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S.H. Ridgway
cells in the dolphin ear suggests a high order of auditory proficiency in
general, and especially a marked ability of tonal differentiation. The large
ratio of ganglion cells to hair cells suggest unusual capabilities in the utilization of auditory information." Further, they add, "The representation of
detailed information about high-frequency sounds, and especially their time
and phase relations, may well be the basis for the dolphin's remarkable
facility in echolocation."
2.1 The Ear and High Frequencies
Sensitivity to very high frequencies, even above 100 kHz, is apparently
important for dolphins making detailed judgments about echolocation
targets (Au 1993). So, how do they do this? The mammalian ear evolved for
hearing in air. In assuming the aquatic mode, cetaceans have undergone
modification of this aerial system, and hearing apparently has become their
most highly developed sense. When the distant ancestors of cetaceans
entered the water, they almost certainly could hear high-frequency sounds
by bone conduction. Human divers can hear ultrasonic frequencies underwater, even above 100kHz (Smith 1985), but, because of the upper frequency limit of human hearing, they have no pitch discrimination in the
ultrasonic region above about 20 kHz. The evolution of the cetacean ear
apparently has been a process of enhancing those characteristics that allow
for greater high-frequency hearing sensitivity and complex auditory processing. Audiograms developed by both behavioral and electrophysiological techniques have shown that T truncatus respond to frequencies as high
as 150 kHz, with greatest sensitivity between about 40 and 100 kHz (Johnson
1966; McCormick et al. 1970).
2.2 Sound Path to the Ear
Without a pinna and with only a vestige of the EAM (Fig. 6.2), there has
been considerable discussion of how sound reaches the cochlea (Fig. 6.3)
deep inside the dolphin's head (Popper 1980). The most sensitive part of
the head for sound reception is the lower jaw (Norris et al. 1961; Bullock
et al. 1968; Norris 1968; McCormick et al. 1970; Brill et al. 1988). The lower
jaw contains a special "acoustic fat body" that abuts both the side of the
jaw and the ear bones sitting directly behind the lower jaw. McCormick et
al. (1970) have conducted physiological studies of sound transmission to the
cochlea by recording electrical potentials from the round window (Fig. 6.4)
of dolphins for frequencies of 5 to 100kHz. Within this frequency range,
their results show that the EAM, the tympanic membrane, and the membrane's transduction to the middle ear bones are largely bypassed in
dolphins. They found, for example, that the malleus is connected to the
eardrum only by a ligament. Removal of the eardrum did not reduce the
S.H. Ridgway
cells in the dolphin ear suggests a high order of auditory proficiency in
general, and especially a marked ability of tonal differentiation. The large
ratio of ganglion cells to hair cells suggest unusual capabilities in the utilization of auditory information." Further, they add, "The representation of
detailed information about high-frequency sounds, and especially their time
and phase relations, may well be the basis for the dolphin's remarkable
facility in echolocation."
2.1 The Ear and High Frequencies
Sensitivity to very high frequencies, even above 100 kHz, is apparently
important for dolphins making detailed judgments about echolocation
targets (Au 1993). So, how do they do this? The mammalian ear evolved for
hearing in air. In assuming the aquatic mode, cetaceans have undergone
modification of this aerial system, and hearing apparently has become their
most highly developed sense. When the distant ancestors of cetaceans
entered the water, they almost certainly could hear high-frequency sounds
by bone conduction. Human divers can hear ultrasonic frequencies underwater, even above 100kHz (Smith 1985), but, because of the upper frequency limit of human hearing, they have no pitch discrimination in the
ultrasonic region above about 20 kHz. The evolution of the cetacean ear
apparently has been a process of enhancing those characteristics that allow
for greater high-frequency hearing sensitivity and complex auditory processing. Audiograms developed by both behavioral and electrophysiological techniques have shown that T truncatus respond to frequencies as high
as 150 kHz, with greatest sensitivity between about 40 and 100 kHz (Johnson
1966; McCormick et al. 1970).
2.2 Sound Path to the Ear
Without a pinna and with only a vestige of the EAM (Fig. 6.2), there has
been considerable discussion of how sound reaches the cochlea (Fig. 6.3)
deep inside the dolphin's head (Popper 1980). The most sensitive part of
the head for sound reception is the lower jaw (Norris et al. 1961; Bullock
et al. 1968; Norris 1968; McCormick et al. 1970; Brill et al. 1988). The lower
jaw contains a special "acoustic fat body" that abuts both the side of the
jaw and the ear bones sitting directly behind the lower jaw. McCormick et
al. (1970) have conducted physiological studies of sound transmission to the
cochlea by recording electrical potentials from the round window (Fig. 6.4)
of dolphins for frequencies of 5 to 100kHz. Within this frequency range,
their results show that the EAM, the tympanic membrane, and the membrane's transduction to the middle ear bones are largely bypassed in
dolphins. They found, for example, that the malleus is connected to the
eardrum only by a ligament. Removal of the eardrum did not reduce the