92
The Biology of Sea Turtles, Vol. II
membrane is a thin partition in the circular fluid pathway, which contains two basic
cell types: hair cells and supporting cells. In most reptiles, and presumably in sea
turtles as well, the tectorial membrane overlies the hair cells of the basilar papilla
(Wever, 1978; Lewis et al., 1985).
3.3.1.2 Water Conduction vs. Bone Conduction Hearing
The functional morphology of the sea turtle ear is still under some debate. Lenhardt
et al. (1985) postulated that the sea turtle ear is a poor aerial receptor. For the
terrestrial vertebrate ear, the middle ear acts as an impedance transformer between
the media by which the sound is propagated (air) and the media by which the receptor
cells reside (fluid). Generally, this impedance mismatch can be overcome by having
a high ratio of the area of the tympanic membrane to that of the oval window, and
by employing a columella lever ratio. Lenhardt et al. (1985) found both of these
ratios to be low in the loggerhead sea turtle compared to its terrestrial counterparts.
They suggested, instead, that the shape of the columella and its interactions with
the cochlea and saccule are not optimized for hearing in air, but rather are adapted
for sound conduction through two media, bone and water. If the turtle uses bone
conduction to process sound, sound flows through the bones and soft tissue to
stimulate the inner ear. The tympanum would act as a release mechanism rather than
a sound receptor. However, if the turtle uses water conduction to process sound, the
tympanum and subtympanal fat could act as low-impedance channels for underwater
sound, resulting in columellar displacement to stimulate the inner ear. Recent imaging data strongly suggest that the fats adjacent to the tympanal plates in at least
three turtle species are highly specialized for underwater sound conduction (Ketten
et al., 1999).
3.3.2 E LECTROPHYSIOLOGY
Electrophysiological studies on hearing have been conducted on juvenile green
turtles ( C. mydas ) (Ridgeway et al., 1969) and on juvenile loggerheads ( C. caretta )
(Bartol, 1999). Ridgeway et al. (1969) used both aerial and vibrational stimuli to
obtain auditory cochlear potentials. The active electrode was placed, using surgical
techniques, in the perilymph spaces of the labyrinth. Sounds were presented either
with a loudspeaker or with a mechanical vibrator. Absolute thresholds were not
measured; instead, cochlear response curves of 0.1 m V potential were plotted for
frequencies ranging from 50 to 2000 Hz. Green sea turtles detect a limited frequency
range (200–700 Hz), with best sensitivity at the low tone region of about 400 Hz.
Although this investigation examined two separate modes of sound reception (air
conduction and bone conduction), sensitivity curves were relatively similar (Figure 3.6). These results suggest that the inner ear is the main structure for determining
frequency sensitivity (Ridgeway et al., 1969).
Bartol et al. (1999) used a second technique for obtaining electrophysiological
responses to sound stimuli from sea turtles, the collection of auditory brainstem
responses (ABRs). ABRs are sequences of events originating in the brain stem and
are generated by separate parts of the auditory pathway in the first 10 msec after
1123 book.book Page 92 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
membrane is a thin partition in the circular fluid pathway, which contains two basic
cell types: hair cells and supporting cells. In most reptiles, and presumably in sea
turtles as well, the tectorial membrane overlies the hair cells of the basilar papilla
(Wever, 1978; Lewis et al., 1985).
3.3.1.2 Water Conduction vs. Bone Conduction Hearing
The functional morphology of the sea turtle ear is still under some debate. Lenhardt
et al. (1985) postulated that the sea turtle ear is a poor aerial receptor. For the
terrestrial vertebrate ear, the middle ear acts as an impedance transformer between
the media by which the sound is propagated (air) and the media by which the receptor
cells reside (fluid). Generally, this impedance mismatch can be overcome by having
a high ratio of the area of the tympanic membrane to that of the oval window, and
by employing a columella lever ratio. Lenhardt et al. (1985) found both of these
ratios to be low in the loggerhead sea turtle compared to its terrestrial counterparts.
They suggested, instead, that the shape of the columella and its interactions with
the cochlea and saccule are not optimized for hearing in air, but rather are adapted
for sound conduction through two media, bone and water. If the turtle uses bone
conduction to process sound, sound flows through the bones and soft tissue to
stimulate the inner ear. The tympanum would act as a release mechanism rather than
a sound receptor. However, if the turtle uses water conduction to process sound, the
tympanum and subtympanal fat could act as low-impedance channels for underwater
sound, resulting in columellar displacement to stimulate the inner ear. Recent imaging data strongly suggest that the fats adjacent to the tympanal plates in at least
three turtle species are highly specialized for underwater sound conduction (Ketten
et al., 1999).
3.3.2 E LECTROPHYSIOLOGY
Electrophysiological studies on hearing have been conducted on juvenile green
turtles ( C. mydas ) (Ridgeway et al., 1969) and on juvenile loggerheads ( C. caretta )
(Bartol, 1999). Ridgeway et al. (1969) used both aerial and vibrational stimuli to
obtain auditory cochlear potentials. The active electrode was placed, using surgical
techniques, in the perilymph spaces of the labyrinth. Sounds were presented either
with a loudspeaker or with a mechanical vibrator. Absolute thresholds were not
measured; instead, cochlear response curves of 0.1 m V potential were plotted for
frequencies ranging from 50 to 2000 Hz. Green sea turtles detect a limited frequency
range (200–700 Hz), with best sensitivity at the low tone region of about 400 Hz.
Although this investigation examined two separate modes of sound reception (air
conduction and bone conduction), sensitivity curves were relatively similar (Figure 3.6). These results suggest that the inner ear is the main structure for determining
frequency sensitivity (Ridgeway et al., 1969).
Bartol et al. (1999) used a second technique for obtaining electrophysiological
responses to sound stimuli from sea turtles, the collection of auditory brainstem
responses (ABRs). ABRs are sequences of events originating in the brain stem and
are generated by separate parts of the auditory pathway in the first 10 msec after
1123 book.book Page 92 Monday, November 11, 2002 11:11 AM