9. Echolocation in Dolphins
365
of the auditory central nervous system. The characteristics of the dolphin
auditory system is discussed by Nachtigall et al. in Chapter 8 and the auditory central nervous system is discussed by Ridgway in Chapter 6.
The capability of a dolphin to detect objects in noise and clutter and
to discriminate between various objects depends to a large extent on the
information-carrying capabilities of the emitted signals. Also important are
the extent to which the dolphin's auditory system can extract pertinent
information from the echoes and the animal's cognitive capabilities. In
order to make optimal use of acoustical information, the dolphin should
have an auditory system that is very sensitive over a wide frequency range.
The dolphin should also be sensitive in both quiet and noisy environments
and should be able to detect short- and long-duration sounds. A good spectral analysis capability is important in discriminating and recognizing predators, prey, and other objects in the environment. Other important
characteristics of a good sonar receiver include the ability to spatially
resolve and localize sounds, reject externally generated interferences, and
recognize temporal and spectral patterns of sounds.
2. The Transmission System
Dolphins produce sounds within their nasal system, and the signals are projected out through the melon (Cranford, Chapter 3; Aroyan et aI., Chapter
10). Although there has been a long-standing controversy over whether
sounds are produced in the larynx or in the nasal system of odontocetes,
almost all experimental data with dolphins indicate that sounds are produced in the nasal system (Au 1993). The melon in front of the nasal plug
may playa role in channeling sounds into the water, a notion first introduced by Wood (1964). Norris and Harvey (1974) found a low-velocity core
extending from just below the anterior surface toward the right nasal plug,
and a graded outer shell of high-velocity tissue. Such a velocity gradient
could channel signals originating in the nasal region in both the vertical and
horizontal planes. Using both a two-dimensional (Aroyan et al. 1992) and
a three-dimensional model (Aroyan, 1996; Aroyan et aI., Chapter 10) to
study sound propagation in a dolphin's head, Aroyan has shown that
echolocation signals most likely are generated in the nasal system and are
channeled into the water by the melon. Cranford (Chapter 3) has also collected evidence that suggests that echolocation signals are most likely produced in the nasal system at the location of the monkey lips-dorsal bursae
complex just beneath of the blow hole.
Echolocation signals seem to fall into two general categories. The first
category is associated with signals produced by dolphins that can also
produce whistles. Whistles are frequency-modulated signals of relatively
long duration (1 to 3s) and are described by Tyack and Clark in Chapter 4.
The properties of signals produced by dolphins that can also whistle will be
365
of the auditory central nervous system. The characteristics of the dolphin
auditory system is discussed by Nachtigall et al. in Chapter 8 and the auditory central nervous system is discussed by Ridgway in Chapter 6.
The capability of a dolphin to detect objects in noise and clutter and
to discriminate between various objects depends to a large extent on the
information-carrying capabilities of the emitted signals. Also important are
the extent to which the dolphin's auditory system can extract pertinent
information from the echoes and the animal's cognitive capabilities. In
order to make optimal use of acoustical information, the dolphin should
have an auditory system that is very sensitive over a wide frequency range.
The dolphin should also be sensitive in both quiet and noisy environments
and should be able to detect short- and long-duration sounds. A good spectral analysis capability is important in discriminating and recognizing predators, prey, and other objects in the environment. Other important
characteristics of a good sonar receiver include the ability to spatially
resolve and localize sounds, reject externally generated interferences, and
recognize temporal and spectral patterns of sounds.
2. The Transmission System
Dolphins produce sounds within their nasal system, and the signals are projected out through the melon (Cranford, Chapter 3; Aroyan et aI., Chapter
10). Although there has been a long-standing controversy over whether
sounds are produced in the larynx or in the nasal system of odontocetes,
almost all experimental data with dolphins indicate that sounds are produced in the nasal system (Au 1993). The melon in front of the nasal plug
may playa role in channeling sounds into the water, a notion first introduced by Wood (1964). Norris and Harvey (1974) found a low-velocity core
extending from just below the anterior surface toward the right nasal plug,
and a graded outer shell of high-velocity tissue. Such a velocity gradient
could channel signals originating in the nasal region in both the vertical and
horizontal planes. Using both a two-dimensional (Aroyan et al. 1992) and
a three-dimensional model (Aroyan, 1996; Aroyan et aI., Chapter 10) to
study sound propagation in a dolphin's head, Aroyan has shown that
echolocation signals most likely are generated in the nasal system and are
channeled into the water by the melon. Cranford (Chapter 3) has also collected evidence that suggests that echolocation signals are most likely produced in the nasal system at the location of the monkey lips-dorsal bursae
complex just beneath of the blow hole.
Echolocation signals seem to fall into two general categories. The first
category is associated with signals produced by dolphins that can also
produce whistles. Whistles are frequency-modulated signals of relatively
long duration (1 to 3s) and are described by Tyack and Clark in Chapter 4.
The properties of signals produced by dolphins that can also whistle will be