184
P.L. Tyack and c.w. Clark
3.1.3 Phocoenids and Cephalorhynchus
Phocoenid porpoises and the delphinids of the genus Cephalorhynchus are
both small coastal cetaceans that look somewhat similar and appear to have
convergent ecological niches. Both species appear to feed on benthic and
demersal prey. They have been described as less social than delphinid
odontocetes of other than Cephalorhynchus, but their social behavior has
been little studied. They are not known to produce tonal calls such as
whistles, but have only been recorded producing high-frequency pulsed
sounds. These pulsed signals of Phocoenid porpoises and Cephalorhynchus
dolphins are 5 to 10 times longer (150 to 600 Ils) than those of bottlenose
dolphins, roughly half the bandwidth (a bandwidth of 40 kHz spanning from
120 to 160 kHz) of bottlenose dolphin clicks, and with source levels between
150 to 170 dB relllPa at 1 m, several orders of magnitude weaker than the
loudest bottlenose dolphin clicks (Phocoena phocoena: Amundin 1991,
Kamminga and Wiersma 1981, M~hl and Andersen 1973; Phocoenoides
dalli: Hatekayama and Soeda 1990; Cephalorhynchus commersonii: Kamminga and Wiersma 1982, Evans et al. 1988; C. hectori: Dawson and Thorpe
1990). Figure 4.6 illustrates the waveform and spectrum of one click from
a young harbor porpoise. While bottlenose dolphins may produce echolocation clicks with spectral peaks well below 100kHz, porpoise and
Cephalorhynchus clicks tend to have peaks above 100kHz. Ketten (1994)
has identified porpoises and Cephalorhynchus as having inner ears that are
particularly specialized for high-frequency audition above 100kHz.
It seems very likely that Phocoenid porpoises and the delphinids of the
genus Cephalorhynchus use high-frequency clicks for echolocation, but
there is little experimental evidence that they in fact do so. Little is known
about how they use echolocation, or how they process sonar information.
However, the longer duration and narrower bandwidth of their signals and
specialized high-frequency hearing suggest a possible analogy with bats.
Some bats have evolved a highly specialized biosonar system in order to
resolve subtle differences in frequency induced by Doppler shifts from
movements of their prey. This ability to detect Doppler shifts appears to
help them detect their prey in a cluttered environment (Neuweiler 1990).
The auditory systems of these bats are specialized for detecting a narrow
range of frequencies near the central frequency of their biosonar signal.
This has been called an acoustic fovea, in analogy to the fovea of the eye,
which is the area of most acute vision (Schuller and Pollak 1979). Doppler
processing puts high demands on the frequency resolution of an auditory
system. The change in frequency from a Doppler shift is approximately
equal to two times the relative velocity V r between signal and target times
the frequency of the signal f s divided by the speed of sound c: ~ = 2V r f,lc.
A relative velocity difference of 1 mis, which seems reasonable for a swimming dolphin would yield a Doppler shift of only 160 Hz for a sonar signal
at 120kHz. The ability to detect such a small shift would require a special-
P.L. Tyack and c.w. Clark
3.1.3 Phocoenids and Cephalorhynchus
Phocoenid porpoises and the delphinids of the genus Cephalorhynchus are
both small coastal cetaceans that look somewhat similar and appear to have
convergent ecological niches. Both species appear to feed on benthic and
demersal prey. They have been described as less social than delphinid
odontocetes of other than Cephalorhynchus, but their social behavior has
been little studied. They are not known to produce tonal calls such as
whistles, but have only been recorded producing high-frequency pulsed
sounds. These pulsed signals of Phocoenid porpoises and Cephalorhynchus
dolphins are 5 to 10 times longer (150 to 600 Ils) than those of bottlenose
dolphins, roughly half the bandwidth (a bandwidth of 40 kHz spanning from
120 to 160 kHz) of bottlenose dolphin clicks, and with source levels between
150 to 170 dB relllPa at 1 m, several orders of magnitude weaker than the
loudest bottlenose dolphin clicks (Phocoena phocoena: Amundin 1991,
Kamminga and Wiersma 1981, M~hl and Andersen 1973; Phocoenoides
dalli: Hatekayama and Soeda 1990; Cephalorhynchus commersonii: Kamminga and Wiersma 1982, Evans et al. 1988; C. hectori: Dawson and Thorpe
1990). Figure 4.6 illustrates the waveform and spectrum of one click from
a young harbor porpoise. While bottlenose dolphins may produce echolocation clicks with spectral peaks well below 100kHz, porpoise and
Cephalorhynchus clicks tend to have peaks above 100kHz. Ketten (1994)
has identified porpoises and Cephalorhynchus as having inner ears that are
particularly specialized for high-frequency audition above 100kHz.
It seems very likely that Phocoenid porpoises and the delphinids of the
genus Cephalorhynchus use high-frequency clicks for echolocation, but
there is little experimental evidence that they in fact do so. Little is known
about how they use echolocation, or how they process sonar information.
However, the longer duration and narrower bandwidth of their signals and
specialized high-frequency hearing suggest a possible analogy with bats.
Some bats have evolved a highly specialized biosonar system in order to
resolve subtle differences in frequency induced by Doppler shifts from
movements of their prey. This ability to detect Doppler shifts appears to
help them detect their prey in a cluttered environment (Neuweiler 1990).
The auditory systems of these bats are specialized for detecting a narrow
range of frequencies near the central frequency of their biosonar signal.
This has been called an acoustic fovea, in analogy to the fovea of the eye,
which is the area of most acute vision (Schuller and Pollak 1979). Doppler
processing puts high demands on the frequency resolution of an auditory
system. The change in frequency from a Doppler shift is approximately
equal to two times the relative velocity V r between signal and target times
the frequency of the signal f s divided by the speed of sound c: ~ = 2V r f,lc.
A relative velocity difference of 1 mis, which seems reasonable for a swimming dolphin would yield a Doppler shift of only 160 Hz for a sonar signal
at 120kHz. The ability to detect such a small shift would require a special-