132
T.w. Cranford
potential flaw. Clay and Medwin (1977) note the caustic effect that cavitation bubbles have on nearby objects such as ship propeller blades. If dolphins actually produce cavitation bubbles, we must question how adjacent
tissues remain resilient in the face of repeated physical and chemical insults
brought about by these extreme conditions (Crum and Fowlkes 1986;
Maddox 1993). On the other hand, it may not be necessary to invoke or
unleash such powerful forces to produce the loudest clicks yet recorded
(Aroyan 1990, appendix 1.2).
Generally, and in order to address impedance-matching concerns, a
cavitation-based sound generation mechanism would require that minute
bubbles form in the fluid that bathes the phonic lips. Ostensibly, sounds
would be generated as these bubbles collapse. Copious amounts of serous
fluid are produced in a healthy animal from a mass of glandular tissue
located just below the phonic lips (Evans and Maderson 1973). These
fluids might also protect the tissues from the insults of a cavitation mechanism. Bubble formation should occur in the low-pressure phase created
as the lips are pulled or pushed apart. For every repetition of the parting
lips, two seemingly extraordinary conditions would need to be met.
The bubbles need to be roughly the same size and collapse more or less
simultaneously. Bubble size will influence acoustic frequency composition
and the degree of synchronicity in collapse will determine signal duration. It might be possible that uniform bubble size is determined by the
regular size of furrows in the nasal mucosa (Cranford et al. 1996; Cranford
et al. 1997) and that synchronous collapse could result from consistent
geometry and pressure relationships. These possibilities remain to be
substantiated.
At this point, our limited knowledge of sound generation physiology does
not permit us to confirm or reject the notion of a cavitation-based sound
generation mechanism. Until there is some direct evidence upon which to
base such a decision, it seems prudent to take a skeptical view of a cavitation mechanism that requires a particular set of extraordinary circumstances in favor of a simple pneumatic mechanism.
4.3 Pneumatic Mechanism
It would seem that the most parsimonious explanation for the mechanism
of odontocete sonar signal generation is one that requires the least complex
or extreme conditions. A simple "slapping" mechanism is just such a Candidate (Cranford et al. 1987). It has the appeal of efficient energy transfer
from a compressed-air power source to the requisite tissue-borne vibrations, without the need for rapid-reciprocating muscle action. This type of
mechanism waS first proposed by EG. Wood (Handley 1966, p. 66) and has
since been reinforced by the observations of Norris (1969, p. 406), Amundin
and Andersen (1983), Ridgway and colleagues (1980), and Ridgway and
Carder (1988).
T.w. Cranford
potential flaw. Clay and Medwin (1977) note the caustic effect that cavitation bubbles have on nearby objects such as ship propeller blades. If dolphins actually produce cavitation bubbles, we must question how adjacent
tissues remain resilient in the face of repeated physical and chemical insults
brought about by these extreme conditions (Crum and Fowlkes 1986;
Maddox 1993). On the other hand, it may not be necessary to invoke or
unleash such powerful forces to produce the loudest clicks yet recorded
(Aroyan 1990, appendix 1.2).
Generally, and in order to address impedance-matching concerns, a
cavitation-based sound generation mechanism would require that minute
bubbles form in the fluid that bathes the phonic lips. Ostensibly, sounds
would be generated as these bubbles collapse. Copious amounts of serous
fluid are produced in a healthy animal from a mass of glandular tissue
located just below the phonic lips (Evans and Maderson 1973). These
fluids might also protect the tissues from the insults of a cavitation mechanism. Bubble formation should occur in the low-pressure phase created
as the lips are pulled or pushed apart. For every repetition of the parting
lips, two seemingly extraordinary conditions would need to be met.
The bubbles need to be roughly the same size and collapse more or less
simultaneously. Bubble size will influence acoustic frequency composition
and the degree of synchronicity in collapse will determine signal duration. It might be possible that uniform bubble size is determined by the
regular size of furrows in the nasal mucosa (Cranford et al. 1996; Cranford
et al. 1997) and that synchronous collapse could result from consistent
geometry and pressure relationships. These possibilities remain to be
substantiated.
At this point, our limited knowledge of sound generation physiology does
not permit us to confirm or reject the notion of a cavitation-based sound
generation mechanism. Until there is some direct evidence upon which to
base such a decision, it seems prudent to take a skeptical view of a cavitation mechanism that requires a particular set of extraordinary circumstances in favor of a simple pneumatic mechanism.
4.3 Pneumatic Mechanism
It would seem that the most parsimonious explanation for the mechanism
of odontocete sonar signal generation is one that requires the least complex
or extreme conditions. A simple "slapping" mechanism is just such a Candidate (Cranford et al. 1987). It has the appeal of efficient energy transfer
from a compressed-air power source to the requisite tissue-borne vibrations, without the need for rapid-reciprocating muscle action. This type of
mechanism waS first proposed by EG. Wood (Handley 1966, p. 66) and has
since been reinforced by the observations of Norris (1969, p. 406), Amundin
and Andersen (1983), Ridgway and colleagues (1980), and Ridgway and
Carder (1988).