminae have virtually the same rigidity as solid
bone but with less mass. He concluded, based on changes in the solidity and
cross-sectional area of the inner laminar plates in the typical T. truncatus
inner ear, that the stability gradient of the inner osseous lamina changes
one hundred-fold from base to apex in dolphins. The outer osseous spiral
lamina, by comparison, is largely solid compact bone at the basal end with
noticeable fibrous inclusions only as it begins to disappear apically. Fleischer therefore estimated a magnitude greater; i.e., a thousand-fold baseapex stiffness gradient, for dolphin outer laminae. If these observations and
gradient estimates are even vaguely correct, they suggest that differences
amongst species in both the mass and stiffness of the outer versus inner suspension of the basilar membrane are highly significant elements affecting
membrane motion that are generally overlooked in basilar membrane
models (see de Boer 1996).
In low-frequency mammals, the inner laminae are poorly developed and
outer laminae are reduced or absent. Mysticetes are no exception to this
pattern. The cross-sectional separation of the tympanal and vestibular
plates is large in mysticetes (300 11m, 60 11m at the apex), but the struts are
so thin that the two lightweight laminar plates appear to be disjunct in many
places (Fig. 2.6C,D) (Norris and Leatherwood 1981). Towards the apex the
struts disappear, leaving only parallel, uncoupled laminae or, in some cases,
a single plate for support in the upper apical turn. The outer lamina is absent
or reduced to a disjointed thread. It is assumed to be dysfunctional and is
probably vestigial in mysticetes.
One other point on laminar construction should be made. In part because
of their rarity and post-mortem condition, whale ears, like fossil material,