212
PL. Tyack and c.w. Clark
every doubling of pressure. Since different parts of the vocal tract are more
or less elastic, changes in volume will lead to changes in shape. These depthinduced changes in the vocal tract are likely to outweigh the subtle developmental differences that lead to voice differences. For example, the
whistles of a beluga whale recorded at different depths show strong differences in their frequency spectra (Ridgway 1997). If diving animals rely upon
individually distinctive calls, they may be unable to use voice cues and may
need to create distinctive calls by learning to modify acoustic features under
voluntary control, such as the frequency modulation of whistles.
6. Summary
The ocean has acoustic properties that open wonderful opportunities for
human ingenuity and for evolutionary innovations in marine organisms.
Over the past century, acoustic engineers have developed hundreds of
devices to communicate under the sea, to listen for and track sound sources,
to measure the distance to the sea floor, to find fish, to map bathymetry, and
to orient at long ranges. When cetaceans entered the sea, they started with
a well-developed system for auditory processing typical of mammals. Since
entering the sea, they have become even more specialized in audition
(Ketten, Chapter 2). Odontocetes have specialized in high-frequency
acoustics, with most calls above 2kHz, while mysticetes have specialized in
low-frequency sounds, with most sounds below 2 kHz (Thompson, Winn and
Perkins 1979; Watkins and Wartzok 1985; Clark 1991a; Matthews 1999).
Dolphins evolved high-frequency echolocation systems matched only by
bats. The demands of echolocation appear to have selected for auditory
systems that are sensitive to high frequencies and that can discriminate
short time intervals, but not enough is known about the details of sonar processing in odontocetes to highlight more neurophysiological specializations
of the sort well known for bats (e.g., Suga 1977). Many odontocetes show
patterns of variation in signal by individual, by group, and by geographical
area that suggest the evolution of complex recognition systems involving
contact calls. Demands of discriminating subtle differences in tonal whistles may be responsible for the evolution of precise frequency resolution in
the whistle frequency band in dolphins. Mysticetes have specialized in lowfrequency calls, many of which can be detected at ranges of hundreds of
kilometers. The advantageous propagation of low-frequency sound in the
ocean may have selected for low-frequency vocal signals and auditory processing in these whales. Several species have evolved complex reproductive
advertisement displays that rival those of songbirds for complexity and
musicality. Sexual selection may lead to the evolution of better-developed
abilities of auditory discrimination in females at the same time as it selects
for more complex displays in males. While there are intriguing suggestions
that some cetaceans might use low-frequency sound to explore their
PL. Tyack and c.w. Clark
every doubling of pressure. Since different parts of the vocal tract are more
or less elastic, changes in volume will lead to changes in shape. These depthinduced changes in the vocal tract are likely to outweigh the subtle developmental differences that lead to voice differences. For example, the
whistles of a beluga whale recorded at different depths show strong differences in their frequency spectra (Ridgway 1997). If diving animals rely upon
individually distinctive calls, they may be unable to use voice cues and may
need to create distinctive calls by learning to modify acoustic features under
voluntary control, such as the frequency modulation of whistles.
6. Summary
The ocean has acoustic properties that open wonderful opportunities for
human ingenuity and for evolutionary innovations in marine organisms.
Over the past century, acoustic engineers have developed hundreds of
devices to communicate under the sea, to listen for and track sound sources,
to measure the distance to the sea floor, to find fish, to map bathymetry, and
to orient at long ranges. When cetaceans entered the sea, they started with
a well-developed system for auditory processing typical of mammals. Since
entering the sea, they have become even more specialized in audition
(Ketten, Chapter 2). Odontocetes have specialized in high-frequency
acoustics, with most calls above 2kHz, while mysticetes have specialized in
low-frequency sounds, with most sounds below 2 kHz (Thompson, Winn and
Perkins 1979; Watkins and Wartzok 1985; Clark 1991a; Matthews 1999).
Dolphins evolved high-frequency echolocation systems matched only by
bats. The demands of echolocation appear to have selected for auditory
systems that are sensitive to high frequencies and that can discriminate
short time intervals, but not enough is known about the details of sonar processing in odontocetes to highlight more neurophysiological specializations
of the sort well known for bats (e.g., Suga 1977). Many odontocetes show
patterns of variation in signal by individual, by group, and by geographical
area that suggest the evolution of complex recognition systems involving
contact calls. Demands of discriminating subtle differences in tonal whistles may be responsible for the evolution of precise frequency resolution in
the whistle frequency band in dolphins. Mysticetes have specialized in lowfrequency calls, many of which can be detected at ranges of hundreds of
kilometers. The advantageous propagation of low-frequency sound in the
ocean may have selected for low-frequency vocal signals and auditory processing in these whales. Several species have evolved complex reproductive
advertisement displays that rival those of songbirds for complexity and
musicality. Sexual selection may lead to the evolution of better-developed
abilities of auditory discrimination in females at the same time as it selects
for more complex displays in males. While there are intriguing suggestions
that some cetaceans might use low-frequency sound to explore their