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Chapter 60., (pp. 425-431) in “Echolocation in Bats and Dolphins”
The University of Chicago Press, Chicago, ©2004, ISBN: 0-226-79599-3
Chapter title:
High-Frequency Burst-Pulse Sounds in Agonistic/Aggressive
Interactions in Bottlenose Dolphins, Tursiops truncatus
Christer Blomqvist and Mats Amundin
Introduction
Most studies on dolphin communication have focused on whistles (e.g.,
Caldwell and Caldwell 1965). Whistles are omnidirectional (Evans, Sutherland,
and Beil 1964) and convey information to all members of a dolphin school about
identity, relative position, and, to some extent, emotional state of the whistler
(Caldwell and Caldwell 1972). Pulsed sounds, on the other hand, have mainly
been investigated in connection with echolocation (e.g., Au 1993), but a few
studies suggest that pulsed sounds are also used in social contexts. Dawson
(1991) found a significantly greater abundance of high-repetition-rate burst-
pulse sounds, labeled “cries,” during aerial and aggressive behavior situations
than during feeding in Hector’s dolphin (Cephalorhynchus hectori), suggesting
that these cries were social rather than echolocation sounds. He also claimed it
to be highly improbable that whistles would constitute the entire basis for
intraspecific communication in odontocetes, since this would imply that
nonwhistling species do not communicate acoustically at all. Amundin (1991)
reported that burst-pulsed sounds in agonistic and distress situations have
context-specific repetition rate patterns in the nonwhistling harbor porpoise
(Phocoena phocoena). Connor and Smolker (1996) reported that a pulsed “pop”
sound was correlated with courtship and/or dominance in the bottlenose dolphin
(Tursiops truncatus). Overstrom (1983) reported pulsed sounds correlated with
aggressive behaviors in the same species.
The objectives of this study were to investigate whether burst-pulse sounds
emitted in aggressive interactions contain ultrasonic frequencies similar to the
sonar sounds and to describe their repetition rate patterns and concurrent visual
behavior patterns.
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
2
Materials and Methods
In a previous study (Karlsson 1997), conducted in the Kolmården captive
dolphin colony, aggressive sounds in the human audible range (i.e., <20 kHz)
were recorded together with concurrent behaviors of two adult female bottlenose
dolphins. The females and their calves, 2 and 3 years old, were the only dolphins
present in the display pool. These recordings were included (courtesy of T.
Karlsson) and used to define aggressive behavior patterns to be recorded in this
study (see below). The sounds in Karlsson’s study were picked up by a Sonar
International Hydrophone (frequency response +2 dB between 10 Hz and 20
kHz), and recorded, together with concurrent visual behaviors, on a
videocassette recorder (Panasonic NV-FS 1 HQ; HiFi stereo audio frequency
range 0.5–20 kHz). Spectrograms of the audible sounds emitted in selected
aggressive encounters were produced using Spectra Plus (Prof. edition, v. 3.0a,
Pioneer Hill Software).
New recordings were made during six consecutive days, between 16 and 21
February 1998, in the same captive colony, including all 12 dolphins in the
Kolmarden colony. They were kept in a 6400 m
3
pool complex consisting of
three pools with a surface area of 900, 800, and 185 m
2
, respectively. Five of the
dolphins were born in the facility, and the age span in the whole group ranged
from 2 to 35 years. During the recordings the animals were temporarily
separated into two subgroups by means of a net barrier placed in a 4.0 m long ×
1.8 m wide × 2.3 m deep channel connecting the 900 m
2
display pool with the
185 m
2
holding pool (fig. 60.1). The net barrier prevented physical but not visual
or acoustical contact. The channel restricted the lateral movements of dolphins
engaged in social interactions across the net barrier. It thereby increased the
probability, with a fixed hydrophone (fig. 60.1), of recording sounds along the
axis of the rostrum where the higher frequencies (>100 kHz) would be expected
if the aggressive sounds were broadband and directional similar to sonar sounds
(Au 1993).
Fig. 60.1. Schematic view of the channel
connecting the display pool (900 m2) with
the holding pool (185 m2) at the Kolmården
dolphinarium, Kolmården Wild Animal
Park, Sweden. The hydrophone is attached
to the net barrier (white line) at mid-depth,
i.e., 1.5 m. The corner at the end of the
channel (A) was used occasional by
dolphins in the display pool to hide behind
during aggressive interactions over net
barrier (see Results).
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
3
Envelope Detector Recordings
The behavior of the dolphins was recorded using a b/w Ikegami video camera,
suited with a wide-angle lens and placed in an underwater housing mounted in
the channel. The video images were stored on a NV FS90 HQ Panasonic VHS
videocassette recorder. Underwater sounds were picked up by means of a Sonar
Products HS/70 hydrophone (frequency response ±14 dB between 5 and 150
kHz), mounted on the net barrier. The hydrophone was suspended at
approximately 1.5 m below the water surface.
The hydrophone was connected to an envelope detector, custom-made by
Loughborough University, UK. The internal band-pass filter of the detector was
supplemented with an external high-pass filter (HP-ITHACO 4302; 24
dB/octave). The frequency response of this recording system was ±2 dB
between 70 and 100 kHz. Between 110 and 150 kHz it was ±3.5 dB, albeit 12
dB lower than in the 70–100 kHz range. Below 70 kHz there was a 42 dB/octave
cutoff, with the sensitivity at 60 kHz in level with that at 120 kHz. The output of
the envelope detector, which was in the audible frequency range, was recorded
on the HiFi audio channel of the Panasonic VCR (frequency range 20 Hz to 20
kHz).
An interaction was classified as aggressive when two animals were in a face-
to-face position on opposite sides of the net barrier, emitting burst-pulse sounds
and showing concurrent visual aggressive behavior patterns—that is, head jerks,
pectoral fin jerks, “S”-shaped body postures, and jaw claps (DeFran and Pryor
1980; Overstrom 1983). On some occasions only one of two interacting animals
was in view of the underwater camera, because other dolphins, not engaged in
the interaction, were playing with the camera, thus concealing the other
individual. In spite of this, those encounters were included based on the sounds
and the visual behavior of the individual in view.
Burst-pulse durations were measured manually from spectrograms produced
by means of Spectra Plus (Prof. edition, v. 3.0a, Pioneer Hill Software).
Full-Bandwidth Recordings
In parallel with the envelope detector recordings, a selection of sounds, picked
up by the HS/70 hydrophone, was also recorded using a broadband DSP card
(model SPB2 from Signal-data, DK-2840 Holte, Denmark) and a Toshiba 3200
laptop. The frequency response was effectively determined by the hydrophone.
The DSP card was controlled by means of custom-made software (SBP Bat
Recorder v. 1.1, 11–96 CSC-OU, Odense University, Denmark). The maximum
duration of each recording was 590 or 655 ms, depending on the sampling
frequency used—that is, 333 and 300 kHz, respectively. The onset of each
recording was manually trigged, based on the character of the sounds, which
were transformed to audible range via an envelope detector and played through a
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
4
speaker, as well as on the continuous sound time series displayed on the
computer screen. An effort was made to get full-bandwidth samples of all the
sound types shown in fig. 60.2—that is, slow-repetition-rate click trains, as well
as medium- to high-repetition-rate burst pulses. Each recording was manually
stored on the hard disk of the computer as a separate file.
Fig. 60.2 Continuous spectrogram (FFT = 4096, overlap = 95 %) of the audible sounds
recorded in a typical aggressive interaction between two female bottlenose dolphins in the
display pool. Each square in the figure represents ~ 2.3 s and the total duration of the
sounds is 34.2 s. Y-axis = 3 kHz. The encounter contains slow click trains (squares 7-8
and 9-10), and pulse-bursts with low pulse-repetition-rates (squares 1-3, 6, and 11-15),
medium pulse-repetition-rate (squares 4-5, 11 and 14), and fast repetition-rates (squares 7-
8, 11). “Jaw claps” (see: Marten and Norris, 1988) are seen in squares 3, 8, 9, 10, 12, and
15. The short and very low frequency sounds in squares 14-15 are from the hydrophone
hitting the pool wall due to wave action.
The average power spectrum of 3–6 pulses (selected from the beginning,
middle, and end of each full-bandwidth recording of burst pulses) was calculated
using Waterfall (v. 3.18) software (Cambridge Electronic Design Ltd.). After
corrections for the hydrophone frequency response curve, the power spectra
were plotted against relative amplitude. Pulse repetition rate analysis was made
using MATLAB for Windows (v. 4.2c.1, MathWorks Inc.).
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
5
Results
Analysis of the recordings made by Karlsson (1997) revealed that burst pulse
sounds, with pulse repetition rates from 100 to over 900 pps, occurred frequently
in aggressive interactions between the two adult female bottlenose dolphins. Fig.
60.2 shows a 34.2 s continuous spectrogram of the audible sounds emitted in a
typical aggressive interaction between these two females. The frequency scale
was reduced to 3 kHz in order to better display the repetition rate patterns, as
revealed by the harmonic interval (Watkins 1967) and hence make them
comparable to the envelope detector recordings. The interaction contained slow
click trains and burst pulses with low, medium, and high pulse repetition rates.
The violent head jerks, pectoral fin jerks, S-shaped body postures, and jaw
claps (see DeFran and Pryor 1980; Overstrom 1983) seen in these free-
swimming aggressive encounters also occurred between dolphins interacting
across the net barrier.
The distance between animals interacting across the net barrier was estimated
to be between 1 and 4 m, whereas in free-swimming individuals involved in
such encounters, the distance initially was in the order of 10–20 m. Often there
was an escalation of the aggressive behaviors leading up to a climax of
simultaneous emissions of intensive burst pulses with medium to high repetition
rates and jaw claps, in concert with high-intensity aggressive behaviors. An
example of such burst pulses (recorded via the envelope detector and thus
representing the pulse frequency content between 60 and 150 kHz) is shown as a
spectrogram in fig. 60.3.
Fig. 60.3. Spectrogram of pulse-bursts recorded in an aggressive interaction between
two bottlenose dolphins. The sounds were band-pass filtered between 100 and 160
kHz and recorded on a VCR using an envelope detector. Burst durations: A – 200 ms,
B – 230 ms, C – 230 ms, D – 900 ms, E – 670 ms, F – 380 ms, G – 630 ms. Peak
pulse-repetition-rates: A – 380 pps, B – 415 pps, C – 940 pps, D – 195 pps, E – 200
pps, F – 100 pps, G – 195 pps.
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
6
An interaction over the net barrier most often started with two dolphins
approaching the net barrier from either side. Occasionally, it seemed to be
initiated by one animal closer to the net barrier, emitting click trains with a low
pulse repetition rate while apparently pointing its rostrum toward animals
passing by on the other side of the net barrier. The intensity and pulse repetition
rate, as judged by the human ear, often increased each time another dolphin
passed the net barrier. After a varying number of times ignoring this, the passing
dolphin could suddenly turn and approach the net barrier, in what appeared to be
a response to the other’s provocation.
During long aggressive interactions, in both the free-swimming and net
barrier situations, the dolphins often turned on their side or fully upside down
(i.e., rotated 90–180° along their longitudinal body axis). It was obvious that
both animals pointed their rostrum in the general direction of the other, and in
the gate situation more than what seemed to be inevitable due to the physical
restraints of the channel. In one interaction over the net barrier one of the
animals made short body jerks in synchrony with intense, short burst pulses
emitted by the other. On a few occasions the dolphin in the display pool was
also seen to hide behind the corner at the end of the channel (see fig. 60.1A),
seemingly trying to keep out of sight as well as out of the sound emission of the
other animal. From time to time it exposed its head to the aggressive burst
pulses of the antagonist, pointed its rostrum toward the other, and responded
with similar aggressive burst pulses.
Both visual behaviors and acoustic signals were immediately interrupted if
the net barrier suddenly was removed during an interaction. On such occasions,
the animals in the holding pool swam silently and at high speed through the
channel into the larger display pool. Continued fighting or any other aggressive
behavior was never seen immediately after the animals were reunited. With the
net barrier left in place an aggressive climax usually ended with slow to medium
pulse rate emissions from one or both of the animals, followed by one or both of
them leaving the net barrier. However, in the free-swimming encounters
between the two adult females, similar aggressive climaxes sometimes resulted
in both animals charging toward each other, apparently trying to bite and/or hit
each other with rostrum and/or tail fin. These physical encounters were very
short and did not result in any injuries. In other free-swimming encounters, one
of the females fled, chased by the other, or the interaction ended with both
animals just swimming away from each other, often after a final, intensive, low-
repetition-rate pulse train.
Envelope Detector Recordings
A total of 222 aggressive interactions were recorded across the net barrier with
the envelope detector setup, ranging between 0.4 and 37.3 s in duration. They
included 3706 burst pulses, and the presence of acoustic energy in the 60–150
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
7
kHz frequency range was confirmed in all these interactions. The average
number of bursts per interaction was 16.7, ranging from 1 to 49. A total of 3435
(92.7%) burst pulses were less than 500 ms in duration, 185 (5.0%) were
between 500 and 1000 ms, and 86 (2.3%) had a duration over 1 s. The mean
duration of the burst pulses within each of these classes was 130 ms, 680 ms,
and 1.39 s, respectively.
Full-Bandwidth Recordings
A total of 80 s of full-bandwidth recordings, divided into 234 data files, were
stored on the Toshiba hard disk. Twenty-six of these recordings were of
occasional single pulses or other types of sounds (e.g., jaw claps). Forty-one
recordings contained samples of pulse trains with a repetition rate below 100
pps. The remaining 167 recordings contained 249 burst pulses with a pulse
repetition rate of more than 100 pps. Of these burst pulses, 136 (55%) had a
peak pulse repetition rate between 100 and 250 pps, 64 (26%) had a repetition
rate between 251 and 500 pps, and 49 (20%) had a repetition rate between 501
and 940 pps.
Four of the full-bandwidth recordings included what appeared to be two
overlapping pulse trains. In each case, there was a fixed time lag between the
pulses in the two trains—that is, they had identical pulse repetition rates (153–
413 pps range). However, the amplitude changes were apparently independent.
All individual pulses had the same phase, indicating that they were either direct
sounds or direct sounds blended with a reflection from a hard surface (the pool
wall or floor).
Of the 249 burst pulses, 193 were recorded without overload, and thus
allowed for frequency analysis. Fig 60.4 shows the power spectrum of three
typical pulses chosen from the beginning, middle, and end of an aggressive
burst. It was 80 ms in duration and had a pulse repetition rate around 500 pps.
The –3 dB bandwidth was 20 kHz, centered on 120 kHz. There was a strong
component (–12 dB re to the 120 kHz peak) in the audible frequency range, with
a peak at 15 kHz.
Average power spectra of 3–6 pulses (1–2 selected from the start, middle,
and end of each burst, respectively) were calculated for all 193 pulse bursts. One
hundred and thirty-one (68%) of these bursts had an average frequency peak
above 100 kHz. Generally there was a second lower peak in the 20 kHz to 85
kHz range, although spectra with a single frequency peak above 100 kHz also
occurred (fig. 60.4). Sixty-two (32%) of the bursts had pulses with a peak
frequency below 100 kHz. The strong frequency component in the audible range
(fig. 60.4) was found in most of the burst pulses.
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
8
-50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
0 20406080100120140160
Fr e quenc y ( kHz )
Relative amplitude (dB)
Beginning
Middle
End
0
10 0
200
300
400
50 0
600
0 20406080100
Time (msec)
Pulse repetition rate (pps)
Fig. 60.4. Frequency spectrum (FFT size: 512) of three different pulses, selected
from the start, middle and end of an aggressive pulse-burst, 80 ms in duration. The
pulse-repetition-rate was around 500 pps. The sound was recorded digitally with an
A/D sampling rate of 333 kHz. The spectra are corrected for the frequency response
curve of the hydrophone.
Discussion
This study shows that the burst pulses, which occurred in aggressive encounters
between captive bottlenose dolphins, were broadband and contained strong
frequency components between 60 and 150 kHz, as well as in the audible
frequency range. All the characteristic pulse repetition rate patterns of these
bursts were only observed in situations containing aggressive behavior elements
(cf. DeFran and Pryor 1980; Overstrom 1983). Also, the increased energy
content in the audible frequency range (<20 kHz) of these sounds and the
synchronous escalation of concurrent aggressive behavior elements makes it
likely that these sounds were social signals.
The net barrier, separating the dolphins and protecting them from the
immediate consequences of their actions, may have amplified and even triggered
the aggressive behaviors. However, similar encounters were frequently observed
in these dolphins when swimming in the same pool (Karlsson 1997) and have
also been seen in wild Atlantic spotted dolphins, Stenella frontalis (Dudzinski
1995). Hence, the behavior recorded in this restricted situation may still
represent a sample of the normal, species-specific behavior repertoire.
The highest pulse repetition rate in the burst pulses recorded across the net
barrier was 940 pps. This corresponds to a pulse interval of a little over 1 ms,
that is, much shorter than that of close range sonar click “buzzes” (Evans and
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
9
Powell 1967). Also the estimated distances between the dolphins involved in
these interactions were 1–4 m—much longer than would be anticipated if the
burst pulses were close-range sonar signals. Thus, on the basis of pulse
repetition rate, it is unlikely that these very high pulse repetition rate bursts were
used for echolocation.
With only one hydrophone placed between the interacting individuals, it was
not possible to test whether the pulses in these social sounds were directional.
However, the similarity between their frequency spectrum and that of sonar
clicks (Au 1993) suggests that this may be the case. If so, the pulses with a
single frequency peak in the 115–135 kHz range may be from the beam core,
whereas those with dual frequency peaks may be from the beam periphery (see
Au 1993). It was also not possible to determine whether the overlapping pulse
trains in the full-bandwidth recordings originated from two different animals or
were a direct signal from one animal blended with a reflection from the pool
wall or floor. If the latter was the case, the amplitude difference between the two
overlapping pulse trains could be explained by the dolphin making small
scanning movements of the head and the hydrophone, and/or the reflective
surface was in the periphery of a sound beam similar to that found in the sonar
clicks (Au 1993). It is also possible that one of the dolphins synchronized its
pulse repetition rate to that of the other. Dolphins are adapted to match their
sonar click train to the returning echoes of moving targets (Au 1993), and
matching its repetition rate to that of another dolphin should not be an
impossible task. If this is the case, the social significance of such a
synchronization remains to be revealed. A third possibility is that the same
animal operated two independent pulse sound generators (Cranford et al. 1997).
The apparently deliberate pointing of the rostrum toward the antagonist may
be an indication that the proposed directional characteristic was used. This may
be to ensure that maximum sound energy reaches the other individual or to
address the aggressive signals to a selected individual. The more omni-
directional low-frequency components (<20 kHz) would allow the rest of the
group to hear the entire interaction, but unless hit by the high-frequency beam
core, they would know they were not the target for the aggression. Such a
directional acoustic signaling would be a potentially powerful communication
tool in a species lacking conspicuous directional visual signals, considered to be
highly important in social terrestrial mammals (e.g., Altmann 1967; Goodall
1968). This aspect is currently being investigated in our dolphins.
Burst-pulse emissions and conspicuous behavior displays dominated the free-
swimming aggressive interactions, and a climax including physical fighting
constituted only a very small part. Thus the sounds as well as the visual behavior
patterns may be part of a ritualized behavior sequence, with the purpose to settle
rank conflicts or other disagreements between herd members with a minimum of
physical fighting. Such physical fights may not only be dangerous to the
combatants, but may also be potentially dangerous to the herd (Lorenz 1969).
Submissive behaviors resulting in the inhibition of physical aggression have
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
10
been seen in many group-living terrestrial mammals—for example, the wolf,
Canis lupus (Schenkel 1967) and the chimpanzee, Pan troglodytes (Goodall
1986).
Although no accurate source levels were obtained, the aggressive burst
pulses sounded loud to the human ear, relative to sonar click trains emitted by
these animals. This may have been an artifact, due to the social sounds having
more energy in the audible range, as compared to sonar clicks. However, the
bottlenose dolphin has been demonstrated to be capable of producing very high
sound pressure levels (230 dB p-p re 1 µPa at 1 m during echolocation tasks; Au
1993). It has even been suggested that dolphins may be capable of debilitating
prey with intense sounds (Marten et al. 1988), a possibility originally suggested
for sperm whales, Physeter macrocephalus (Norris and Møhl 1983). Taking this
into consideration and the fact that hearing is extremely sensitive in dolphins
(Au 1993), it is possible that intense burst pulses, in aggressive interactions like
those reported here, can be used with the intent to cause auditory discomfort or
even pain in the antagonist. The temporal summation in the ear with increasing
pulse rates (Vel’min, Titov, and Yurkevich 1975 in Au 1993) may add to this
effect and favor the use of high pulse repetition rates in aggressive encounters.
The burst pulses, especially if they are directional, may then function as a safer
alternative to physically hitting an opponent with, for example, the rostrum or
the tail fin. The avoidance behaviors, such as “open mouth threat” or “tail
blow,” observed in response to the aggressive burst pulses, supports this
hypothesis. Amundin (1991) found similar avoidance in harbor porpoises
(Phocoena phocoena) in response to aggressive “sideward turn threats,”
including burst pulses with very high repetition rates (400–1000 pps). Such an
acoustic “weapon” would work equally well at night, in the dark at great depths,
or in murky waters, and would also allow the dolphin to keep track of where its
antagonist is under these circumstances.
From this point of view, it is easy to comprehend how this aggressive use of
pulse trains may have evolved from the original sonar function. The fact that
some of the interactions between the two females, who were close in rank
(Dolphinarium staff, pers. comm., 1998), resulted in direct, physical fights are in
no conflict with this interpretation. It may be compared with, for example, the
rare fights between impala (Aepyceros melampus) territorial males, taking place
in spite of conspicuous displays of neck and horn development, in combination
with a powerful roaring display (Estes 1991). These displays are usually enough
to discourage weaker opponents from daring a fight with them, but may not be
sufficient to intimidate equally strong males. The absence of injuries after fights
between the two dolphin females in this study may be due to them not really
trying to bite each other, but only performing a ritualized display fight. Such
ritualized fighting is found in antelope species with potentially lethal horns—for
example, the impala, A. melampus, and the oryx antelope, Oryx gazella (Estes
1991). Another example is the “bite inhibition” seen in wolves, Canis lupus, in
connection with “passive submission,” where the subordinate wolf rolls onto its
Blomqvist et al., High-frequency burst-pulse sounds in agonistic/aggressive interactions in the Bottlenose dolphin (Tursiops truncatus)
11
back, presenting its throat and abdomen, a posture that in effect prevents a
dominant wolf to kill a weaker pack mate (Mech 1970).
To study these social sounds in more detail, new methods have to be adopted
where free-swimming animals can interact with each other without being
restricted by a narrow channel, as in this study. At present, a sound recording
unit, attached by means of suction cups to the dorsal fin of our dolphins, is being
tested. It will record, in any social interaction, directional pulse sounds received
by the dolphin carrying the unit.
Acknowledgments
Thanks to Ericsson Mobile Communications for funding the project. Special
thanks to Lee Miller, Odense University, Denmark, for lending us the broadband
PC sound card and the Toshiba laptop, and for valuable technical advice. Thanks
also to Dave Goodson, Brian Woodward, Paul Lepper, Paul Connelly, and
Darryl Newborough at the Underwater Acoustics Group, Loughborough
University, UK, for technical support and equipment. Whitlow Au, Hawaii
Institute for Marine Biology, University of Hawaii, provided prompt and helpful
advice. Finally, thanks to the Kolmården Dolphinarium staff for being so
tolerant and helpful and always coming up with practical solutions to problems
during the recordings.
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