142
T.w. Cranford
composition. If there are two sound generation complexes in the foreheads
of nonphyseterid odontocetes, as suggested by Cranford and colleagues
(Cranford 1988, 1992a; Amundin and Cranford 1990; Cranford 1992a;
Cranford et al. 1996; Cranford et al. 1997), then it is important to know how
much flexibility there is in the frequency emphasis of each set of sound
sources. A few reports apparently suggest that some delphinoids can
operate one or both sound generators simultaneously, with considerable
control over the intensity at which they operate (Au et al.1985, 1995; Moore
and Pawloski 1990). The issue of the degree of control and flexibility over
frequency composition is in need of investigation.
For any particular odontocete sonar source, it is reasonable to suggest
that the oscillator needs to be near the phonic lips, which actuate the
process. Unfortunately, our ability to resolve the timing of events is currently coarse enough that an oscillator could conceivably be some distance
away. The most likely candidates for the high-frequency oscillators are the
small omnipresent fat bodies, the dorsal bursae or "bursae cantantes,"
(Cranford, Amundin, and Norris 1996) because of their material composition, size, and position. It is also conceivable that the cartilaginous blades,
which can be found embedded within the fibers of the blowhole ligament
just behind each posterior bursa, could instead be responsible for the brief
high-frequency vibrations. At this point, we are incapable of making this
distinction.
The recent development of an ultrasound endoscope (Chang and Flores
1994; Chang et al. 1994) could provide the technical means toward a solution to this question. The question might also be addressed (in smaller odontocetes) by physical manipulation of post-mortem tissues,4 or measurement
of the response to perturbation using a variety of tools, like shaking tables,
and tiny sensors built by engineers. The structure of the sperm whale's
nose may also shed light on the question of what is shaking, quaking, or
quivering. Since the fat-filled structures have grown to such enormous relative proportions in the sperm whale's nose (Cranford 1999), it seems reasonable and appropriate to begin by looking at these lipid structures for
insights into understanding the sound generation process.
6.1.2 The Number of Sonar Sources
The number of sonar sources in the odontocete head is an open question
and one that, when answered, will define certain limits of the system. Ultimately, the answer may depend upon the species in question. Examination
of the nasal anatomy in at least one specimen from every major lineage
within the Odontoceti confirms the existence of two pairs (left and right)
of phonic lips in every group except the Physeteridae (Cranford 1992a,
4It should be noted that working with post-mortem tissues is fraught with peril from
degenerative processes that begin shortly after death (Mackay 1984, 1988).
T.w. Cranford
composition. If there are two sound generation complexes in the foreheads
of nonphyseterid odontocetes, as suggested by Cranford and colleagues
(Cranford 1988, 1992a; Amundin and Cranford 1990; Cranford 1992a;
Cranford et al. 1996; Cranford et al. 1997), then it is important to know how
much flexibility there is in the frequency emphasis of each set of sound
sources. A few reports apparently suggest that some delphinoids can
operate one or both sound generators simultaneously, with considerable
control over the intensity at which they operate (Au et al.1985, 1995; Moore
and Pawloski 1990). The issue of the degree of control and flexibility over
frequency composition is in need of investigation.
For any particular odontocete sonar source, it is reasonable to suggest
that the oscillator needs to be near the phonic lips, which actuate the
process. Unfortunately, our ability to resolve the timing of events is currently coarse enough that an oscillator could conceivably be some distance
away. The most likely candidates for the high-frequency oscillators are the
small omnipresent fat bodies, the dorsal bursae or "bursae cantantes,"
(Cranford, Amundin, and Norris 1996) because of their material composition, size, and position. It is also conceivable that the cartilaginous blades,
which can be found embedded within the fibers of the blowhole ligament
just behind each posterior bursa, could instead be responsible for the brief
high-frequency vibrations. At this point, we are incapable of making this
distinction.
The recent development of an ultrasound endoscope (Chang and Flores
1994; Chang et al. 1994) could provide the technical means toward a solution to this question. The question might also be addressed (in smaller odontocetes) by physical manipulation of post-mortem tissues,4 or measurement
of the response to perturbation using a variety of tools, like shaking tables,
and tiny sensors built by engineers. The structure of the sperm whale's
nose may also shed light on the question of what is shaking, quaking, or
quivering. Since the fat-filled structures have grown to such enormous relative proportions in the sperm whale's nose (Cranford 1999), it seems reasonable and appropriate to begin by looking at these lipid structures for
insights into understanding the sound generation process.
6.1.2 The Number of Sonar Sources
The number of sonar sources in the odontocete head is an open question
and one that, when answered, will define certain limits of the system. Ultimately, the answer may depend upon the species in question. Examination
of the nasal anatomy in at least one specimen from every major lineage
within the Odontoceti confirms the existence of two pairs (left and right)
of phonic lips in every group except the Physeteridae (Cranford 1992a,
4It should be noted that working with post-mortem tissues is fraught with peril from
degenerative processes that begin shortly after death (Mackay 1984, 1988).