Sperm whale sound production studied with ultrasound time/depth-recording tags
1 Department of Zoophysiology, Institute of Biological Sciences, University
of Aarhus, Building 131, 8000 Aarhus C, Denmark
2 The Ocean Alliance/The Whale Conservation Institute, 191 Weston Road,
Lincoln, MA 01775, USA
* e-mail: Peter.teglberg{at}biology.au.dk
Accepted 15 April 2002
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Summary |
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Key words: sperm whale, Physeter macrocephalus, pneumatic, sound production, click, tag, biosonar, communication, nasal complex, spermaceti organ, diving, monkey lips
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Introduction |
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Norris and Harvey (1972)
proposed that the sperm whale nose, homologous with the sound-producing nasal
complex of smaller odontocetes (Cranford et
al., 1996
), is a pneumatic sound generator
(Fig. 1). Recent investigations
have corroborated some of the basic concepts of the Norris and Harvey theory
by showing that clicks are produced in the anterior part of the nasal complex
(Ridgway and Carder, 2001
) and
that sound can be transmitted through the spermaceti compartments
(Møhl, 2001
). The sperm
whale sound generator is believed to be driven by air which, when recycled,
allows for continuous sound production throughout a dive
(Norris and Harvey, 1972
).
However, air volumes contained in soft structured tissue
(Ridgway et al., 1969
) are
reduced in proportion to increasing ambient pressure (Boyle's law:
PV=C, where P is pressure, V is volume and
C is a constant), so the available volume for sound production varies
considerably with depth.
Sperm whales are vociferous animals and, unlike most odontocete species
that have been investigated, their vocal repertoire is made up solely of
clicks. It has been suggested that the so-called usual clicks
(Weilgart and Whitehead, 1988)
are involved in echolocation (Gordon,
1987
), whereas stereotyped patterns of clicks, termed codas
(Watkins and Schevill, 1977
),
are allegedly involved in communication to maintain the complex social
structure in female groups (Weilgart and
Whitehead, 1993
). Recent investigations have demonstrated that
sperm whale usual clicks are highly directional and have the highest
biologically produced source levels ever recorded
(Møhl et al., 2000
).
Clicks of high sound pressure levels and directionality serve biosonar
purposes well (Au, 1993
) but
seem a poor choice for communication because directionality reduces the
communicative space.
Because of the directional properties of sperm whale usual clicks,
far-field recordings cannot quantify changes in the acoustic output of the
sound generator since scanning movements of a directional source rather than
output modulations may be the cause of the observed changes. By placing a
calibrated recording unit in a fixed position on a phonating sperm whale,
directional and/or hydro-acoustic effects on the recorded signals can be ruled
out, and any observed changes will reflect actual changes in the acoustic
output of the sound generator. Sound-recording tags have successfully been
placed on elephant seals (Fletcher et al.,
1996; Burgess et al.,
1998
) and sperm whales
(Malakoff, 2001
) to register
levels of low-frequency noise impinging on the tagged animal and how the
behaviour of the animal is affected. Of interest in the present study are the
acoustics and biomechanics of the sperm whale sound generator. To study these,
we developed a tag that allows for absolute sound pressure recordings of
clicks for 30 min and combination of these data with the real time and depth
of the whale.
Here, we report that sperm whales can maintain and regulate acoustic outputs even when they have a very limited volume of air in the nasal complex. We also present evidence to suggest that the sound-generating mechanism has a bimodal function that allows for the production of clicks suited for biosonar and clicks more suited for communication.
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Materials and methods |
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The tag
The tag was based on an aluminium housing (diameter 100 mm) with a
Syntactic foam tail (MacArtney, Denmark) pressure-tested to a depth of 1100m.
Signals from a custom-built hydrophone were highpass-filtered (-12 dB per
octave, fundamental frequency 1 kHz) and relayed, via an adjustable
gain/anti-alias filter unit, to a 12-bit ADC (Analog Devices: AD7870) and
µcontroller (Maxim Integrated Products, Inc. DS5000T) unit (sampling at
62.5 kHz) writing acoustic, real-time and depth data to a 192 Mb Sandisk
Compact flash card. The hydrophone was calibrated relative to a B&K 8101
hydrophone in an anechoic tank before and after deployment. Sound recording
(bandwidth 30 kHz) was triggered at a depth of 20 m. The depth transducer was
a calibrated Keller PA-7-200 transducer providing depth information in the
range 0-1500 m with an accuracy of 3 m. The suction cup (diameter 25 cm) was
moulded from Wacker silicone (Elastosil M-4440) in a custom-built cast.
Attachment and retrieval
The tags was deployed with a 4.5 m pole from a special boom rigged on the
R/V Odyssey; the tag was attached to the whale with a suction cup
(Fig. 2). The whales were
approached from behind, and the ship drifted the last 30-50 m with the engine
turned off to make a silent approach. Four whales were successfully tagged in
45 trials. After detachment, the tag was retrieved by taking a bearing with
four-element Yagi antennae (Televilt, Y-4FL) to signals from a Telonic
MOD-305, Cast 3C, transmitter integrated in the Syntactic foam tail. A B&K
8101 hydrophone was deployed to record the far-field signatures of the clicks
recorded by the tag. Signals from the B&K 8101 hydrophone were recorded on
a Sony TCD-D8 DAT recorder. This recording chain had a flat (within 2 dB)
frequency response from 0.01 kHz to 22 kHz. From video footage of the tag
attachments, it was possible to calculate the size of the whale from the
diameter of the attached suction cup
(Whitehead and Payne,
1981).
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Analysis
Data were transferred via the Flash card and a PCMCIA slot to a
laptop. The anti-alias filter was compensated for during analysis, giving a
flat frequency response of the tag in the range 0.1-30 kHz. Analysis was
performed with Cool edit 2000 (Syntrilium) and routines written in Matlab 5.3
(MathWorks). Inter-click intervals (ICI) were derived with a peak detector
looking for suprathreshold values of the envelope of the recorded signals. The
spectral content of the clicks was described by the end points of the -10 dB
bandwidth. Centroid frequency was derived as the frequency dividing the
spectrum into halves of equal energy. The duration of a click was defined as
the interval between the -10 dB points relative to the peak of the envelope
function.
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Results |
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The production of usual clicks is initiated with an ICI of approximately 1 s, but as the whale approaches the depth at which its dive levels off, the ICIs drop to a stable 0.5 s (Fig. 4). During descent, the ICIs decrease by 100-200 ms and subsequently increase almost back to the starting level in 3-4 repeated cycles (Fig. 4). Click trains are interrupted by periods of silence lasting 5-30 s.
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Recorded levels of all 1804 usual clicks are plotted in Fig. 5. The recorded levels of the first usual clicks are less than 170 dB re. 1 µPa (peak to peak, pp), and the amplitudes of the following clicks increase to approximately 178 dB re. 1 µPa (pp). The acoustic output is independent of depth within a 20 dB range from 170 to 190 dB re. 1 µPa (pp) (Fig. 5).
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As seen from the data presented in Table 1, there are marked differences between the waveforms of usual clicks and coda clicks. The coda clicks (N=54) have a mean recorded level of 165±5 dB re. 1 µPa (pp), which is significantly lower than the mean recorded level of usual clicks (N=1804) of 178±4 dB re. 1 µPa (pp) (P<0.001). Also, the centroid frequency of the coda clicks is 7-9 kHz with a -10 dB bandwidth of 3-4 kHz, compared with a higher and more variable centroid frequency for the usual clicks between 8 and 25 kHz and a -10 dB bandwidth of 10-15 kHz.
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The duration of the individual pulses within a click is approximately 100 µs for the initial sound pulse (p0) in usual clicks and approximately 300 µs for p0 in coda clicks. A distinct difference between usual clicks and coda clicks is seen in the decay rate (peak amplitude) between the successive pulses within a click (Fig. 6). It is evident from Fig. 6A that there is a decay rate of the order of 20 dB between p0 and the second pulse (p1), and that no third pulse (p2) can be detected above background noise in usual clicks. The decay rate of usual clicks is largest for the most powerful clicks but independent of depth because both low (15 dB) and high (23 dB) decay rates between p0 and p1 are seen at the deepest part of the dive. In coda clicks, the decay rate is approximately 4-8 dB between p0 and p1 (Fig. 6B) irrespective of the whale's depth.
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The far-field signature of the clicks was recorded from the research
vessel. The waveforms of usual clicks differed significantly from the tag
recordings, with the centroid frequency occurring at lower frequencies. The
inter-pulse interval (IPI) denotes the period between two successive pulses
within a click (Norris and Harvey,
1972). The IPI of both coda clicks and usual clicks was 3.4 ms
irrespective of depth. The centroid frequency of usual clicks is independent
of depth because both high and low centroid frequencies are found in clicks
during shallow and deeper parts of the dive. There is, however, a positive
relationship (r=0.70, P<0.001) between the acoustic
output (recorded level) and centroid frequency
(Fig. 7).
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Discussion |
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The adjustment in ICI with depth during a dive
(Fig. 4) may be explained by a
longer sonar range at the beginning of the dive and by the fact that the ICI
is reduced as the whale approaches sonar targets (e.g. prey or bottom),
thereby reducing the two-way travel time of the clicks and the echo
(Au, 1993). This adjustment in
ICI has also been reported in other sperm whale studies (e.g.
Gordon, 1987
; M. Wahlberg,
manuscript submitted), suggesting that it is an integrated part of sperm whale
ecophysiology during feeding dives. However, the sound pressure levels are not
reduced accordingly (Fig. 5),
indicating that sonar range alone does not dictate the magnitude of the
acoustic outputs.
In the near field of what is considered to be 180° off the acoustic
axis of the sound generator (Møhl
et al., 2000), the mean recorded level of usual clicks is
178±4 dB re. 1 µPa (pp). This is consistent with off-axis levels
reported from array recordings of usual clicks made by male sperm whales
(Møhl et al., 2000
).
The recorded levels are within a 20 dB range of 170-190 dB re. 1 µPa (pp)
(Fig. 5), and it is feasible
that the source levels (the sound pressure at a distance of 1 m on the
acoustic axis) are emitted within the same 20 dB dynamic range but that they
are some 40 dB higher (Møhl et al.,
2000
). There is no apparent link between available volumes of air
and sound pressure since both high- and low-sound-pressure clicks are produced
during the deepest part of the dive (Fig.
5). Thus, sperm whales can regulate the sound pressure levels of
their clicks, and it is sonar or feeding demands rather than available air
volume that dictate acoustic output levels at these depths.
Data from Møhl
(2001) suggests that the
multipulses in sperm whale clicks are the result of a single pulse
(p0) being reflected on the air surfaces of the distal and frontal
air sacs (Fig. 1). From this,
it can be inferred from the decay rate data presented here that the bulk of
the energy of the initial pulse, p0, in usual clicks is directed
forwards into the water after a single round trip through the spermaceti organ
and the junk, and that only a small fraction is intercepted by the distal air
sac, giving rise to the low amplitude of p1 shown in
Fig. 6A.
As noted above, the recorded levels of p0 in coda clicks are 20 dB
less intense than those of usual clicks, suggesting that the overall acoustic
output in coda clicks is reduced compared with that of usual clicks or that a
smaller fraction of the initial energy is directed backwards into the
spermaceti organ and consequently towards the recording tag. When generating
coda clicks, a large fraction of the returning pulse (p1) from the
frontal sac appears to be intercepted by the distal air sac and contained in
the nasal complex for further round trips, thereby giving rise to a large
number of pulses with small decay rates within each coda click. We propose
that these two different ways of handling the initial sound pulse represent a
bimodal generation of clicks depending on whether they are intended by the
whale for use in biosonar or for communication. In usual clicks, most of the
energy is put into a single pulse, directed into the water in front of the
whale after traversing the spermaceti complex twice. In coda clicks, the
energy is recycled in the nasal complex by multiple reflections that seem to
result in less-directional clicks that are better suited for communication. In
addition to the inferred low directionality, the narrow-band nature, longer
pulse duration and low decay rate of coda clicks may offer useful information
about the transmitter to conspecifics. We suggest that the initial pulse of
the two click types is generated in the same way and that the marked
differences between coda clicks and usual clicks are caused by different sound
propagation in the nasal complex. The difference in click structure and the
inferred difference in directionality between coda clicks and usual clicks may
also explain in part the substantial discrepancy between reports of low
directionality in clicks from coda-producing sperm whales
(Watkins, 1980) and the high
directionality observed in usual clicks from foraging male sperm whales
(Møhl et al.,
2000
).
If the distinct multipulse structure of the coda clicks is generated by
repetitive reflections on the air sacs, it may explain why coda clicks are
produced in the shallow part of the dive cycle when more than 4% of the
initial air volume is still present. It is possible that a certain air volume
is needed to maintain the production of coda clicks and that sperm whales are
accordingly limited by depth in coda production. However, the fact that the
whale switched from the production of coda clicks to usual clicks within 10 s,
at a depth of 265 m suggests that shifts between the two modes of click
generation are not determined solely by the available air volume. It is
feasible that, during the formation of a usual click, muscle action in the
complex muscle/tendon system covering the dorso-lateral part of the spermaceti
organ could be changing the conformation of the sound-transmitting structures
and the distal air sac, thereby causing most of the energy to be projected
forwards into the water after one round trip through the spermaceti complex.
On the basis of observations of several other pulsed sound types from sperm
whales (Gordon, 1987;
Weilgart and Whitehead, 1988
),
the possibility that the sperm whale sound generator may have additional modes
from the two deduced from this study cannot be excluded.
The far-filed signature of the clicks revealed a different waveform and
emphasis at lower frequencies compared with the tag recording. The waveform
differences between the nearfield (the tag) and the far field cannot be
explained solely by surface reflections and hydrodynamic effects because the
decay rate of the usual clicks was lower in the far field than when recorded
in the near field from the crest of the skull. It is tempting to suggest that
the lower centroid frequency observed in the far field relates to
lowpass-filtering of the clicks by frequency-dependent absorption. However,
considering the physical limits of the range between the tagged animal and the
research vessel during 10-20 min of swimming (1-5 m s-1),
frequency-dependent absorption in the relevant frequency range of sperm whale
clicks cannot account entirely for the observed changes
(Urick, 1983). It appears that
the main contributing factor to the waveform and frequency differences is the
directional effects of the sperm whale sound generator.
The Gordon equation (Gordon,
1991) describes the relationship between IPI and the size of a
whale. With an IPI of 3.4 ms, the Gordon equation predicts a body length of
9.8 m, which matches the visual estimate of 10 m from video recordings of the
whale and the tag. Consequently, the data presented here lend weight to the
Gordon equation as a reliable acoustic means of measuring the size of sperm
whales from their clicks.
The inter-pulse interval (IPI) is 3.4 ms in both click types and constant
throughout the dive. Clarke
(1970) has proposed that the
nasal complex of the sperm whale is a buoyancy regulator that facilitates
descent and ascent during dives by cooling and heating the spermaceti oil.
Assuming a pressure range of 7000 kPa (70 atmospheres) (0-700 m depth) and a
temperature difference of 22-37°C, it can be calculated that the sound
speed would differ by 7% between the start and the deepest point of a dive (on
the basis of data from Goold et al.,
1996
). In a sperm whale with an estimated two-way sound travel
path of 4.7 m (Fig. 1), such
differences in sound speed would change the IPI by more than 200 µs during
a dive to 700 m. We did not observe IPI fluctuations of that order of
magnitude, so the theory (Clarke,
1970
) proposing that ascent and descent of sperm whales are
assisted by changes in buoyancy of the head due to heating and cooling of the
spermaceti oil is not supported.
The centroid frequencies of the usual clicks vary between 8 and 26 kHz.
These values are consistent with previous reports on the frequency content of
sperm whale clicks (Watkins,
1980; Madsen and Møhl,
2000
). It is, however, surprising that centroid frequencies above
10 kHz can be found in usual clicks recorded from what is believed to be
180° off the acoustic axis of the sound generator
(Møhl et al., 2000
). It
can be conjectured that the high centroid frequencies recorded from the crest
of the skull are due to near-field phenomena and the peculiar sound
transmission in the sperm whale nasal complex, where the bulk of the initial
pulse is directed backwards into the spermaceti organ by the distal sac and
anatomy of the monkey lips. This problem calls for further investigations.
There are no apparent correlations between the spectrum of the usual clicks
and the whale's depth because both high and low centroid frequencies were
recorded from clicks at the deepest part of the dive. This contrasts with
investigations on white whale (Delphinapterus leucas) whistles at
depth (Ridgway et al., 2001).
Ridgway and co-workers found that the peak frequency of whistle spectra
increased with depth and proposed that this effect is the result of increased
air density and a reduction in total air volume at depth. The absence of a
similar effect in sperm whale clicks emphasises, in our view, the difference
in how clicks and whistles are generated in the odontocete nasal complex.
When centroid frequency is plotted against recorded sound level
(Fig. 7), it appears that there
is a positive correlation between acoustic output and frequency. This
correlation should not be confused with the fact that the on-axis parts of the
clicks contain more high-frequency components than the off-axis parts
(Møhl et al., 2000).
Investigations on smaller odontocetes have revealed a positive correlation
between acoustic output and centroid frequency in clicks from D. leucas,
P. crassidens and Tursiops truncatus
(Au, 2001
). That a similar
relationship has been found in the present study supports the conclusion that
sound production in sperm whales is based on the same fundamental biomechanics
as in smaller odontocetes and that the nasal complexes are, therefore, not
only anatomically (Cranford,
1999
) but also functionally homologous in generating the initial
sound pulse.
In conclusion, sperm whale click production in terms of output and frequency content is unaffected by hydrostatic reductions in available air volume down to depths of at least 700 m. Evidence is presented to suggest that the sound-generating mechanism has a bimodal function, allowing for the production of clicks suited for biosonar and clicks more suited for communication. Shared click features suggest that sound production in sperm whales is based on the same fundamental biomechanics as in smaller odontocetes. This project has shown that it is possible to gather information about the physiology and biomechanics of sound production from free-ranging animals not suited for study in captivity. Together with other approaches, the development of this technique can provide further insight into the mechanics of the largest biological sound generator, the sperm whale nose, and may prove to be heuristic in the development of biomimetic sound sources in man-made sonars.
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Acknowledgments |
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