3. Impulse Sound Sources
135
A similar simplistic interpretation might explain why we observe only a
single (tissue-borne) spectral peak in the echolocation pulses of particular
delphinoids (e.g., the porpoises and Cephalorhynchus spp.) as discussed
earlier. These odontocetes also possess bilateral sound generation complexes, but they are the same size. While the uniform size of these putative
bilateral sonar signal generators may explain the single-peak spectral
characteristics of the pulses, the causes of their long-duration, polycyclic
structure needs additional examination. There are some intriguing structural correlates, like symmetrical bilateral placement of the sources, or their
geometric relationships with other elements (Cranford 1992a), that cry out
for inquiry by computer simulations. If the peculiar (directional) asymmetry in odontocete foreheads is related to the spectral quality of the echolocation pulses, it raises interesting questions about delphinoid feeding
ecology and evolution (Cranford 1992a, b, 1999).
Aside from the phonic lips, the endoscope work did not show other
structures in the airways vibrating synchronously with each acoustic pulse
generation event, although some interesting collateral movements were
discovered (Cranford et al. 1997). For example, the palatopharyngeal
muscle complex apparently compresses the air column that drives the sound
generation system, as proposed some years ago by Norris (1969). During
some phases of the sound generation process, a vapor or mist of droplets
forms, often resembling snow flurries, in the nasopharynx and allows
visualization of airflow. Air flows dorsally just before, and during, sound
production (whistles and pulses).
Perhaps the most intriguing endoscopic observations are those concerning the dynamic parallel furrows that form in the surface of the nasal
mucosa, between the nasal plugs and the phonic lips. The direction and
depth of the furrows can be changed instantaneously, apparently by muscle
action. These furrows may function to direct the flow of air precisely across
the phonic lips during sound generation and could explain how these
animals can dynamically alter the pattern of airflow supplying the sound
generation apparatus. The underlying structural basis for this behavior is
currently unknown.
These endoscopic observations largely settle the long-standing debate
over the site of biosonar signal generation in odontocetes. They also open
a vista of potential avenues for future investigations.
5. Signal Propagation
Any deliberation over odontocete pulse generation mechanisms is incomplete without some discussion of the effects of sound propagation pathways
because the original or initial tissue-borne signal, produced by the generator(s), encounters various tissue interfaces and impedance gradients as it
propagates out of the head. These interactions between the initial signal
135
A similar simplistic interpretation might explain why we observe only a
single (tissue-borne) spectral peak in the echolocation pulses of particular
delphinoids (e.g., the porpoises and Cephalorhynchus spp.) as discussed
earlier. These odontocetes also possess bilateral sound generation complexes, but they are the same size. While the uniform size of these putative
bilateral sonar signal generators may explain the single-peak spectral
characteristics of the pulses, the causes of their long-duration, polycyclic
structure needs additional examination. There are some intriguing structural correlates, like symmetrical bilateral placement of the sources, or their
geometric relationships with other elements (Cranford 1992a), that cry out
for inquiry by computer simulations. If the peculiar (directional) asymmetry in odontocete foreheads is related to the spectral quality of the echolocation pulses, it raises interesting questions about delphinoid feeding
ecology and evolution (Cranford 1992a, b, 1999).
Aside from the phonic lips, the endoscope work did not show other
structures in the airways vibrating synchronously with each acoustic pulse
generation event, although some interesting collateral movements were
discovered (Cranford et al. 1997). For example, the palatopharyngeal
muscle complex apparently compresses the air column that drives the sound
generation system, as proposed some years ago by Norris (1969). During
some phases of the sound generation process, a vapor or mist of droplets
forms, often resembling snow flurries, in the nasopharynx and allows
visualization of airflow. Air flows dorsally just before, and during, sound
production (whistles and pulses).
Perhaps the most intriguing endoscopic observations are those concerning the dynamic parallel furrows that form in the surface of the nasal
mucosa, between the nasal plugs and the phonic lips. The direction and
depth of the furrows can be changed instantaneously, apparently by muscle
action. These furrows may function to direct the flow of air precisely across
the phonic lips during sound generation and could explain how these
animals can dynamically alter the pattern of airflow supplying the sound
generation apparatus. The underlying structural basis for this behavior is
currently unknown.
These endoscopic observations largely settle the long-standing debate
over the site of biosonar signal generation in odontocetes. They also open
a vista of potential avenues for future investigations.
5. Signal Propagation
Any deliberation over odontocete pulse generation mechanisms is incomplete without some discussion of the effects of sound propagation pathways
because the original or initial tissue-borne signal, produced by the generator(s), encounters various tissue interfaces and impedance gradients as it
propagates out of the head. These interactions between the initial signal