4. Acoustic Communication in Whales and Dolphins
207
kilometers. The fin whale vocal characteristics (very low frequency,
high intensity, simple, and redundant) are all well adapted for long-range
transmission and detection through a deep ocean habitat, but poorly
adapted for a shallow water habitat. This supports the conclusion that
these signals function for long-range communication in this pelagic
species.
Some bioacousticians have also emphasized the potential utility of these
low-frequency signals for use in detecting echoes from the sea floor, echoes
that might be useful for orientation as these animals navigate through the
deep sea (Norris 1966, 1969; Payne and Webb 1971; Thompson et al. 1979;
Clark 1994a; Clark and Ellison 1997). The long wavelengths of these signals
are not useful for detecting small objects, but do reflect off of large bathymetric features. As was discussed in the section on attributes of fish as
targets, the amount of sound energy reflecting off a rigid object drops
rapidly if the circumference of the object is less than the wavelength of the
sound. The speed of sound in seawater, denoted by the variable c, is about
1,500m/s. The relationship between wavelength, A, speed, c, and frequency,
f, is: A = c/f. This means that the wavelength in seawater of a 15 Hz whale
call would be 100m; that of a 150Hz call, 10m (Table 4.1). The loud, lowfrequency calls of whales would be relatively well suited to detecting and
resolving bathymetric features with scales larger than 100m or so. If the
whale sounds were not directional, then the whale might receive a series of
echo returns from the sea floor and bathymetric features such as seamounts,
the continental slope, or islands. For example, Figure 4.16 shows echoes at
4 and 5s from the fin whale call produced between 1 and 2 s. Clark and
Ellison (1997) present results from propagation models showing that a blue
whale vocalizing 650km away from Bermuda would have been able to hear
an echo reflecting from the Bermuda seamount in the quiet ocean before
the era of motorized ships. It would take more than 14 min for this sound
to reach Bermuda, reflect, and make the return trip to the whale. If the
whale were able to separate the superposition of echo returns from different bearings and ranges, it might be able to form some sort of bathymetric
map or image from the directional pattern of echoes. If a whale were
able to update bathymetric maps from successive sounds, this might be
integrated with vestibular input for acoustic orientation. Such a hypothetical orientation mechanism would make quite specific demands upon the
auditory system. The evolution of such a system might well select for
specialized auditory processing quite different from the high-frequency
echolocation system of dolphins. For example, the neural processing for
range estimation in bats operates on delay times of milliseconds, but
acoustic orientation in the sea could require delay times of minutes. We
know that the low-frequency calls of whales often do create detectable
echoes from the sea floor, but nothing is known about whether whales
use this information about their environment. The challenge is to design
experiments capable of testing whether and how they do so.
207
kilometers. The fin whale vocal characteristics (very low frequency,
high intensity, simple, and redundant) are all well adapted for long-range
transmission and detection through a deep ocean habitat, but poorly
adapted for a shallow water habitat. This supports the conclusion that
these signals function for long-range communication in this pelagic
species.
Some bioacousticians have also emphasized the potential utility of these
low-frequency signals for use in detecting echoes from the sea floor, echoes
that might be useful for orientation as these animals navigate through the
deep sea (Norris 1966, 1969; Payne and Webb 1971; Thompson et al. 1979;
Clark 1994a; Clark and Ellison 1997). The long wavelengths of these signals
are not useful for detecting small objects, but do reflect off of large bathymetric features. As was discussed in the section on attributes of fish as
targets, the amount of sound energy reflecting off a rigid object drops
rapidly if the circumference of the object is less than the wavelength of the
sound. The speed of sound in seawater, denoted by the variable c, is about
1,500m/s. The relationship between wavelength, A, speed, c, and frequency,
f, is: A = c/f. This means that the wavelength in seawater of a 15 Hz whale
call would be 100m; that of a 150Hz call, 10m (Table 4.1). The loud, lowfrequency calls of whales would be relatively well suited to detecting and
resolving bathymetric features with scales larger than 100m or so. If the
whale sounds were not directional, then the whale might receive a series of
echo returns from the sea floor and bathymetric features such as seamounts,
the continental slope, or islands. For example, Figure 4.16 shows echoes at
4 and 5s from the fin whale call produced between 1 and 2 s. Clark and
Ellison (1997) present results from propagation models showing that a blue
whale vocalizing 650km away from Bermuda would have been able to hear
an echo reflecting from the Bermuda seamount in the quiet ocean before
the era of motorized ships. It would take more than 14 min for this sound
to reach Bermuda, reflect, and make the return trip to the whale. If the
whale were able to separate the superposition of echo returns from different bearings and ranges, it might be able to form some sort of bathymetric
map or image from the directional pattern of echoes. If a whale were
able to update bathymetric maps from successive sounds, this might be
integrated with vestibular input for acoustic orientation. Such a hypothetical orientation mechanism would make quite specific demands upon the
auditory system. The evolution of such a system might well select for
specialized auditory processing quite different from the high-frequency
echolocation system of dolphins. For example, the neural processing for
range estimation in bats operates on delay times of milliseconds, but
acoustic orientation in the sea could require delay times of minutes. We
know that the low-frequency calls of whales often do create detectable
echoes from the sea floor, but nothing is known about whether whales
use this information about their environment. The challenge is to design
experiments capable of testing whether and how they do so.