High-Frequency Burst-Pulse Sounds in · PDF fileHigh-Frequency Burst-Pulse Sounds in Agonistic ... sonar sounds and to describe their repetition rate patterns and ... During the recordings

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)

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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 m3 pool complex consisting of three pools with a surface area of 900, 800, and 185 m2, 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 m2 display pool with the 185 m2 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).


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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


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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.).


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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.


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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


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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.


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-50-45-40-35-30-25-20-15-10-50

0 20 40 60 80 100 120 140 160

Frequency (kHz)

Rel

ativ

e am

plitu

de (d

B)

BeginningMiddleEnd

0

10 0

2 0 0

3 0 0

4 0 0

50 0

6 0 0

0 2 0 4 0 6 0 8 0 10 0T ime ( msec)Pu

lse

repe

titio

n ra

te (p

ps)

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


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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


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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


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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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High-Frequency Burst-Pulse Sounds in · PDF fileHigh-Frequency Burst-Pulse Sounds in Agonistic ... sonar sounds and to describe their repetition rate patterns and ... During the recordings