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Ontogeny of Acoustic and Feeding Behaviour in the Grey Gurnard,
M. Clara P. Amorim & Anthony D. Hawkins
FRS Marine Laboratory, Victoria Road, Aberdeen, UK
Although sound production in teleost ﬁsh is often associated with territorial
behaviour, little is known of ﬁsh acoustic behaviour in other agonistic contexts
such as competitive feeding and how it changes during ontogeny. The grey
gurnard, Eutrigla gurnardus, frequently emits knock and grunt sounds during
competitive feeding and seems to adopt both contest and scramble tactics under
defensible resource conditions. Here we examine, for the ﬁrst time, the eﬀect of
ﬁsh size on sound production and agonistic behaviour during competitive feeding.
We have made sound (alone) and video (synchronized image and sound)
recordings of grey gurnards during competitive feeding interactions. Experimental
ﬁsh ranged from small juveniles to large adults and were grouped in four size
classes: 10–15, 15–20, 25–30 and 30–40 cm in total length. We show that, in this
species, both sound production and feeding behaviour change with ﬁsh size.
Sound production rate decreased in larger ﬁsh. Sound duration, pulse duration
and the number of pulses increased whereas the peak frequency decreased with
ﬁsh size, in both sound types (knocks and grunts). Interaction rate and the
frequency of agonistic behaviour decreased with increasing ﬁsh size during
competitive feeding sessions. The proportion of feeding interactions accompanied
by sound production was similar in all size classes. However, the proportion of
interactions accompanied by knocks (less aggressive sounds) and by grunts (more
aggressive) increased and decreased with ﬁsh size, respectively. Taken together,
these results suggest that smaller grey gurnards compete for food by contest
tactics whereas larger specimens predominantly scramble for food, probably
because body size gives an advantage in locating, capturing and handling prey.
We further suggest that sounds emitted during feeding may potentially give
information on the motivation and ability of the individual to compete for food
Correspondence: M. Clara P. Amorim, Unidade de Investigac¸ a
˜o em Eco-
Etologia, ISPA, Rua Jardim do Tabaco 34, 1149-041 Lisboa, Portugal. E-mail:
Ethology 111, 255—269 (2005)
2005 Blackwell Verlag, Berlin
Aggression is often observed in animals while competing for limited and
unevenly distributed resources like food (Archer 1988). Feeding success may
reﬂect the individual’s ability to either compete or to scramble for food items,
depending on the patterns of resource distribution in space and time. If food
is presented in a way that enables one or few animals in a group to
monopolize the resource (contest competition), feeding success should be
proportional to the individual’s ﬁghting ability (Grant 1993; McCarthy et al.
1999). However, when resources are indefensible, the proportion of food
obtained by an individual in a foraging group may reﬂect its capacity to be
faster or more eﬃcient in ﬁnding and exploiting food under conditions of
scramble competition (Grant 1993; Weir & Grant 2004). Animals may show
behavioural plasticity and switch between scramble and contest tactics
depending on the opportunity to monopolize the resource (e.g. Goldberg
et al. 2001; Grant et al. 2002). Hence, resource defence theory (sensu Brown
1964) predicts that when animals forage in groups, aggressiveness should
increase as resources become more defensible, i.e. more clumped in space, less
clumped in time and more predictable in both space and time (Grant 1993;
Goldberg et al. 2001).
Acoustic signalling is known to play an important role in agonistic contexts
in ﬁsh (reviewed in Ladich 1997), and may inﬂuence the outcome of contests
(Valinski & Rigley 1981; Ladich et al. 1992a). Acoustic cues from sounds
emitted during confrontations may provide information on the individual’s
relative ﬁghting ability and may mediate decisions to quit or escalate ﬁghts
(Clutton-Brock & Albon 1979; Myrberg 1997). For example, the dominant
frequency of sounds may give indication of body size in ﬁsh and in other
animals (Davies & Halliday 1978; Clutton-Brock & Albon 1979; Myrberg et al.
1993). Little is known about agonistic sound production outside territorial
defence (Ladich 1997), but sound production has been observed in competitive
feeding contests in triglids and other ﬁshes, and may signal feeding arousal and
diﬀerent levels of aggression (Hawkins 1993; Amorim & Hawkins 2000; Amorim
et al. 2004).
The characteristics of sounds may change throughout the life span of ﬁshes
¨ller & Ladich 1999; Wysocki & Ladich 2001). For example, the
dominant frequency of sounds decreases with increasing body size in a number of
ﬁsh species (e.g. Ladich et al. 1992b; Myrberg et al. 1993; Amorim et al. 2003).
The frequency and intensity of agonistic behaviour associated with competitive
feeding may also change with ﬁsh ontogeny (Ryer & Olla 1991). However,
ontogenetic changes of sound production and agonistic behaviour during
competitive feeding have never been studied so far. Our aim was to investigate
the eﬀect of ﬁsh size on sound production and feeding interactions in the grey
gurnard, Eutrigla gurnardus, and to ﬁnd out variations in the characteristics of the
emitted sounds during ontogeny, from small juveniles [10 cm in total length (TL)]
to large adult ﬁsh (40 cm in TL).
256 M. C. P. Amorim & A. D. Hawkins
The Study Species
The grey gurnard is a marine demersal ﬁsh commonly found in coastal waters
of the eastern North Atlantic at depths from approx. 20 m down to 140 m
(Wheeler 1969), in small (Protasov 1965) to occasionally extremely large shoals
(Heesen & Daan 1994). A previous study has shown that the grey gurnard seems
to both scramble and contest for food under defensible conditions (Amorim et al.
2004). Whether grey gurnards scramble or contest for food, their production of
sounds increases substantially compared with non-feeding situations (Amorim
et al. 2004). Competing ﬁsh emit knocks predominantly while grasping food and
during non-agonist behaviour, and they emit grunts mainly while performing
frontal displays to opponents, suggesting that knocks and grunts are associated
with diﬀerent levels of aggression. A third sound type, the growl, is heard typically
at the end of grunt sequences but is emitted only rarely (Amorim et al. 2004). In
the absence of conspeciﬁcs, this species rarely emits sounds during feeding bouts
indicating that sound emission is part of the social/agonistic behavioural
repertoire of this species (Amorim 1996). Typically, knocks are composed of 12
pulses, grunts of 48 and growls of more than 10 pulses, and also diﬀer in their
duration and pulse repetition rate (Fig. 1; Amorim et al. 2004).
Fish Collection and Maintenance
Fish were trawled at depths of 15–40 m in the North Sea and taken to the
aquarium facilities of the FRS Marine Laboratory, Aberdeen (UK). We grouped
ﬁsh into the following size classes: 10–15 cm (small, S), 15–20 cm (medium, M),
25–30 cm (large, L), and 30–40 cm (extra-large, XL) in total length. According to
length–age data found in the literature, S, M, L and XL ﬁsh were probably £1, 2,
Fig. 1: Oscillograms of a sequence of knocks (a), a grunt (b), and part of a growl (c), emitted by the
grey gurnards during competitive feeding
257Ontogeny of Acoustic Behaviour in Foraging Fish
4–5, and >6 yr old, respectively (Damn 1987). Two sets were used for the Ôsound
recordingÕand for the Ôfeeding interactionsÕexperiments. The established groups
were composed of three to eight individuals of both sexes and were sexually
inactive, although L and XL individuals had reached the size of maturity
(Papaconstantinou 1983). We maintained small and medium specimens in 1.5-
and 3-m-diametre ﬁbreglass tanks, respectively, and larger specimens in a
swimming pool of 7.0 m ·3.5 m ·1.5 m (length ·breadth ·depth). Experi-
mental tanks were provided with a sand substrate and with ﬁltered and sterilized
re-circulated seawater, with temperatures ranging from 7C (winter) to 12C
(summer). The light : dark illumination cycle resembled the natural photoperiod.
We fed ﬁsh three times a week, with ﬁsh or shrimp.
Recording and Analysis of Sounds
Sounds were recorded two to three times a week in 5-min feeding sessions in all
groups, with a hydrophone (Plessey, MS83; Plessey Company Ltd, London, UK), a
low-noise ampliﬁer (Brookdeal, model 450; Brookdeal Electronics Ltd, London,
UK) and a DAT recorder (Casio, model DA1; Casio Electronics Co. Ltd, London,
UK), as described in Amorim et al. (2004). We dropped food every minute through
a feeding tube throughout the recording session. The amount of food was scarce and
the presentation of food (clumped in space and predictable in time) was in such a
way as to promote competitive feeding (Grant 1993; Weir & Grant 2004). A total of
11, 20, 10, 11 sound recording sessions were obtained for S, M, L and XL sized ﬁsh
respectively. These recordings were obtained during diﬀerent periods of the year
according to the availability of ﬁsh: Jun.–Jul. (water temperature: 11.8C) for S,
May–Dec. (9.4–12.4C) for M, Mar.–Apr. (11.3–11.6C) for L, and Oct.–Nov.
(10–11.6C) for XL ﬁsh. Recordings for medium ﬁsh covered water temperatures
and seasons of the year similar to the other ﬁsh size classes, to allow ﬁsh size
comparisons. Fish group sizes used for sound recordings varied: four to ﬁve, three to
four, three to four and eight individuals for S, M, L and XL ﬁsh respectively.
We measured sound production rate (number of sounds emitted per minute)
from each recording session. We analysed knocks and grunts (described in Amorim
et al. 2004) with the Loughborough Sound Images software (version 2.0; 1986
Metagraphics Software Corporation
) for: sound duration: time elapsed from the
start of the ﬁrst to the end of the last pulse in a sound, ms; pulse duration: time
elapsed between the start and the end of a pulse, averaged for a maximum of 10
pulses, ms; number of pulses: total number of pulses within a sound; pulse period:
mean time elapsed between the peak amplitude of two consecutive pulses within a
sound, ms; and peak frequency: the frequency component with the highest energy
in the entire sound, measured for each pulse of sound, Hz.
We studied feeding interactions in S, M, XL size classes using synchronized
image and sound recordings obtained with a Sony video 8 camcorder (CCD-
258 M. C. P. Amorim & A. D. Hawkins
FX500E Pal 8; Sony Video 8, Sony, London, UK) as described by Amorim et al.
(2004). Recordings of approx. 15 min were obtained for each ﬁsh group three times
a week. Video recordings for each size class were performed at diﬀerent times of the
year because of ﬁsh availability but feeding activity and behaviour did not show any
noticeable seasonal variation (M. C. P. Amorim & A. D. Hawkins unpubl. data). Six
video sessions were obtained for small and medium ﬁsh and nine sessions for extra-
large ﬁsh. We fed ﬁsh with few items of food (fewer than the number of ﬁsh) every
minute throughout the ﬁlming session (from minute 0 to minute 14) to promote
competitive feeding, as explained above. Fish group sizes used for this part of the
study were six, four and eight individuals for S, M, and XL ﬁsh respectively.
We considered ﬁsh to be interacting with one another when they were in close
proximity and altered one another’s behaviour. We registered the succession of
behavioural acts for each ﬁsh participating in each interaction and considered the
following behavioural categories: dash, circle, grasp, orient, approach + chase,
frontal display and ﬂee, as described by Amorim et al. (2004). We measured
interaction duration (s) in 50 interactions taken at random from the three size
classes considered. We also took the following measurements for size class
comparisons: number of behavioural acts per interaction (per ﬁsh): measured for
each sequence of behaviours observed for each ﬁsh in an interaction; frequency of
a behaviour act in interactions (per ﬁsh): number of occurrences of a particular
behaviour per minute observed for each ﬁsh in an interaction; interaction rate:
number of interactions per min observed per session; number of ﬁsh involved in
an interaction: averaged per session; proportion of interactions accompanied by
sound production: averaged per session.
As sound emissions could not always be attributed to individual ﬁsh
(Amorim et al. 2004) we tested the eﬀect of ﬁsh size on the diﬀerent sound and
feeding interactions variables with ﬁsh size classes as the grouping variable.
Parametric statistics were generally used, but the eﬀect of ﬁsh size on sound
characteristics was tested with the nonparametric Kruskal–Wallis statistics since
none of the commonly used transformations managed to meet the normality and
homoscedasticity assumptions of the anova. We used Dunn tests as a posteriori
tests to examine diﬀerences between groups.
For the analysis of sound production rate, we tested the eﬀect of ﬁsh length
on normalized data for the eﬀect of group size by dividing the number of sounds
produced by minute by the number of ﬁsh in each group. For the analysis of
feeding interactions a diﬀerent approach was used, given that competitor density
(deﬁned as the number of competitors on a patch), and not group size, is
considered an important variable during competitive feeding (Goldberg et al.
2001). In this case, we tested the eﬀect of ﬁsh size with ancova, where the mean
number of ﬁsh involved per interaction and session (i.e. the mean number of
competitors per session) was used as an explanatory covariate. When the
mean number of ﬁsh involved per interaction was not signiﬁcantly correlated
Ontogeny of Acoustic Behaviour in Foraging Fish
(Spearman correlation) with the dependent variable, we performed one-way
anova instead. We used 95% conﬁdence intervals as a posteriori tests to examine
diﬀerences between groups.
Fish emitted sounds frequently during feeding sessions. Fish from S, M, L,
and XL groups produced on average 5.8 (4.6/0.8), 4.3 (3.0/1.2), 2.9 (1.8/1.1), 1.1
(0.6/0.5) sounds (knocks/grunts) per ﬁsh per minute respectively (Fig. 2). Smaller
individuals emitted signiﬁcantly more sounds per minute than larger ones (group
level: one-way anova:F
¼7.74, p ¼0.0003; ﬁsh level: one-way anova:
¼10.10, p ¼0.00003; Fig. 2). Individual size also had an eﬀect on the
characteristics of the emitted sounds. Sound duration, pulse duration and the
number of pulses increased with ﬁsh size whereas the peak frequency decreased
with ﬁsh size, for both knocks and grunts (Kruskal–Wallis test: all diﬀerences
were signiﬁcant at p < 0.01; Fig. 3). Sound duration increased by a magnitude of
7 ms in knocks and in 34 ms (median values) in grunts with increasing body size.
Median pulse duration increased from S to XL ﬁsh by 1.3 ms for knocks and by
1.7 ms for grunts. Knocks and grunts of XL grey gurnards had one or two pulses
more than their smaller counterparts. Knock peak frequency decreased 265 Hz
from S to XL ﬁsh and grunt peak frequency decreased by 31 Hz (median values).
No. of sounds/min
No. of sounds per fish/min
Fig. 2: Sound production rate (mean±SE) observed at the group and at the individual level for small
(S), medium (M), large (L) and extra-large (XL) size classes of ﬁsh during feeding sessions. Numbers
are sample sizes and are the same for both graphs. Groups that are signiﬁcantly diﬀerent (a¼0.05) are
indicated by diﬀerent letters (results from 95% conﬁdence intervals)
260 M. C. P. Amorim & A. D. Hawkins
Sound duration (ms)
Pulse duration (ms)
Pulse duration (ms)
Number of pulses
Number of pulses
Peak frequency (Hz)
Peak frequency (Hz)
Sound duration (ms)
200 248 150 125
Fig. 3: Comparison of sound and pulse durations, number of pulses and peak frequency of knocks
and grunts observed for small (S), medium (M), large (L) and extra-large (XL) size classes of
individuals during feeding sessions. Small squares represent the median, the open rectangles show the
25 and 75 quartiles and bars indicate range. Numbers are sample sizes and are the same for all graphs.
Groups that are signiﬁcantly diﬀerent (a¼0.05) are indicated by diﬀerent letters (results from Dunn
261Ontogeny of Acoustic Behaviour in Foraging Fish
At the sight of food, most ﬁsh swam rapidly towards the feeding area and
started interacting with one another often producing sounds, as described in
Amorim et al. (2004). The mean number of competitors per interaction was
signiﬁcantly higher in M than in S or XL ﬁsh (anova:F
¼8.58, p ¼0.002;
Fig. 4), although biological diﬀerences were probably negligible as the mean
number of interacting ﬁsh varied between 2.3 for S and XL, and 2.7 for M ﬁsh.
Feeding interactions had a mean duration of 4.2 s (±0.28 SE). The number
of behavioural categories per interaction increased with the number of ﬁsh per
interaction (for all size classes together – Spearman correlation: r
¼0.23, n ¼
698, p < 0.001; Fig. 5) and diﬀered signiﬁcantly between size classes (ancova:
¼10.90, p < 0.001, data transformed by 1/x), although there was no trend
with ﬁsh size, i.e., there was no increase or decrease (of no. of behaviours) with
increasing ﬁsh size. The mean number of behavioural categories observed per
interaction (±SE) were 2.9 (±0.07), 3.4 (±0.10) and 2.7 (±0.07) for S, M and
XL grey gurnards, respectively.
Mean no. of fish/interaction
135 90 108
Fig. 4: Number of ﬁsh (mean±SE) involved per interaction for small (S), medium (M) and extra-large
(XL) individuals. Numbers are sample sizes. Groups that are signiﬁcantly diﬀerent (a¼0.05) are
indicated by diﬀerent letters (results from 95% conﬁdence intervals)
Number of fish per interaction
No. of behaviours per
Fig. 5: Relation between the number of ﬁsh and the number of behavioural categories observed per
262 M. C. P. Amorim & A. D. Hawkins
All behaviours except circle, orient and approach + chase (for all size classes
together – Spearman correlation: r
¼0.07–0.40, n ¼21, p > 0.05), increased
with the mean number of competitors per interaction (for all size classes
together – Spearman correlation: r
¼0.60–0.81, n ¼21, p < 0.01). All agonistic
behaviours and dash decreased in frequency with ﬁsh size, although orient showed
only a marginal non-signiﬁcant diﬀerence between size classes (Table 1; Fig. 6).
Interaction rate increased with the mean number of ﬁsh involved in
interactions per session (for all size classes together Spearman correlation: r
0.66, n ¼21, p < 0.01).
Controlling for the number of competitors involved in interactions,
interaction rate decreased signiﬁcantly with ﬁsh size (ancova:F
p¼0.005; Fig. 7), from an average of 3.0 interactions per minute for S to 1.7 for
Feeding interactions were frequently accompanied by bursts of knocks and
grunts [n (recording sessions) ¼21;
x ± SE for all ﬁsh sizes: 87.9 ± 0.02%). The
average number of competitors per interaction did not have any eﬀect on the
proportion of interactions accompanied by sound, or speciﬁcally by knocks or
grunts (for all size classes together Spearman’s correlation: r
¼0.10–0.39, n ¼
21, p > 0.08). Overall, ﬁsh size had no signiﬁcant eﬀect on the proportion of
feeding interactions accompanied by sound production (one-way anova:F
0.09, p ¼0.91; Fig. 8). However, the trends observed for knocks and grunts were
opposite and cancelled each other out. The proportion of interactions accom-
panied by knocks were smaller in S than in larger ﬁsh (one-way anova:F
4.28, p ¼0.03; Fig. 8), but the reverse was observed for the proportion of
interactions accompanied by grunts, which decreased with ﬁsh size (one-way
¼6.58, p ¼0.007; Fig. 8).
Table 1: Eﬀect of ﬁsh size on the frequency of behaviours observed during feeding
Behaviour Test df F-value p-value
Orient anova 2, 18 10.90 0.097
Approach + chase anova 2, 18 7.18 0.005
Frontal display ancova 2, 17 7.39 0.005
No. of ﬁsh 1 8.53 0.010
Flee ancova 2, 17 11.88 0.0006
No. of ﬁsh 1 26.62 0.00008
Dash ancova 2, 17 6.88 0.006
No. of ﬁsh 1 10.17 0.005
Circle anova 2, 18 0.91 0.42
Grasp ancova 2, 17 2.09 0.15
No. of ﬁsh 1 18.34 0.0005
When the mean number of ﬁsh involved per interaction had a signiﬁcant eﬀect on the
dependent variable it was used as an explanatory covariate.
263Ontogeny of Acoustic Behaviour in Foraging Fish
In this study, we have shown that both sound production and associated
feeding interactions changed with ﬁsh size in the grey gurnard. Smaller ﬁsh were
Approach + chase
Fig. 6: Frequencies (mean±SE) for the behavioural acts observed for small (S), medium (M) and
extra-large (XL) size classes of grey gurnards during feeding interactions. Numbers are sample sizes
and are the same for all graphs. Groups that are signiﬁcantly diﬀerent (a¼0.05) are indicated by
diﬀerent letters (results from 95% conﬁdence intervals)
264 M. C. P. Amorim & A. D. Hawkins
more active sound producers than larger ones as they produced more sounds per
minute during feeding sessions. The acoustic features of knocks and grunts, the
most commonly emitted sounds, also changed with ﬁsh size. Sound duration,
pulse duration and the number of pulses increased with ﬁsh size whereas the peak
frequency decreased with ﬁsh size, for both knocks and grunts. The change in the
predominant frequency of the sounds during ontogeny was almost certainly the
result of the increase in body size.
Sounds from diﬀerent size classes of grey gurnards were recorded at diﬀerent
time of the year because of their availability and therefore at diﬀerent
temperatures. Water temperature is known to inﬂuence diﬀerent parameters of
sounds such as sound duration and pulse period (e.g. Connaughton et al. 2000)
and may have inﬂuenced our results. We suggest, however, that the detected
diﬀerences between ﬁsh groups were because of ﬁsh size. S ﬁsh were recorded at
very similar temperatures to L gurnards. Moreover, temperatures for M gurnards
covered the range of all groups, thus covering all variability caused by any
temperature eﬀect. Thus, any variability in sound features caused by temperature
would have decreased the probability of ﬁnding a size eﬀect rather than creating a
spurious size eﬀect.
Interaction rate decreased signiﬁcantly with ﬁsh size during competitive
feeding sessions. Moreover, agonistic behaviour including frontal displays
decreased in frequency with ﬁsh size. These results suggest that smaller ﬁsh
interact more often and are more aggressive during competitive feeding than
larger ones. We do not believe that diﬀerences in group size among size classes
inﬂuenced these results as variability in the mean number of interacting ﬁsh was
small among size classes and was accounted for as a covariate in ancova.
Diﬀerences in ﬁsh density were also unlikely to have aﬀected ﬁsh interactions
because tank space was plentiful and, when feeding occurred, ﬁsh concentrated
around the feeding area where social interactions took place.
There were no diﬀerences between the proportion of feeding interactions
accompanied by sound production between ﬁsh size classes. However, the
percentage of interactions accompanied by knocks and by grunts increased and
Fig. 7: Interaction rate (mean ± SE) observed for small (S), medium (M) and extra-large (XL) grey
gurnards during feeding interactions. Numbers are sample sizes and are the same for all graphs.
Groups that are signiﬁcantly diﬀerent (a¼0.05) are indicated by diﬀerent letters (results from 95%
265Ontogeny of Acoustic Behaviour in Foraging Fish
decreased with ﬁsh size respectively. Smaller ﬁsh performed more agonistic acts
including frontal displays, which are associated with the emission of grunts
(Amorim et al. 2004). Conversely, the emission of knocks is associated with
grasping food and nonagonistic acts (Amorim et al. 2004) and was frequent in
larger, less aggressive grey gurnards.
The fact that smaller ﬁsh emitted sounds at a higher rate, interacted more
frequently during feeding, performed more agonistic acts, and showed a lower
and higher proportion of interactions accompanied by knocks and grunts than
larger animals, respectively, suggest that smaller grey gurnards competed for food
by contest tactics whereas larger specimens predominantly scrambled for food.
According to the resource defence theory (Brown 1964), animals will only defend
resources and adopt aggressive tactics when the net beneﬁts of defence are greater
Interactions with sound
Interactions with knocks
Interactions with grunts
Fig. 8: Proportion of feeding interactions (mean ± SE) accompanied with sounds (a), and speciﬁcally
with knocks (b) and grunts (c) for small (S), medium (M) and extra-large (XL) grey gurnards. Numbers
are sample sizes and are the same for all graphs. Groups that are signiﬁcantly diﬀerent (a¼0.05) are
indicated by diﬀerent letters (results from 95% conﬁdence intervals)
266 M. C. P. Amorim & A. D. Hawkins
than those of alternative tactics such as scrambling for the resource. As ﬁsh with
larger bodies are more eﬃcient in locating (e.g. Browman & O’Brien 1992),
capturing (e.g. Wanzenbo
¨ck 1992) and handling prey (e.g. Mittelbach 1981), it is
probably more economically advantageous for larger grey gurnards to scramble
than to compete aggressively for food than it is for smaller ones, under similar
patterns of food availability.
Grey gurnards make sounds by contracting sonic muscles attached to the
gas-ﬁlled swimbladder. The vocal apparatus increases with ﬁsh size but does not
show evident macroscopic structural changes (Amorim 1996). The peak frequency
of sounds produced by a swimbladder mechanism is expected to decrease with ﬁsh
size as the resonance frequency of the swimbladder reduces with size (e.g.
Myrberg et al. 1993; Amorim et al. 2003). Lower peak frequencies and longer
pulse durations may also result from muscle-scaling eﬀects (Wainwright & Barton
1995), as larger ﬁsh with larger sound-producing muscles would take longer to
complete a muscle twitch, resulting in longer pulse durations and lower peak
frequencies (Connaughton et al. 2000, 2002). The ability of larger ﬁsh to make
longer sounds with a higher number of pulses (i.e. higher number of contractions
of the sonic muscle), could reﬂect the fact that they had reached the size of sexual
maturity (Papaconstantinou 1983) and thus may have suﬀered physiological
changes associated with the increase of sonic muscle fatigue resistance for the
production of courtship sounds (Fine et al. 1990; Brantley et al. 1993; Con-
naughton & Taylor 1995; Modesto & Cana
´rio 2003). Fish courtship sounds are
typically longer and with a faster pulse repetition rate than agonistic sounds (Gray
& Winn 1961; Hawkins 1993; Brantley & Bass 1994). Another species of ﬁsh that
has been studied for ontogenetic changes in sound production (the croaking
gourami, Trichopsis vittata) also showed an increase of the temporal character-
istics (total duration, number of pulses, pulse interval) and a decrease of dominant
frequencies of the agonistic croaking sounds (Henglmu
¨ller & Ladich 1999;
Wysocki & Ladich 2001).
This study shows that sound production in ﬁsh in agonistic contexts is not
limited to territorial defence and that sound-producing behaviour shows plasticity
with ﬁsh ontogeny and with the feeding strategy adopted. In the grey gurnard, the
rate of sound production, the sound type emitted, and the temporal and frequency
characteristics of knocks and grunts are potentially able to give information on
the level of individual motivation and ability to contest food resources.
Our study also adds to the literature on sound production in juvenile ﬁsh.
Data on acoustic behaviour of juvenile poikilothermic vertebrates is scarce and
has been only mentioned for juvenile ﬁsh in four species other than the grey
gurnard (reviewed in Henglmu
¨ller & Ladich 1999 and Wysocki & Ladich 2001).
The ontogeny of sound production from hatching to maturation has only been
investigated in the croaking gourami (Henglmu
¨ller & Ladich 1999; Wysocki &
Ladich 2001) and in the cichlid ﬁsh, Tramitichromis intermedius (Ripley & Lobel
2004), although 40 families of ﬁsh are known to vocalize during agonistic
interactions (Ladich 1997) suggesting that many ﬁshes are likely to emit sounds
Ontogeny of Acoustic Behaviour in Foraging Fish
This research was ﬁnancially supported by a grant (BD/2346/92-IG, Programa Cieˆ ncia) and by
the pluriannual programme (UI&D 331/94) of FCT, Portugal. We thank D. Urquhart and M. Burns
for their technical support, V. Almada for valuable suggestions for the behavioural analysis, Y.
Stratoudakis for comments on this manuscript. We are also thankful to M. Fine and another
anonymous referee for their comments.
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Received: April 14, 2004
Initial acceptance: June 8, 2004
Final acceptance: October 22, 2004 (L. Sundstro
269Ontogeny of Acoustic Behaviour in Foraging Fish