162
PL. Tyack and c.w. Clark
of ambient noise is much higher at low frequencies than high (Fig. 4.1), in
part because sound from distant noise sources travel so well at low
frequency. This means that an underwater sonar or acoustic communication
system faces tradeoffs for operating at different ranges. Low-frequency
sound propagates better over long range, but requires a higher received
level to be detected against the higher noise level compared to highfrequency sound. If animals relied upon narrow-band low-frequency sounds
to function over long ranges, then the detection range for signals would be
limited by the bandwidth at which the ear integrates sound energy. This
masking bandwidth has been well studied in mammals, and is often close
to a constant percentage of the center frequency, but becomes wider at low
frequencies (Nachtigall et aI., Chapter 8, this volume). In humans, one of
the best studied species, the equivalent rectangular bandwidth of the
masking band at 100Hz is less than 40Hz (Moore 1993). While this bandwidth is smallest at this low frequency, it is considerably larger as a percentage of the center frequency. Baleen whales appear to use relatively
narrow-band low frequency sounds for long-range communication. Their
inner ears appear to be specialized for low-frequency hearing (Ketten,
Chapter 2), but nothing is known about their auditory thresholds or
masking bandwidths. If the range at which they can detect a signal is limited
by the bandwidth of hearing, then selection might have favored specializations in the inner ear that would narrow the bandwidth over which lowfrequency sound energy was integrated.
2.3 Detection of Predators
When people think about how animals use sound, they are most likely to
think of echolocation and communication with conspecifics, but it is
often important for animals to intercept the sounds of other species, such
as predators or prey. The most common predator for many cetaceans is
another cetacean: the killer whale, Orcinus orca (Jefferson et ai. 1991).
Killer whales vocalize while foraging (Ford 1989; Barrett-Lennard et ai.
1996), making both clicks for echolocation and other calls used for communication. Killer whale echolocation clicks are reported to emphasize the
12 to 25 kHz range (Diercks et ai. 1971), while their communication calls
range from 500 Hz to 35 kHz. It can be a life-or-death matter for potential
prey to be able to detect killer whales early enough to be able to avoid
them. Many prey species of killer whales monitor for killer whale sounds
and respond strongly when they hear them. This has been clearly demonstrated in playback experiments. Gray whales migrating along the coast of
California respond to playback of killer whale calls by swimming rapidly
inshore into beds of kelp (Cummings and Thompson 1971; Malme et ai.
1983). Beluga whales (Delphinapterus leucas) , feeding on salmon in an
Alaskan river, also showed a strong avoidance response to playback of
killer whale calls (Fish and Vania 1971).
PL. Tyack and c.w. Clark
of ambient noise is much higher at low frequencies than high (Fig. 4.1), in
part because sound from distant noise sources travel so well at low
frequency. This means that an underwater sonar or acoustic communication
system faces tradeoffs for operating at different ranges. Low-frequency
sound propagates better over long range, but requires a higher received
level to be detected against the higher noise level compared to highfrequency sound. If animals relied upon narrow-band low-frequency sounds
to function over long ranges, then the detection range for signals would be
limited by the bandwidth at which the ear integrates sound energy. This
masking bandwidth has been well studied in mammals, and is often close
to a constant percentage of the center frequency, but becomes wider at low
frequencies (Nachtigall et aI., Chapter 8, this volume). In humans, one of
the best studied species, the equivalent rectangular bandwidth of the
masking band at 100Hz is less than 40Hz (Moore 1993). While this bandwidth is smallest at this low frequency, it is considerably larger as a percentage of the center frequency. Baleen whales appear to use relatively
narrow-band low frequency sounds for long-range communication. Their
inner ears appear to be specialized for low-frequency hearing (Ketten,
Chapter 2), but nothing is known about their auditory thresholds or
masking bandwidths. If the range at which they can detect a signal is limited
by the bandwidth of hearing, then selection might have favored specializations in the inner ear that would narrow the bandwidth over which lowfrequency sound energy was integrated.
2.3 Detection of Predators
When people think about how animals use sound, they are most likely to
think of echolocation and communication with conspecifics, but it is
often important for animals to intercept the sounds of other species, such
as predators or prey. The most common predator for many cetaceans is
another cetacean: the killer whale, Orcinus orca (Jefferson et ai. 1991).
Killer whales vocalize while foraging (Ford 1989; Barrett-Lennard et ai.
1996), making both clicks for echolocation and other calls used for communication. Killer whale echolocation clicks are reported to emphasize the
12 to 25 kHz range (Diercks et ai. 1971), while their communication calls
range from 500 Hz to 35 kHz. It can be a life-or-death matter for potential
prey to be able to detect killer whales early enough to be able to avoid
them. Many prey species of killer whales monitor for killer whale sounds
and respond strongly when they hear them. This has been clearly demonstrated in playback experiments. Gray whales migrating along the coast of
California respond to playback of killer whale calls by swimming rapidly
inshore into beds of kelp (Cummings and Thompson 1971; Malme et ai.
1983). Beluga whales (Delphinapterus leucas) , feeding on salmon in an
Alaskan river, also showed a strong avoidance response to playback of
killer whale calls (Fish and Vania 1971).