126
T.w. Cranford
3.1 Sound Generation Anatomy
Nineteenth-century authors wrote at length about cetacean anatomy and
authors from the early part of the twentieth century have since added to
our knowledge about how odontocete heads are constructed (see review in
Mead 1975). Only after dolphins were suspected of echolocation did combined studies of the structure and function of odontocete cephalic anatomy
flourish. It is clear that the anatomy in the forehead of odontocetes has been
dramatically reshaped to accommodate the biosonar function. We can be
certain of this because mysticetes also lead aquatic lives yet their foreheads
are much less complex, even though their foreheads have been rearranged
somewhat to allow them to breath from the top of their heads and suspend
a large feeding apparatus from the rostrum.
Fleischer (1976) and Wiirsig (1989) suggest that the ability to produce
impulse sounds and receive and interpret reflected echoes from targets contributed significantly to the radiation of the toothed whales. If all odontocetes are using homologous structures (and presumably a functionally
similar mechanism) to generate sonar signals (Cranford et al. 1996), then
we may assume that the structural precursors and rudimentary ability probably existed in an early common ancestor, almost certainly as far back as
the Oligocene (Fordyce and Barnes 1994). Fleischer's study (1976) of
fossil odontocete cochleas suggests that echolocation probably existed in
the earliest odontocetes for which he had specimens, the Oligocene
squalodontoids.
Modern anatomic studies have used traditional dissection/description
techniques (Lawrence and Schevill 1956; Schenkkan 1973; Mead 1975;
Heyning 1989). The introduction of modern medical imaging and computer
graphic technology to study the intact geometry of the odontocete sonar
apparatus began in 1988. Medical remote imaging techniques (X-ray,
computed tomography, and magnetic resonance scanners) and traditional
methods (histology, dissection, cryosectioning) were combined to describe
the craniofacial origins of biosonar signals in toothed whales (Cranford
1988, 1992a, 1999). A published account of the anatomic geometry in the
odontocete forehead and a proposal for their sonar signal generation site
can be found in Cranford, Amundin, and Norris (1996). The strength of our
proposal is that the anatomic site for sound generation includes structures
that are homologous in all odontocetes and thereby "unifies" the sound
generation hypothesis across the entire suborder (Fig. 3.1).
Since that time, two tests have verified the sound source location in dolphins and thereby strengthened the "unified hypothesis." The first test simulated sound production in a computer-based two-dimensional numerical
model of a dolphin's head (Aroyan et al. 1992). In the second test of the
unified hypothesis, high-speed video endoscopy was used to observe the
process of sound generation in live, phonating dolphins (Cranford et al.
1997). This technique substantially answered a question that has perplexed
T.w. Cranford
3.1 Sound Generation Anatomy
Nineteenth-century authors wrote at length about cetacean anatomy and
authors from the early part of the twentieth century have since added to
our knowledge about how odontocete heads are constructed (see review in
Mead 1975). Only after dolphins were suspected of echolocation did combined studies of the structure and function of odontocete cephalic anatomy
flourish. It is clear that the anatomy in the forehead of odontocetes has been
dramatically reshaped to accommodate the biosonar function. We can be
certain of this because mysticetes also lead aquatic lives yet their foreheads
are much less complex, even though their foreheads have been rearranged
somewhat to allow them to breath from the top of their heads and suspend
a large feeding apparatus from the rostrum.
Fleischer (1976) and Wiirsig (1989) suggest that the ability to produce
impulse sounds and receive and interpret reflected echoes from targets contributed significantly to the radiation of the toothed whales. If all odontocetes are using homologous structures (and presumably a functionally
similar mechanism) to generate sonar signals (Cranford et al. 1996), then
we may assume that the structural precursors and rudimentary ability probably existed in an early common ancestor, almost certainly as far back as
the Oligocene (Fordyce and Barnes 1994). Fleischer's study (1976) of
fossil odontocete cochleas suggests that echolocation probably existed in
the earliest odontocetes for which he had specimens, the Oligocene
squalodontoids.
Modern anatomic studies have used traditional dissection/description
techniques (Lawrence and Schevill 1956; Schenkkan 1973; Mead 1975;
Heyning 1989). The introduction of modern medical imaging and computer
graphic technology to study the intact geometry of the odontocete sonar
apparatus began in 1988. Medical remote imaging techniques (X-ray,
computed tomography, and magnetic resonance scanners) and traditional
methods (histology, dissection, cryosectioning) were combined to describe
the craniofacial origins of biosonar signals in toothed whales (Cranford
1988, 1992a, 1999). A published account of the anatomic geometry in the
odontocete forehead and a proposal for their sonar signal generation site
can be found in Cranford, Amundin, and Norris (1996). The strength of our
proposal is that the anatomic site for sound generation includes structures
that are homologous in all odontocetes and thereby "unifies" the sound
generation hypothesis across the entire suborder (Fig. 3.1).
Since that time, two tests have verified the sound source location in dolphins and thereby strengthened the "unified hypothesis." The first test simulated sound production in a computer-based two-dimensional numerical
model of a dolphin's head (Aroyan et al. 1992). In the second test of the
unified hypothesis, high-speed video endoscopy was used to observe the
process of sound generation in live, phonating dolphins (Cranford et al.
1997). This technique substantially answered a question that has perplexed