4. Acoustic Communication in Whales and Dolphins
157
Just as human technology has diversified to take advantage of the special
acoustic properties of the ocean, the mammals that entered the sea also
appear to have evolved many adaptations to use sound for communication
and for exploring their aquatic environment. One crude way to measure the
relative investment in audition versus vision is to compare the number of
fibers in the auditory and optic nerves. Ketten (1997) points out that most
cetaceans have auditory/optic ratios of fiber counts that are two to three
those of land mammals, suggesting an increased investment in audition
compared to vision. There is also evidence that cetaceans use sound to solve
problems in ways similar to those just described for human engineers. Dolphins have a large repertoire of vocalizations spanning frequencies from
below 100 Hz to more than 100 kHz. As Au describes in Chapter 9, dolphins
have evolved high-frequency echolocation similar to some human-made
sonars and to the biosonar used by bats. Large baleen whales have evolved
abilities to produce and to hear low-frequency calls well suited for longrange communication (Payne and Webb 1971; Ketten, Chapter 2). All of
these observations suggest that cetaceans will be fascinating subjects
for comparative study of how mammalian hearing may adapt to a very
different environment.
Most specialists in audition and psychophysics emphasize commonalities
in hearing across different vertebrates. The structure and function of hair
cells appear to be highly conserved across animal groups separated by hundreds of millions of years of evolution. While there are some differences in
the frequency ranges of hearing in different taxa, there are few examples
of specialized auditory processing in vertebrates. Perhaps the best known
example is the specialized hearing of some bats, such as the greater horseshoe bat, Rhinolophus ferrumequinum, where half of the auditory neurons
are tuned to only 12% of the audible frequency region (Schuller and Pollak
1979). While these bats can hear from 9 to 96kHz, more than 16% of auditory neurons are tuned to the 83 to 84.5 kHz region. This "acoustic fovea"
allows these bats to discriminate very small differences in frequency at this
sensitive band. The mustache bat, Pteronotus parnelli, has a sensitive band
near 60 kHz, where it can detect differences in frequency of less than about
10Hz (Bodenhamer and Pollak 1983). To understand the auditory specialization in these bats, it was necessary to know the structure and function of
their vocalizations. Each of these bat species produces a narrow-band tonal
vocalization tuned to the same frequency as the acoustic fovea typical of
that species. When these bats echolocate on a target moving in a cluttered
environment, they rely upon the Doppler shift of the echo to detect and
classify the target. The acoustic fovea allows them to discriminate small
Doppler shifts that are critical for their sonar processing.
In this chapter, we explore the structure and function of cetacean
vocalizations in order to provide background on possible selection pressures for auditory specializations in cetaceans. For example, the selection
pressures for echolocation in some odontocetes may have led to auditory
157
Just as human technology has diversified to take advantage of the special
acoustic properties of the ocean, the mammals that entered the sea also
appear to have evolved many adaptations to use sound for communication
and for exploring their aquatic environment. One crude way to measure the
relative investment in audition versus vision is to compare the number of
fibers in the auditory and optic nerves. Ketten (1997) points out that most
cetaceans have auditory/optic ratios of fiber counts that are two to three
those of land mammals, suggesting an increased investment in audition
compared to vision. There is also evidence that cetaceans use sound to solve
problems in ways similar to those just described for human engineers. Dolphins have a large repertoire of vocalizations spanning frequencies from
below 100 Hz to more than 100 kHz. As Au describes in Chapter 9, dolphins
have evolved high-frequency echolocation similar to some human-made
sonars and to the biosonar used by bats. Large baleen whales have evolved
abilities to produce and to hear low-frequency calls well suited for longrange communication (Payne and Webb 1971; Ketten, Chapter 2). All of
these observations suggest that cetaceans will be fascinating subjects
for comparative study of how mammalian hearing may adapt to a very
different environment.
Most specialists in audition and psychophysics emphasize commonalities
in hearing across different vertebrates. The structure and function of hair
cells appear to be highly conserved across animal groups separated by hundreds of millions of years of evolution. While there are some differences in
the frequency ranges of hearing in different taxa, there are few examples
of specialized auditory processing in vertebrates. Perhaps the best known
example is the specialized hearing of some bats, such as the greater horseshoe bat, Rhinolophus ferrumequinum, where half of the auditory neurons
are tuned to only 12% of the audible frequency region (Schuller and Pollak
1979). While these bats can hear from 9 to 96kHz, more than 16% of auditory neurons are tuned to the 83 to 84.5 kHz region. This "acoustic fovea"
allows these bats to discriminate very small differences in frequency at this
sensitive band. The mustache bat, Pteronotus parnelli, has a sensitive band
near 60 kHz, where it can detect differences in frequency of less than about
10Hz (Bodenhamer and Pollak 1983). To understand the auditory specialization in these bats, it was necessary to know the structure and function of
their vocalizations. Each of these bat species produces a narrow-band tonal
vocalization tuned to the same frequency as the acoustic fovea typical of
that species. When these bats echolocate on a target moving in a cluttered
environment, they rely upon the Doppler shift of the echo to detect and
classify the target. The acoustic fovea allows them to discriminate small
Doppler shifts that are critical for their sonar processing.
In this chapter, we explore the structure and function of cetacean
vocalizations in order to provide background on possible selection pressures for auditory specializations in cetaceans. For example, the selection
pressures for echolocation in some odontocetes may have led to auditory