3. Impulse Sound Sources
143
1999). Sperm whales and their allies are so asymmetric that only one sound
generation complex (right side) can be recognized. This single complex
is probably homologous to the right side in other odontocetes (Cranford
et al. 1996).
Endoscopic observations (Cranford et al. 1997) have confirmed that T.
truncatus possesses at least two click generators (each pair of phonic lips),
and acoustic recordings suggest that they can activate them simultaneously
or independently (Sigurdson 1997a). In fact, the activation of multiple pulse
generators is perhaps the only means for explaining reports of very high
pulse repetition rates, some in excess of a thousand per second. We can tentatively extrapolate this ability to all nonphyseterid odontocetes because
they possess bilateral pairs of phonic lips. Even though the anatomic and
physiologic evidence confirms the existence of at least two sonar sources,
the apparent fine control over the movement of air seen with the endoscope
makes it conceivable that, for example, multiple sites may be activated
along each pair of phonic lips. There is one report of at least three simultaneous click generators in a single dolphin (Markov and Ostrovskaya
1990), but this has yet to be independently corroborated.
In light of this existing evidence, it is curious that the acoustic simulation
work reported by Aroyan (Chapter 10) indicates a sound source only on
the right side and not on the left in his reverse propagation models. There
are a few possible explanations for these contrasting results. First, and
perhaps most plausible, is that some error is inherent in this model or
its initial conditions, assumptions, and/or estimates. There is at least some
error present, otherwise it is difficult to reconcile why the simulated source
location for the right side of the head is not coincident with the location
pinpointed by previous studies using the very same specimen of D. de/phis
(Aroyan et al. 1992; Cranford 1992a; Cranford et al. 1996). Another possible explanation concerns the largely incomplete nature of the acoustically
reflective air sac system. Aroyan admits that his model of the air sac system
is incomplete, but he apparently does not consider that this inadequacy
resulted in the absence of a simulated source on the left side. I disagree.
In fact, it may be that his results for the right side are robust primarily due
to skull geometry, particularly in the absence of a complete model of the
air sacs.
Finally, from a biological perspective, Aroyan's reasoning to explain the
lack of a simulated sound source on the left side is unsatisfying. He suggests that the moderate asymmetry in the nasal apparatus of this specimen
indicates atrophy or degeneration of the left side. Functional questions
concerning odontocete cephalic asymmetry are admittedly complex, but
this explanation seems to ignore the fact that there are a variety of delphinoids where asymmetry is relatively slight (Cranford 1992a, b; Cranford et
al. 1996). In addition, Aroyan vaguely suggests that the atrophy of the left
side is the result of the negative effects of interference. However, the causes
and effects of interference are complicated and may not be adequately
143
1999). Sperm whales and their allies are so asymmetric that only one sound
generation complex (right side) can be recognized. This single complex
is probably homologous to the right side in other odontocetes (Cranford
et al. 1996).
Endoscopic observations (Cranford et al. 1997) have confirmed that T.
truncatus possesses at least two click generators (each pair of phonic lips),
and acoustic recordings suggest that they can activate them simultaneously
or independently (Sigurdson 1997a). In fact, the activation of multiple pulse
generators is perhaps the only means for explaining reports of very high
pulse repetition rates, some in excess of a thousand per second. We can tentatively extrapolate this ability to all nonphyseterid odontocetes because
they possess bilateral pairs of phonic lips. Even though the anatomic and
physiologic evidence confirms the existence of at least two sonar sources,
the apparent fine control over the movement of air seen with the endoscope
makes it conceivable that, for example, multiple sites may be activated
along each pair of phonic lips. There is one report of at least three simultaneous click generators in a single dolphin (Markov and Ostrovskaya
1990), but this has yet to be independently corroborated.
In light of this existing evidence, it is curious that the acoustic simulation
work reported by Aroyan (Chapter 10) indicates a sound source only on
the right side and not on the left in his reverse propagation models. There
are a few possible explanations for these contrasting results. First, and
perhaps most plausible, is that some error is inherent in this model or
its initial conditions, assumptions, and/or estimates. There is at least some
error present, otherwise it is difficult to reconcile why the simulated source
location for the right side of the head is not coincident with the location
pinpointed by previous studies using the very same specimen of D. de/phis
(Aroyan et al. 1992; Cranford 1992a; Cranford et al. 1996). Another possible explanation concerns the largely incomplete nature of the acoustically
reflective air sac system. Aroyan admits that his model of the air sac system
is incomplete, but he apparently does not consider that this inadequacy
resulted in the absence of a simulated source on the left side. I disagree.
In fact, it may be that his results for the right side are robust primarily due
to skull geometry, particularly in the absence of a complete model of the
air sacs.
Finally, from a biological perspective, Aroyan's reasoning to explain the
lack of a simulated sound source on the left side is unsatisfying. He suggests that the moderate asymmetry in the nasal apparatus of this specimen
indicates atrophy or degeneration of the left side. Functional questions
concerning odontocete cephalic asymmetry are admittedly complex, but
this explanation seems to ignore the fact that there are a variety of delphinoids where asymmetry is relatively slight (Cranford 1992a, b; Cranford et
al. 1996). In addition, Aroyan vaguely suggests that the atrophy of the left
side is the result of the negative effects of interference. However, the causes
and effects of interference are complicated and may not be adequately