2. Cetacean Ears
85
where both major cochlear ducts connect in whales and whether they serve
equivalent functions in land and aquatic animals.
Disproportionately large cochlear aqueducts have also been noted in
constant frequency/frequency modulated (CFIFM) bats (Kossl and Vater
1995). Kossl and Vater suggest that large cochlear aqueducts may prevent
damage from intracochlear reverberant oscillations, but if that were the
function, why is the cochlear aqueduct significantly larger in all cetaceans?
Do cetaceans have greater amplitude oscillations because of greater
acoustic pressures? Is this another pointer to low-frequency sonar in baleen
whales? It is tempting to draw functional conclusions, but current data are
essentially anecdotal. Comprehensive morphometric analyses of the
cochlear duct and other canal structures in cetaceans, particularly in mysticetes, may help determine whether similarities between cetaceans and
microchiropteran bats are functionally important. If similarities were found
among some Type M species, the data would serve also as a valuable guide
to which species are worth investigating behaviorally for "mega" sonar.
Even more important, this is a clear case where deciphering the anomalous
canal structures could lead to a solution for the general case.
The hallmark of mysticete cochlear ducts is bigger structures with fewer
cells. Mysticete cochleae (Figs. 2.4, 2.6C,D), with a few exceptions, have cellular trends that are clearly and consistently the opposite of those in odontocetes, even taking into account cellular losses because of poor
preservation. Inner ear material from odontocetes with similar postmortem times retain clear evidence of hypercellularity and, even with
advanced decay, do not resemble the ears of mysticetes. There is insufficient
data to make definitive comments on the cellular distributions in any mysticete organ of Corti. The stria and supporting cells are unremarkable in
comparison to the average human cochlea, and the spiral ligament has poor
cellular development in comparison to many land mammal cochleae. The
same scalar trends are found in mysticetes as in odontocetes. Scala tympani
is inflated in the basal turn and there is an exceptionally large cochlear
aqueduct. As noted in the previous section, the decrease in scala vestibuli
is more pronounced in mysticetes than in odontocetes. Unfortunately, no
complete cochlear studies are available for elephants, hippopotami, or other
exceptionally large land mammals, which are the species most appropriate
for comparing with mysticete whales. Such comparisons are crucial for
answering definitively whether mysticetes are on a continuum with larger
land mammal ears or if they represent a leap to exceptional dimensions.
Clearly there is a distinct hole in the auditory database for larger mammalian ears that hampers our ability to understand and interpret the
breadth of cochlear adaptations for low-frequency hearing.
5.3.3 Basilar Membrane Shape and Support
The foundation of frequency analysis in the cochlea is the basilar membrane. The spiral shape of the auditory organ in mammals is generally con-
85
where both major cochlear ducts connect in whales and whether they serve
equivalent functions in land and aquatic animals.
Disproportionately large cochlear aqueducts have also been noted in
constant frequency/frequency modulated (CFIFM) bats (Kossl and Vater
1995). Kossl and Vater suggest that large cochlear aqueducts may prevent
damage from intracochlear reverberant oscillations, but if that were the
function, why is the cochlear aqueduct significantly larger in all cetaceans?
Do cetaceans have greater amplitude oscillations because of greater
acoustic pressures? Is this another pointer to low-frequency sonar in baleen
whales? It is tempting to draw functional conclusions, but current data are
essentially anecdotal. Comprehensive morphometric analyses of the
cochlear duct and other canal structures in cetaceans, particularly in mysticetes, may help determine whether similarities between cetaceans and
microchiropteran bats are functionally important. If similarities were found
among some Type M species, the data would serve also as a valuable guide
to which species are worth investigating behaviorally for "mega" sonar.
Even more important, this is a clear case where deciphering the anomalous
canal structures could lead to a solution for the general case.
The hallmark of mysticete cochlear ducts is bigger structures with fewer
cells. Mysticete cochleae (Figs. 2.4, 2.6C,D), with a few exceptions, have cellular trends that are clearly and consistently the opposite of those in odontocetes, even taking into account cellular losses because of poor
preservation. Inner ear material from odontocetes with similar postmortem times retain clear evidence of hypercellularity and, even with
advanced decay, do not resemble the ears of mysticetes. There is insufficient
data to make definitive comments on the cellular distributions in any mysticete organ of Corti. The stria and supporting cells are unremarkable in
comparison to the average human cochlea, and the spiral ligament has poor
cellular development in comparison to many land mammal cochleae. The
same scalar trends are found in mysticetes as in odontocetes. Scala tympani
is inflated in the basal turn and there is an exceptionally large cochlear
aqueduct. As noted in the previous section, the decrease in scala vestibuli
is more pronounced in mysticetes than in odontocetes. Unfortunately, no
complete cochlear studies are available for elephants, hippopotami, or other
exceptionally large land mammals, which are the species most appropriate
for comparing with mysticete whales. Such comparisons are crucial for
answering definitively whether mysticetes are on a continuum with larger
land mammal ears or if they represent a leap to exceptional dimensions.
Clearly there is a distinct hole in the auditory database for larger mammalian ears that hampers our ability to understand and interpret the
breadth of cochlear adaptations for low-frequency hearing.
5.3.3 Basilar Membrane Shape and Support
The foundation of frequency analysis in the cochlea is the basilar membrane. The spiral shape of the auditory organ in mammals is generally con-