4. Acoustic Communication in Whales and Dolphins
177
vocalization as an indicator of body size (e.g., Davies and Halliday 1978 for
anurans; Clutton-Brock and Albon 1979 for a terrestrial mammal). If males
judge the size of opponents by listening to the lowest peak frequency of
their calls, then this will create a selection pressure for males to make the
lowest frequency sounds they can. Morton (1977) suggests that sounds used
in aggressive interactions in a broad range of birds and mammals tend to
be low in frequency. If the minimum peak frequency a male can produce
is constrained by the volume of a resonating cavity and if the resonator
volume correlates with body size, then males may be constrained to produce
an honest advertisement of their body size.
The second model emphasizes how the receiver may use threat signals
to predict the next actions of an opponent or to assess the opponent's willingness to escalate. Some biologists have suggested that a contestant might
incur a higher cost than benefit if it produces signals that help an opponent
to predict behaviors (e.g., Maynard Smith 1974). This perspective highlights
the potential benefits of strategic bluffing or tailoring signals so they do not
allow an opponent to predict immediate responses. However, there is evidence of predictive signaling from a diverse array of species. The potentially
high risks of making a mistake in assessing an opponent in an all out fight
may encourage a more or less "honest" assessment phase (Hauser and
Nelson 1991). As animals escalate in this assessment phase, they often
produce louder, more intense threats. When the signaler puts more energy
into the display, it may be broadcasting its ability as well as its willingness
to escalate. If the signal is much louder than is required to just be detected
at typical ranges of receivers, then detection of this kind of signal is less
likely to be noise or hearing limited than, say, detecting an echo from a
distant echolocation target.
3. Descriptions and Proposed Functions for
Vocalizations of Different Cetaceans
3.1 Toothed Whales
3.1.1 Bottlenose Dolphins
Of all cetaceans, bottlenose dolphins, T. truncatus, have the best studied
vocal repertoire. Tens of studies have reported on vocalizations of these
animals both in the wild and in captivity. Yet even for this species, there are
real problems with describing the vocal repertoire. Bottlenose dolphins
are reported to produce several different kinds of pulsed calls and several
different tonal calls.
The clicks used by dolphins for echolocation have been well described.
The echolocation clicks of bottlenose dolphins are very short «100 ~s), with
a rapid rise-time and a relatively broad bandwidth from several tens of kHz
177
vocalization as an indicator of body size (e.g., Davies and Halliday 1978 for
anurans; Clutton-Brock and Albon 1979 for a terrestrial mammal). If males
judge the size of opponents by listening to the lowest peak frequency of
their calls, then this will create a selection pressure for males to make the
lowest frequency sounds they can. Morton (1977) suggests that sounds used
in aggressive interactions in a broad range of birds and mammals tend to
be low in frequency. If the minimum peak frequency a male can produce
is constrained by the volume of a resonating cavity and if the resonator
volume correlates with body size, then males may be constrained to produce
an honest advertisement of their body size.
The second model emphasizes how the receiver may use threat signals
to predict the next actions of an opponent or to assess the opponent's willingness to escalate. Some biologists have suggested that a contestant might
incur a higher cost than benefit if it produces signals that help an opponent
to predict behaviors (e.g., Maynard Smith 1974). This perspective highlights
the potential benefits of strategic bluffing or tailoring signals so they do not
allow an opponent to predict immediate responses. However, there is evidence of predictive signaling from a diverse array of species. The potentially
high risks of making a mistake in assessing an opponent in an all out fight
may encourage a more or less "honest" assessment phase (Hauser and
Nelson 1991). As animals escalate in this assessment phase, they often
produce louder, more intense threats. When the signaler puts more energy
into the display, it may be broadcasting its ability as well as its willingness
to escalate. If the signal is much louder than is required to just be detected
at typical ranges of receivers, then detection of this kind of signal is less
likely to be noise or hearing limited than, say, detecting an echo from a
distant echolocation target.
3. Descriptions and Proposed Functions for
Vocalizations of Different Cetaceans
3.1 Toothed Whales
3.1.1 Bottlenose Dolphins
Of all cetaceans, bottlenose dolphins, T. truncatus, have the best studied
vocal repertoire. Tens of studies have reported on vocalizations of these
animals both in the wild and in captivity. Yet even for this species, there are
real problems with describing the vocal repertoire. Bottlenose dolphins
are reported to produce several different kinds of pulsed calls and several
different tonal calls.
The clicks used by dolphins for echolocation have been well described.
The echolocation clicks of bottlenose dolphins are very short «100 ~s), with
a rapid rise-time and a relatively broad bandwidth from several tens of kHz