3. Impulse Sound Sources
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latex blindfold
2
for a dolphin that aquatic echolocation was unequivocally
demonstrated (Norris et al.1961). In that paper, they also suggested a novel
pathway for sounds leaving or entering the dolphin's head. Their suggestion led to a series of scientific experiments and sparked a debate that lasted
for more than two decades. The essence of the debate is whether echolocation sounds were being produced in the larynx or the nose. At first, the
question about the site of sound generation appeared to be a simple one.
2.1 Nasal Versus Laryngeal Sound Source Debate
It was assumed, a priori, that the larynx was the source of some or all odontocete sounds (Schevill and Lawrence 1956). The larynx seemed a logical
site for sound production because most other mammals, including many
echolocating bats, produce sound from the larynx. In addition, the odontocete larynx is structurally complex (Reidenberg and Laitman 1988; Harrison 1995) and heavily muscled (Lawrence and Schevill 1965; Green et al.
1980). It also simplified matters somewhat that there is more overt morphological similarity in laryngeal structure across the odontocete suborder
than can be found in their foreheads. The "laryngeal phonation hypothesis"
was put forward largely based on anatomy (Purves 1966; Blevins and
Parkins 1973; Purves and Pilleri 1973; Schenkkan 1973).
An odontocete larynx is generally larger than that of a similarly sized
human. The larynx is unusual in shape, characterized by the presence of an
intranarial epiglottic spout (also called the "goosebeak") that is formed by
the elongation of the cuneiform and epiglottal cartilages. The epiglottic
spout is held in the basicranial space by a powerful sphincter-like striated
muscle complex, the palatopharyngeal muscles. Purves and Pilleri (1983)
and Pilleri (1990) reviewed the evidence for how this curious larynx might
produce sound. They reasoned that the larynx was the most parsimonious
choice for the source of odontocete echolocation signals since most
mammals use laryngeal phonation, and because of its unique structure and
heavily muscled appearance.
Consequently, there was understandable skepticism when Norris and colleagues (1961) proposed that the bottlenose dolphin either sent or received
sounds (or both) through its bulbous forehead. In this series of experiments,
Norris and his colleagues listened to an echolocating dolphin through two
spatially separated hydrophones and compared the sounds received at each
as the animal swam toward one or the other. The sounds they heard were
loudest when the dolphin pointed its rostrum at the hydrophone. They
suggest, "the use of a beam or cone of sound emanating from the forehead
2 A vivid and amusing account of this and other discoveries made during the first
years of cetology can be found in the popular book, The Porpoise Watcher by Dr.
Kenneth S. Norris (1974).
119
latex blindfold
2
for a dolphin that aquatic echolocation was unequivocally
demonstrated (Norris et al.1961). In that paper, they also suggested a novel
pathway for sounds leaving or entering the dolphin's head. Their suggestion led to a series of scientific experiments and sparked a debate that lasted
for more than two decades. The essence of the debate is whether echolocation sounds were being produced in the larynx or the nose. At first, the
question about the site of sound generation appeared to be a simple one.
2.1 Nasal Versus Laryngeal Sound Source Debate
It was assumed, a priori, that the larynx was the source of some or all odontocete sounds (Schevill and Lawrence 1956). The larynx seemed a logical
site for sound production because most other mammals, including many
echolocating bats, produce sound from the larynx. In addition, the odontocete larynx is structurally complex (Reidenberg and Laitman 1988; Harrison 1995) and heavily muscled (Lawrence and Schevill 1965; Green et al.
1980). It also simplified matters somewhat that there is more overt morphological similarity in laryngeal structure across the odontocete suborder
than can be found in their foreheads. The "laryngeal phonation hypothesis"
was put forward largely based on anatomy (Purves 1966; Blevins and
Parkins 1973; Purves and Pilleri 1973; Schenkkan 1973).
An odontocete larynx is generally larger than that of a similarly sized
human. The larynx is unusual in shape, characterized by the presence of an
intranarial epiglottic spout (also called the "goosebeak") that is formed by
the elongation of the cuneiform and epiglottal cartilages. The epiglottic
spout is held in the basicranial space by a powerful sphincter-like striated
muscle complex, the palatopharyngeal muscles. Purves and Pilleri (1983)
and Pilleri (1990) reviewed the evidence for how this curious larynx might
produce sound. They reasoned that the larynx was the most parsimonious
choice for the source of odontocete echolocation signals since most
mammals use laryngeal phonation, and because of its unique structure and
heavily muscled appearance.
Consequently, there was understandable skepticism when Norris and colleagues (1961) proposed that the bottlenose dolphin either sent or received
sounds (or both) through its bulbous forehead. In this series of experiments,
Norris and his colleagues listened to an echolocating dolphin through two
spatially separated hydrophones and compared the sounds received at each
as the animal swam toward one or the other. The sounds they heard were
loudest when the dolphin pointed its rostrum at the hydrophone. They
suggest, "the use of a beam or cone of sound emanating from the forehead
2 A vivid and amusing account of this and other discoveries made during the first
years of cetology can be found in the popular book, The Porpoise Watcher by Dr.
Kenneth S. Norris (1974).