4. Acoustic Communication in Whales and Dolphins
169
seawater is usually around 1,500m/sec. If a dolphin echolocated to detect
rigid spherical targets with a radius a of 0.5 cm, it would do well to use a
frequency f = C/21Ul = 1,500/(21t x 0.005) = near 50kHz. As you can see
from the right-hand side of Figure 4.3, higher frequencies (higher ka) than
this would still be effective sonar signals for a rigid target, but frequencies
below f would show a sharp decrease in effectiveness with decreasing
frequency. Fifty kilohertz is in fact on the low end of the typical frequency
range of dolphin echolocation clicks (Au 1993). The echolocation signals of
dolphins include energy up to about 150kHz, which would be well suited
to detecting spherical targets with radii as small as 1.5 mm.
While the high-frequency sounds of dolphin echolocation appear well
suited to Rayleigh scattering from small rigid targets, many biologically
important targets may reflect sound energy at much lower frequencies.
Marine mammals and many fish species have gas-filled organs that may
provide another important kind of sonar target with sonar characteristics
very different from a rigid sphere and quite similar to the bubble shown on
the left side of Figure 4.3. Many fish species that are prey for cetaceans have
gas-filled swim bladders that are used for buoyancy regulation, as well as
sound production and reception. The resonant frequencies of these swim
bladders are surprisingly low given the size of the fish. For example, Batzler
and Pickwell (1970) reported that a small anchovy at one atmosphere of
pressure had a resonant frequency of 1,275 Hz. The lungs of marine
mammals are larger, and presumably have resonance frequencies that are
even lower. These low-frequency echo returns may be better for longerrange detection than the high-frequency Rayleigh scattering returns typically modeled for dolphin sonar. In addition, these gas-filled targets
may produce a low-frequency echo of use to animals that lack the highfrequency sonar of dolphins.
If cetaceans produced echolocation signals designed to match the lowfrequency resonance of fish with swim bladders, then signals with lower frequencies than those typically associated with echolocation might be used
to detect fishes with air-filled bladders. It would be well worth more detailed
effort to compare the spectra of sounds produced by cetaceans against the
target spectra of their prey. Delphinids produce high-frequency echolocation clicks while feeding, but are also reported to produce a variety of
pulsed sounds with energy below 5 to 10 kHz. These have typically been
considered as communicative signals (e.g., Caldwell and Caldwell 1967), but
Marten et al. (1988) suggest that they are also associated with feeding in
bottlenose dolphins and killer whales. Many large whales also feed on small
schooling fish such as anchovy, and many of these whales produce lowfrequency sounds during the feeding season, but little is known about the
function of these sounds. We should not rule out the possibility that even
these species might have a less specialized ability to use lower-frequency
sounds not only to communicate, but also to detect these kinds of sonar
targets.
169
seawater is usually around 1,500m/sec. If a dolphin echolocated to detect
rigid spherical targets with a radius a of 0.5 cm, it would do well to use a
frequency f = C/21Ul = 1,500/(21t x 0.005) = near 50kHz. As you can see
from the right-hand side of Figure 4.3, higher frequencies (higher ka) than
this would still be effective sonar signals for a rigid target, but frequencies
below f would show a sharp decrease in effectiveness with decreasing
frequency. Fifty kilohertz is in fact on the low end of the typical frequency
range of dolphin echolocation clicks (Au 1993). The echolocation signals of
dolphins include energy up to about 150kHz, which would be well suited
to detecting spherical targets with radii as small as 1.5 mm.
While the high-frequency sounds of dolphin echolocation appear well
suited to Rayleigh scattering from small rigid targets, many biologically
important targets may reflect sound energy at much lower frequencies.
Marine mammals and many fish species have gas-filled organs that may
provide another important kind of sonar target with sonar characteristics
very different from a rigid sphere and quite similar to the bubble shown on
the left side of Figure 4.3. Many fish species that are prey for cetaceans have
gas-filled swim bladders that are used for buoyancy regulation, as well as
sound production and reception. The resonant frequencies of these swim
bladders are surprisingly low given the size of the fish. For example, Batzler
and Pickwell (1970) reported that a small anchovy at one atmosphere of
pressure had a resonant frequency of 1,275 Hz. The lungs of marine
mammals are larger, and presumably have resonance frequencies that are
even lower. These low-frequency echo returns may be better for longerrange detection than the high-frequency Rayleigh scattering returns typically modeled for dolphin sonar. In addition, these gas-filled targets
may produce a low-frequency echo of use to animals that lack the highfrequency sonar of dolphins.
If cetaceans produced echolocation signals designed to match the lowfrequency resonance of fish with swim bladders, then signals with lower frequencies than those typically associated with echolocation might be used
to detect fishes with air-filled bladders. It would be well worth more detailed
effort to compare the spectra of sounds produced by cetaceans against the
target spectra of their prey. Delphinids produce high-frequency echolocation clicks while feeding, but are also reported to produce a variety of
pulsed sounds with energy below 5 to 10 kHz. These have typically been
considered as communicative signals (e.g., Caldwell and Caldwell 1967), but
Marten et al. (1988) suggest that they are also associated with feeding in
bottlenose dolphins and killer whales. Many large whales also feed on small
schooling fish such as anchovy, and many of these whales produce lowfrequency sounds during the feeding season, but little is known about the
function of these sounds. We should not rule out the possibility that even
these species might have a less specialized ability to use lower-frequency
sounds not only to communicate, but also to detect these kinds of sonar
targets.