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T.W. Cranford
fore, these events are unequivocally coupled. Apparently, bottlenose dolphins can produce acoustic pulses from both pairs of phonic lips or, possibly, from multiple locations along each of them. This is the first direct
evidence that dolphins possess at least two sonar signal generators.
These results have implications for the number of sonar sources in the
dolphin head and the role of soft tissue asymmetry. A single endoscope was
used in these experiments, thereby preventing examination of multiple
locations simultaneously. Even so, it is clear that dolphins produced pulses
using the phonic lips on the left and/or the right sides in different trials. It
is reasonable, though not certain, that they can actuate both sides simultaneously. After careful consideration of the evidence, it is possible that
the two sides can be pressurized independently or simultaneously, but this
specific conclusion must be postponed until simultaneous pressure records
can be gathered.
Since we can be certain there are at least two pulse generators, one in
the left nasal passage and another in the right, we can begin to think about
which factors affect the signal characteristics. These two pulse generation
complexes will produce signals whose characteristics are determined by
factors like size, shape, material composition, stiffness, tension, damping,
air pressure, and perhaps others. Some of these factors are the same for
both pulse generators (e.g., shape, material composition, and material stiffness) and others are probably under control of the animal (air pressure,
tension, and damping). One factor that is stable, yet different for each side,
is size.
For many modern delphinids, like T. truncatus, the sound generation
complex on the right side is twice the size of the sound generation complex
on the left (Cranford 1992a, 1992b). The size of a sound source is apparently correlated with the dominant frequency it produces. These moderately
asymmetric delphinids can produce pulses with principal (tissue-borne)
spectral peaks at two different frequencies (Au et al. 1995; Cranford et al.
1996). Over the course of a pulse series in a single click train, dolphins may
drop one or the other peak or change the relative amounts of energy in
each (Sigurdson 1997a). These animals apparently also have the ability to
produce two pulse rhythms simultaneously, where the pulses comprising
each rhythm are spectrally distinct (Cranford, personal observations; W.E.
Evans, personal communication). One simple explanation for these observations is that dolphins can change the spectral emphasis by simply activating, deactivating, or damping the process at one, the other, or both (left
and right) sound sources. They may also have the ability to control the mode
of vibration of each sound generation complex separately, even if both sides
are pressurized together. On the other hand, such a simplified explanation
may violate Professor Norris' admonition to refrain from "underestimating
the refinements of animal adaptations" (Norris 1964, p. 324). At the very
least, an explanation for the mechanism of producing multiple stable peaks
in the spectrum of a click deserves exploration.
T.W. Cranford
fore, these events are unequivocally coupled. Apparently, bottlenose dolphins can produce acoustic pulses from both pairs of phonic lips or, possibly, from multiple locations along each of them. This is the first direct
evidence that dolphins possess at least two sonar signal generators.
These results have implications for the number of sonar sources in the
dolphin head and the role of soft tissue asymmetry. A single endoscope was
used in these experiments, thereby preventing examination of multiple
locations simultaneously. Even so, it is clear that dolphins produced pulses
using the phonic lips on the left and/or the right sides in different trials. It
is reasonable, though not certain, that they can actuate both sides simultaneously. After careful consideration of the evidence, it is possible that
the two sides can be pressurized independently or simultaneously, but this
specific conclusion must be postponed until simultaneous pressure records
can be gathered.
Since we can be certain there are at least two pulse generators, one in
the left nasal passage and another in the right, we can begin to think about
which factors affect the signal characteristics. These two pulse generation
complexes will produce signals whose characteristics are determined by
factors like size, shape, material composition, stiffness, tension, damping,
air pressure, and perhaps others. Some of these factors are the same for
both pulse generators (e.g., shape, material composition, and material stiffness) and others are probably under control of the animal (air pressure,
tension, and damping). One factor that is stable, yet different for each side,
is size.
For many modern delphinids, like T. truncatus, the sound generation
complex on the right side is twice the size of the sound generation complex
on the left (Cranford 1992a, 1992b). The size of a sound source is apparently correlated with the dominant frequency it produces. These moderately
asymmetric delphinids can produce pulses with principal (tissue-borne)
spectral peaks at two different frequencies (Au et al. 1995; Cranford et al.
1996). Over the course of a pulse series in a single click train, dolphins may
drop one or the other peak or change the relative amounts of energy in
each (Sigurdson 1997a). These animals apparently also have the ability to
produce two pulse rhythms simultaneously, where the pulses comprising
each rhythm are spectrally distinct (Cranford, personal observations; W.E.
Evans, personal communication). One simple explanation for these observations is that dolphins can change the spectral emphasis by simply activating, deactivating, or damping the process at one, the other, or both (left
and right) sound sources. They may also have the ability to control the mode
of vibration of each sound generation complex separately, even if both sides
are pressurized together. On the other hand, such a simplified explanation
may violate Professor Norris' admonition to refrain from "underestimating
the refinements of animal adaptations" (Norris 1964, p. 324). At the very
least, an explanation for the mechanism of producing multiple stable peaks
in the spectrum of a click deserves exploration.