250
Marine Mammal Physiology: Requisites for Ocean Living
in all mammals, serves as the frequency map of the inner ear and vibrates in response to
changes in the pressure of the cochlear fluids caused by action of the stapes on the oval
window. Differences in the thickness and width of the basilar membrane dictate, at least in
large part, where along the basilar membrane length the maximum response to a particular frequency of stimulation occurs; the basilar membrane is narrower and thicker at the
base, where resonant responses to higher frequencies occur, and wider and thinner at the
apex, where resonant responses to lower frequencies occur. The ratio of the basilar membrane thickness to width (T/W) is one of several determinants of the frequency response
of the membrane (Ketten 2000), the vibration of which activates hair cells along the organ
of Corti and results in neural signals being transmitted via the auditory nerve to the brain.
The T/W ratios along the basilar membranes of odontocetes are consistent with the maximum high and low frequencies reported in these species (Ketten and Wartzok 1990). The
membrane itself is supported and stiffened by outer and inner bony laminae that buttress
the membrane and increase its stiffness. The degree that the outer lamina buttresses the
basilar membrane varies by species with more of the basilar membrane being supported
in odontocetes producing echolocation signals with higher peak frequencies (e.g., Phocoena
phocoena, >100 kHz) than those producing echolocation signals with lower peak frequencies (e.g., Tursiops truncatus, <100 kHz; Ketten 2000).
Auditory ganglion cells of the cochlear nerve are dense in odontocetes with calculated
ratios of ganglion to hair cells of ~6.5–7.3:1 (Ketten 2000). In addition, auditory fiber diameters
tend to be greater in odontocetes than in terrestrial mammals and the number of auditory
cell counts is much higher. These factors have been hypothesized to reduce the latencies of
action potentials and contribute to the ability to extract complex information from received
acoustic signals. The brain of the modern delphinid is large and may have an encephalization quotient (EQ), which is the ratio of actual brain mass to predicted brain mass based
upon total body mass, as high as 6.3 (e.g., Lagenorhyncus albirostris; Manger 2006). This is
close to that of humans, although there is considerable variation across species. The brain
is highly derived and demonstrates regional hypertrophy associated with acoustic processing centers; for example, auditory cortex, cerebellum, and pons (in delphinids). Indeed, the
increase in the size of the odontocete brain may have largely resulted from the adaptive
pressures related to sonar-guided navigation and communication (Oelschlager 2008).
Experimental evidence for mechanisms of hearing in mysticetes is absent, although as
previously noted, the external ear canal does connect to the everted tympanic membrane
and this observation has led to the speculation that the external auditory canal of the
mysticete has a role in sound reception (Ketten 2000). The role may be limited to the production of waxy secretions, which serve to connect the tympanic with surrounding bony
structures and may facilitate sound reception via bone conduction. Spongy bone flanges
that extend posteromedially from the periotic to the skull provide additional anatomical
arguments for bone-conducted sound reception. Recent anatomical evidence also suggests
that the fatty sound reception pathways may not be unique to odontocetes but may also
be present in mysticetes. Postmortem investigations of the minke whale (Balaenoptera acutorostrata) demonstrate that a fatty body exists lateral to the tympanoperiotic complex and
connects with it, as is observed in odontocetes (Yamato et al. 2012). It connects directly to
the ossicles and extends laterally to the blubber suggesting the possible presence of a lateral sound transmission pathway.
The ossicles of the middle ear in the mysticete whale are massive and loosely
c oupled, which is suggestive of specialization for low-frequency sound reception.
In the cochlea, the apical basilar membrane thicknesses and widths are implicative of
high-frequency hearing limits possibly as high as in humans (or higher) (Ketten 2000).
Marine Mammal Physiology: Requisites for Ocean Living
in all mammals, serves as the frequency map of the inner ear and vibrates in response to
changes in the pressure of the cochlear fluids caused by action of the stapes on the oval
window. Differences in the thickness and width of the basilar membrane dictate, at least in
large part, where along the basilar membrane length the maximum response to a particular frequency of stimulation occurs; the basilar membrane is narrower and thicker at the
base, where resonant responses to higher frequencies occur, and wider and thinner at the
apex, where resonant responses to lower frequencies occur. The ratio of the basilar membrane thickness to width (T/W) is one of several determinants of the frequency response
of the membrane (Ketten 2000), the vibration of which activates hair cells along the organ
of Corti and results in neural signals being transmitted via the auditory nerve to the brain.
The T/W ratios along the basilar membranes of odontocetes are consistent with the maximum high and low frequencies reported in these species (Ketten and Wartzok 1990). The
membrane itself is supported and stiffened by outer and inner bony laminae that buttress
the membrane and increase its stiffness. The degree that the outer lamina buttresses the
basilar membrane varies by species with more of the basilar membrane being supported
in odontocetes producing echolocation signals with higher peak frequencies (e.g., Phocoena
phocoena, >100 kHz) than those producing echolocation signals with lower peak frequencies (e.g., Tursiops truncatus, <100 kHz; Ketten 2000).
Auditory ganglion cells of the cochlear nerve are dense in odontocetes with calculated
ratios of ganglion to hair cells of ~6.5–7.3:1 (Ketten 2000). In addition, auditory fiber diameters
tend to be greater in odontocetes than in terrestrial mammals and the number of auditory
cell counts is much higher. These factors have been hypothesized to reduce the latencies of
action potentials and contribute to the ability to extract complex information from received
acoustic signals. The brain of the modern delphinid is large and may have an encephalization quotient (EQ), which is the ratio of actual brain mass to predicted brain mass based
upon total body mass, as high as 6.3 (e.g., Lagenorhyncus albirostris; Manger 2006). This is
close to that of humans, although there is considerable variation across species. The brain
is highly derived and demonstrates regional hypertrophy associated with acoustic processing centers; for example, auditory cortex, cerebellum, and pons (in delphinids). Indeed, the
increase in the size of the odontocete brain may have largely resulted from the adaptive
pressures related to sonar-guided navigation and communication (Oelschlager 2008).
Experimental evidence for mechanisms of hearing in mysticetes is absent, although as
previously noted, the external ear canal does connect to the everted tympanic membrane
and this observation has led to the speculation that the external auditory canal of the
mysticete has a role in sound reception (Ketten 2000). The role may be limited to the production of waxy secretions, which serve to connect the tympanic with surrounding bony
structures and may facilitate sound reception via bone conduction. Spongy bone flanges
that extend posteromedially from the periotic to the skull provide additional anatomical
arguments for bone-conducted sound reception. Recent anatomical evidence also suggests
that the fatty sound reception pathways may not be unique to odontocetes but may also
be present in mysticetes. Postmortem investigations of the minke whale (Balaenoptera acutorostrata) demonstrate that a fatty body exists lateral to the tympanoperiotic complex and
connects with it, as is observed in odontocetes (Yamato et al. 2012). It connects directly to
the ossicles and extends laterally to the blubber suggesting the possible presence of a lateral sound transmission pathway.
The ossicles of the middle ear in the mysticete whale are massive and loosely
c oupled, which is suggestive of specialization for low-frequency sound reception.
In the cochlea, the apical basilar membrane thicknesses and widths are implicative of
high-frequency hearing limits possibly as high as in humans (or higher) (Ketten 2000).