Localisation of an acoustic signal in a noisy environment: the display call of the king penguin Aptenodytes patagonicus
1 CNRS-NAMC, UMR 8620, Université Paris-Sud, F-91405 Orsay Cedex,
France
2 CNRS-CEFE, UPR 9056, 1919 Route de Mende, 34293 Montpellier,
France
* Author for correspondence (e-mail: thierry.aubin{at}ibaic.u-psud.fr)
Accepted 19 September 2002
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Summary |
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Key words: sound localisation, communication in noise, individual recognition, king penguin, Aptenodytes patagonicus
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Introduction |
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The display call of an adult king penguin (male or female) is a succession
of 4-8 similar sound components, the syllables, separated by strong amplitude
declines, which coincide with declines in frequency. The structure of the
syllable is complex, with two fundamental frequencies (the `two-voice'
phenomenon; Greenewalt, 1968)
and their corresponding harmonics, strongly modulated in frequency and
amplitude (Robisson, 1992a
).
It has previously been shown that the signal is highly stereotyped within each
individual but differs noticeably between individuals. Individuality of
information can potentially be supported by either temporal or frequency
parameters (Robisson, 1992b
;
Lengagne et al., 1997
);
however, even though it is possible for the king penguin to distinguish the
acoustic signals of different individuals, how the signal is identified
remains unknown. To investigate this codingdecoding process, we
conducted playback experiments in the field. Birds were tested using temporal
or frequency modifications of their natural call
(Jouventin et al., 1999
;
Lengagne et al., 2000
,
2001
). It appeared that the
vocal signature was based on two complementary codes: (1) frequency modulation
(FM) shape of the syllable, and (2) the beats generated by the two voices. A
chick recognised its parental call and paired mates recognised each other's
calls when only one syllable was played back. Even a modified signal with only
the two fundamental frequencies present was still recognised. In addition,
king penguins paid little attention to the amplitude modulation (AM) structure
of the syllable.
Are the complex AM and harmonic structures completely useless? Is the call organisation in successive syllables unnecessary? In fact, the display call is given in the context of a colony where it is difficult to localise individuals in the moving crowd, and where the acoustics are masked by the noise produced by other individuals. Thus, we hypothesised that these acoustic parameters might aid the emitter (1) to be located easily, and (2) to be discriminated from the background noise of the colony. To test these hypotheses, we played parental calls back to their chicks with either the AM or harmonic or syllabic structures modified. Then, we compared the responses to these signals with those to unmodified (natural) calls.
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Materials and methods |
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Recording and playbacks
Adult king penguins were recorded using an omnidirectional Sennheiser MD211
microphone mounted on a 2.5 m pole and connected to a Sony TCD10 Pro II DAT
recorder (sampling frequency: 44.6 kHz, frequency response flat within the
range 20-20000 Hz). The distance between the beak of the recorded bird and the
microphone was about 1 m.
For playback experiments, we used a 4200 Uher tape-recorder (tape-speed 19
cm s-1) connected to a 50 W Audix PH3 selfpowered loudspeaker
(frequency response 100-5600 Hz ±2 dB). Signals were played at a
natural sound pressure level (Robisson,
1993a; Aubin and Jouventin,
1998
), about 95 dBSPL (sound pressure level), measured 1 m from
the loudspeaker, with a Bruël & Kjaer Sound Level Meter type 2235
(linear scale, slow setting) equipped with a 1 inch condenser microphone type
4176.
Sound synthesis and analysis
Analog signals were digitised with a 16 bit Oros Au21 acquisition card
(equipped with a 120 dB/octave anti-aliasing filter) at a sampling frequency
of 16 kHz. Signals were stored on the hard disk of a computer and then
examined and modified with the Syntana V1.2 analytical software package
(Aubin, 1994).
Three kinds of experimental signals were constructed:
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Playback procedure and criteria of responses
Tests were conducted between 10.00 am and 5.00 pm, during clear and dry
weather, with a wind speed less than 4 m s-1. The chicks were
generally resting in the colony, preening themselves. Series A and series B
experiments were conducted at Crozet island and series C at Kerguelen
island.
To test the locatability of the signals (experiments with signals of series
A and B), the following playback procedure was used. The loudspeaker was first
put in the colony at a distance of 12 m from the test chick. At this distance,
and with a normal density of birds (about 2-3 individuals m-2 in a
colony of king penguins, according to
Barrat, 1976) the chick can
detect, recognize and localise the parental call without ambiguity
(Aubin and Jouventin, 1998
).
Then, the parental call or the experimental signal (on average 3.5 s long) was
broadcast repetitively with a 7 s time interval of silence between each
signal. This repetition rate is similar to that of an adult calling while
searching for its mate or chick (Robisson,
1993b
). The broadcast ended when either 12 signals had been
emitted or as soon as the distance between the tested chick and the
loudspeaker was below 2 m. At this distance, we estimated that the `meeting'
was realised, that is, that the sound source was accurately found by the
chick. The number of calls broadcast was then noted. To avoid habituation, no
more than 12 signals were broadcast. If the distance chick-loudspeaker
remained greater than 2 m after the broadcast of the whole series of signals,
a score of 12 was attributed to this experimental session. For series A, 16
chicks were tested and for series B, 15 chicks. Each chick was tested with two
series of signals, the experimental one (series A or series B signals) and the
control one (natural parental call series) and on 2 consecutive days, with one
experiment each day. The order of presentation of the two series was
randomised for the different chicks tested. Hence, the observed responses for
the whole group of chicks were not dependent on playback order.
To test the ability of chicks to discriminate the signal from the
background noise (experiments with series C signals), the following playback
procedure was used. The loudspeaker was placed in the colony at a distance of
7 m from the test chick, which corresponds to the natural calling distance of
an adult (Robisson, 1993b;
Lengagne et al., 1999a
).
Series C signals were played to the tested chick only when the ambient noise,
measured with the Sound Level Meter, was between 70 and 75 dBSPL, i.e. near
the average values usually observed in a king penguin colony (70 dBSPL,
Robisson, 1993b
; 74 dBSPL,
Aubin and Jouventin, 1998
). All
of the 16 chicks tested received the whole series of four signals. To prevent
habituation, each chick was tested only once a day with one of the signals of
the series. As previously, the order of presentation of the signals from day
to day was randomised for the different chicks. To evaluate how chicks
detected and recognized the playback signal, a four-class behavioural scale
was used. The scale was ranked as follows: class 0: no reaction; class 1:
agitation, head turning; class 2: head turning, calls in reply to the signal
broadcast; class 3: head turning, calls in reply to the signal broadcast,
approaches in the direction of the loudspeaker and stops in the vicinity (less
than 2 m).
This behavioural scale is similar to the one used in our previous playback
studies with king penguins (Aubin and Jouventin,
1998,
2002
;
Jouventin et al., 1999
;
Lengagne et al., 2000
).
Statistical analysis
As data were not normally distributed, non-parametric analyses were
performed. To compare paired samples (series A and B), a MannWhitney
U-test was used. To compare paired samples in more than two
categories (series C), we used a KruskalWallis test together with exact
two-sided P values. As the same marginal distribution was used
through several comparisons, we used the Bonferonni DunnSidak method
(Ury, 1976) to assess the
final significance of the test. All statistics computations were carried out
with Statistica V5 and StaXact V3 (Cytel,
1995
) softwares.
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Results |
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Series C signals
The broadcast of only one syllable, against a normal background noise,
elicited weak responses. Only one chick approached the sound source, two
chicks called in reply and the 13 remaining others were silent and stayed at
the same place. With two syllables broadcast, 75% of birds (12/16) called in
reply and one chick approached the sound source. This difference was
significant compared with when one syllable was broadcast
(Table 1). With four identical
syllables broadcast, all the chicks called in reply, and 75% of them
approached the sound source. Compared to the previous situation, the
difference was highly significant (Table
1). Finally, there was no significant difference in the responses
between broadcasting 4 or 8 syllables
(Table 1).
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Discussion |
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The display call of the king penguin comprises a series of 4-8 syllables
that are more or less similar and, as mentioned previously, only one (for
instance the first) is sufficient to elicit recognition. This was demonstrated
in a previous study (Jouventin et al.,
1999). During these earlier experiments, however, the one syllable
signal was broadcast only during relative periods of silence, that is, when
adults in the vicinity were not emitting calls. In fact, under more usual
conditions, as in the present study (i.e. when the level of the background
noise is about 70-75 dB), the broadcast of only one syllable is not sufficient
to draw the chick towards the loudspeaker. A majority of birds did not react
at all or only raised their heads, while staying in the same place. From two
identical syllables broadcast, almost all the birds call in reply, indicating
that the call has been discriminated and recognised. Nevertheless, a majority
of birds do not move towards the sound source. A possible explanation is that
the time duration of the signal (2 syllables of about 500 ms each) is not
sufficient to localise the sound source. This explanation seems implausible,
since it has been demonstrated that small birds are able to localise a 200 ms
duration pure tone or noise with a precision of about 20° in azimuth
(Klump et al., 1986
;
Park and Dooling, 1991
). Birds
with larger heads, such as the king penguin, are likely to be better for
localisation than small birds, since behavioural and physiological studies
have shown that large birds use mainly the interaural time difference for
localisation in azimuth (studies on the barn owl;
Moiseff and Konishi, 1981
;
Moiseff, 1989
). Another
possible explanation for this lack of movement towards the source after
hearing two syllables could be linked to the gregarious behaviour of the king
penguin chicks. Because of predation risks, chicks are reluctant to leave the
crèche (Stonehouse,
1960
). However, a more insistent appeal, with 4-8 syllables as in
our experiment, would encourage the chick to go out of the crèche, the
parental signal being recognised and localised with certainty. Effectively, it
is only with the broadcast of four syllables that almost all the birds move
closer to the loudspeaker. Thus it appears that at least four syllables are
necessary and sufficient to ensure a successful adultchick meeting.
From this series of experiments, it can be stated that a single syllable,
although containing the whole identity code, is not sufficient to convey with
security the information, even at short range (7 m), given the noisy
environment of the colony. As predicted by the mathematical theory of
information (Shannon and Weaver,
1949
), king penguins must repeat the code to ensure the
communication in context of the colony. In addition, a phenomenon called
temporal summation predicts considerable improvement in the detectability of
signals in a noisy environment by increasing the signal duration
(Klump, 1996
). Temporal
summation seems to be widespread in birds, i.e. budgerigars
(Dooling and Searcy, 1985
),
starlings (Klump and Maier,
1990
), canaries (McGregor et
al., 1997
).
In conclusion, king penguins use a particularly efficient acoustic system of communication, allowing recognition and localisation of individuals within a very constraining environment: a colony of thousands of birds, with a high background noise and a lack of visual cues. The acoustic features of the call that are not directly implicated in the coding process, such as the wide spectrum, the strong amplitude modulations and the redundant syllabic organisation, permit better localisation of the emitter and serve to counteract the masking effect of the background noise of the colony.
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Acknowledgments |
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