ORIGINAL PAPER

Acoustic communication in crocodilians: information encodingand species specificity of juvenile calls

Amelie L. Vergne • Thierry Aubin •

Samuel Martin • Nicolas Mathevon

Received: 21 November 2011 / Revised: 2 July 2012 / Accepted: 2 July 2012 / Published online: 21 July 2012

� Springer-Verlag 2012

Abstract In the Crocodylia order, all species are known

for their ability to produce sounds in several communica-

tion contexts. Though recent experimental studies have

brought evidence of the important biological role of young

crocodilian calls, especially at hatching time, the juvenile

vocal repertoire still needs to be clarified in order to

describe thoroughly the crocodilian acoustic communica-

tion channel. The goal of this study is to investigate the

acoustic features (structure and information coding) in the

contact call of juveniles from three different species (Nile

crocodile Crocodylus niloticus, Black caiman, Melanosu-

chus niger and Spectacled caiman, Caiman crocodilus).

We have shown that even though substantial structural

differences exist between the calls of different species, they

do not seem relevant for crocodilians. Indeed, juveniles and

adults from the species studied use a similar and non-

species-specific way of encoding information, which relies

on frequency modulation parameters. Interestingly, using

conditioning experiments, we demonstrated that this tol-

erance in responses to signals of different acoustic struc-

tures was unlikely to be related to a lack of discriminatory

abilities. This result reinforced the idea that crocodilians

have developed adaptations to use sounds efficiently for

communication needs.

Keywords Crocodiles � Caimans �Acoustic communication � Species recognition �Information coding � Conditioning experiments

Introduction

In crocodiles, acoustic signalling is used for social inter-

actions, particularly between adults during courtship and

territorial defence (Garrick et al. 1982), and within family

groups where survival of the young depends on maternal

care (Campbell 1973; Herzog and Burghardt 1977; Vergne

and Mathevon 2008; Senter 2008; Vergne et al. 2011; for a

review of acoustic communication in crocodilians, see

Vergne et al. 2009). Juvenile crocodilians produce various

sounds that have been classified into three main functional

categories. First, ‘‘hatching calls’’ (with the sub-categories,

pre-hatching, hatching and post-hatching) that are uttered

by embryos and newborns to solicit the mother to open the

nest and have been shown to fine-tune hatching synchrony

among siblings (Vergne and Mathevon 2008); second,

‘‘contact calls’’ produced by juveniles from hatching to

several days or weeks old. These mainly support cohesion

between juveniles (Vergne et al. 2011). And the third is

‘‘distress calls’’ that induce parental (mother) protection

(for an experimental approach of the biological roles of

contact and distress calls, see Vergne et al. 2011). Finally,

juveniles may emit ‘‘threat and disturbance’’ calls when

threatened (Britton 2001; Vergne et al. 2009). Although the

acoustic structure and the biological roles of these juvenile

signals have recently started to be studied (Britton 2001;

A. L. Vergne � N. Mathevon (&)

Equipe de Neuro-Ethologie Sensorielle, CNPS, Universite de

Lyon—Saint-Etienne, CNRS UMR 8195, Saint-Etienne, France

e-mail: mathevon@univ-st-etienne.fr

A. L. Vergne � T. Aubin � N. Mathevon

Centre de Neurosciences Paris-Sud, Centre National de la

Recherche Scientifique, UMR 8195, Paris, France

T. Aubin

Equipe ‘Communications acoustiques’, CNPS, Universite Paris

XI, CNRS UMR 8195, Orsay, France

S. Martin

La Ferme aux Crocodiles, Pierrelatte, France

123

Anim Cogn (2012) 15:1095–1109

DOI 10.1007/s10071-012-0533-7

Vergne and Mathevon 2008; Vergne et al. 2007, 2009,

2011), we still do not know the specific acoustic features

that make these sounds relevant to crocodilians.

Species specificity is one of the most consistent char-

acteristic of animal vocalizations, especially in the context

of reproduction (Dooling et al. 1992). This is related to the

fact that most sounds are directed towards conspecifics

whose reactions should be appropriate to the content of the

message. The species specificity of vocalizations is of

primary importance when two close-related species live in

sympatry and when risks of hybridization or mis-directed

parental care can occur. Among the 23 species of crocod-

ilians spread over the subtropical and tropical regions of

the New and Old World (Trutnau and Sommerlad 2006),

sympatry is rare and most interactions are likely to be intra-

specific (though there are some exceptions, for example,

Spectacled caiman Caiman crocodilus and Black caiman

Melanosuchus niger in South America). Due to genetic

drift, differences in the acoustic structure of vocalizations

may also arise in allopatry. Except Britton (2001) who

conducted a preliminary structural analysis of juvenile calls

from different species, there is no published investigation

on the species specificity of juvenile calls.

In the present study, we focused on juvenile contact

calls. These vocalizations, used by the group to gather,

seem to be mostly directed towards other juveniles (Vergne

et al. 2011). By investigating the acoustic structure of

contact calls from different species, our goal was to dis-

cover whether calls were species-specific or whether they

share a common rule for encoding information. First, we

compared the acoustic structure of contact calls from three

different species (one species belonging to the family

Crocodylidae: Nile crocodile Crocodylus niloticus; two

species from the family Alligatoridae: Black caiman,

Melanosuchus niger and Spectacled caiman, Caiman

crocodilus), and we looked for the presence of species-

specific information in these recorded calls using playback

experiments. Then, modified signals were broadcast to find

the acoustic features that trigger the receiver’s behavioural

response. Finally, conditioning experiments helped to

determine the details of some aspects of crocodile auditory

discriminatory abilities.

Experiment 1: species specificity of juvenile calls

Methods

Sound analysis

We analysed calls from juveniles of 3 different species

(Nile Crocodile, Spectacled Caiman and Black Caiman; 25

calls per species, from 5 individuals/species). Animals were

recorded either at the laboratory (Nile crocodiles and

Spectacled caimans, provided by the zoo ‘‘La Ferme aux

Crocodiles’’, Pierrelatte, France), or in the field (Black

caimans, studied along the Rupununi river, Guyana, South

America). All recorded calls were spontaneously emitted in

contexts where juveniles were gathered as a group, without

any visible disturbance. They thus can be considered as

typical juvenile ‘‘contact calls’’ (Vergne et al. 2009, 2011).

These acoustic signals are ‘‘complex’’ sounds, that is,

composed of a fundamental frequency and a harmonic

series, modulated in frequency and amplitude (Fig. 1).

The Nile crocodiles recorded were 4–6 days old. They

hatched in our laboratory, and vocalizations were recorded

at the time the crocodiles were released into a ‘‘home tank’’

where they could meet other siblings for the first time.

Spectacled caimans were older individuals (2–3 weeks old).

They hatched at the zoo ‘‘La Ferme aux Crocodiles’’ (Pi-

errelatte, France) and were brought to the laboratory

2 weeks later. Their vocalizations were recorded when we

released them together in their own home tank. The Black

caimans’ contact calls were recorded during a field trip to

Guyana (South America). Individuals were about 1 week

old. Contact calls were emitted spontaneously by juveniles

gathered on the riverbank. All recordings were performed at

a distance of 30–40 cm from the animals with an omnidi-

rectional microphone SENNHEISER MD42 connected to a

Marantz PMD670 tape recorder.

We conducted a sound analysis to describe the calls’

acoustic structure. For the Nile crocodile contact calls, the

analysis had already been performed and published else-

where (Vergne et al. 2009). For comparison purposes, we

used the same method to analyse the contact calls of the two

other species. Briefly, 7 acoustic variables were measured in

both temporal and frequency domains using Avisoft-SAS

Lab-Pro (http://www.avisoft.com/) and Praat (http://www.

praat.org) (Fig. 2). The total duration of the call (DT) was

measured from the oscillogram (Fig. 2b). We performed a

spectrographic analysis (window size: 1,024, sampling

frequency: 48 kHz, overlap 90 %) and measured two vari-

ables related to the fundamental frequency: the maximal

frequency value (F0max, Hz) and the ending frequency

value (Fend, Hz; Fig. 2a). To describe the frequency

modulation, we calculated the slope of the first temporal

quartile of the call (Slope 1) and the slope of the last three

temporal quartiles of the call (Slope 2; Fig. 2a). To describe

the spectral energy distribution, we measured two param-

eters on a spectrum of the entire call: the bandpass (Band-

pass, Hz) and the frequency at the maximum amplitude

(PicF, Hz) (Fig. 2c).

Statistical tests were conducted using the Statistica

package (version 6, Statsoft France). First, a one-way

MANOVA based on the 7 measured acoustic variables was

performed to compare the calls between the 3 species. This

1096 Anim Cogn (2012) 15:1095–1109

123

analysis was followed by a one-way ANOVA on each

acoustic variable and a post hoc Fisher’s LSD procedure at a

95 % confidence level to identify statistically homogeneous

groups. A cross-validated discriminant function analysis

using the 7 measured acoustic variables, preceded by a

principal component analysis, provided a classification

procedure that assigned each call to its appropriate species

(correct assignment) or to one of the two others (incorrect

assignment).

Playback experiments

Five young Nile crocodiles (age: 1 month old, sex

unknown) were tested. These individuals were kept

together in a home tank (dimensions: 120 9 180 9 50 cm;

water temperature: 30 �C; air temperature: 28–30 �C;appropriate luminance to this tropical species was pro-

grammed 12 h day/12 h night). During experimental peri-

ods, the tested individual was isolated from the others in a

test tank whose characteristics—particularly temperature,

luminance and feeding conditions—were the same as for

the home tank. Both tanks had a shelter at one end and the

crocodiles hid in it most of the time. The test tank was

located in an acoustically isolated room. To limit the

possible stress on the crocodiles due to carrying them from

on tank to another, we always moved the tested individual

in the test tank on the morning before the playback so that

there were several hours of non-disturbance before the

Fig. 1 Spectrograms (top) and oscillograms (bottom) of juvenile contact calls (a: Nile crocodile; b: spectacled caiman; c: black caiman)

Fig. 2 Acoustic parameters

used for the analysis of juvenile

calls. a Spectrogram.

b Oscillogram. c Average

frequency spectrum. DT (s):

total duration, F0max (Hz):

maximal frequency of the

fundamental, Fend (Hz): final

frequency of the fundamental,

slopes 1 and 2 (Hz/s): frequency

modulation slopes, PicF (Hz):

frequency at the maximum

power amplitude of the

spectrum

Anim Cogn (2012) 15:1095–1109 1097

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tests. All playback tests were conducted during the night

(from 8 p.m. to 2 a.m.)—a preliminary study of the croc-

odile circadian behavioural activity having shown a pref-

erential nocturnal activity. A low intensity red neon light

was placed above the aquarium to allow the monitoring of

behaviours at night.

To test the species specificity of calls, each of the 5

individuals was successively challenged with Nile crocodile

calls, black caiman calls, spectacled caiman calls and a

control noise (white noise band-pass filtered between 100

and 5,000 Hz; duration = 0.15 s; emitted at 50 dBSPL;

built with Syntana software, Aubin 1994). The spectrum

bandwidth, duration and intensity of the control noise were

chosen to mimic those of a juvenile natural contact call. A

loudspeaker was suspended at one end of the test tank (at

the opposite side from the shelter). Bubble wrap covered the

tank’s inner walls in order to minimize reverberation of

acoustic waves. The loudspeaker was connected to a com-

puter located outside the experimental room to minimize

disturbance for the isolated crocodile. This computer was

programmed to control the emission of acoustic stimuli.

Emission level conformed to the natural intensity of calls

(51 ± 4 dBSPL at 1 metre from the loudspeaker, measured

with a sound level meter SL-4001, Digital Instruments).

Every 2 h starting at 8 p.m., the tested young Nile crocodile

was exposed to a 90-second playback of an experimental

acoustic sequence. Each sequence was composed of 5

identical series of experimental signals; interval between

series = 12 s; 1 series = 4 repetitions of a given experi-

mental signal with a natural emission rhythm, approxi-

mately one call every 2 s; total duration of one series = 8 s.

During each experimental night, each tested individual was

challenged with 1 sequence of each species’ calls and 1

sequence of control noise. The order of sequences was

changed for each individual. Pseudo-replication within

sequences of a given species’ calls was avoided by building

sequences using different calls from different individuals.

Analysis of behavioural responses

All behavioural and vocal responses were recorded. We

were able to follow precisely the crocodile’s movements

with two webcams (IP DCS-900) suspended above the two

opposite sides of the tank to get a complete view. Go1984

software (http://www.go1984.com/) was programmed to

activate the webcams and record the crocodile’s activity

during sequences of 15 min (5 min before the start of the

playback sequence, during the 90 s of playback and 10 min

after). Any vocal activity was recorded during the same

duration via a microphone (Labtec desk microphone).

Two parameters were chosen to measure the responses to

playbacks: (1) Latency (t), the time between the first call

played back and the first observable reaction (head or body

movement) of the tested individual. A 4-level scale was

defined: 0 if t[ 60 s or no reaction, 1 if 41 B t B 60 s, 2 if

21 B t B 40 s, and 3 if t B 20 s; (2) animal displacements

were also quantified according to a 4-levels scale: 0 was

scored if the animal did not move at all during the 90 s of

playback, 1 if we observed a head and/or body orientation

towards the loudspeaker but no displacement, 2 if the indi-

vidual moved towards the loudspeaker but stayed at more

than 20 cm from it, and 3 if the individual came at less than

20 cm from the loudspeaker. Note that we only took into

consideration experiments when the crocodile was at the

opposite side of the speaker before the start of a test (that

was the case 99 % of the time). Also, we did not need

negative scores to assess displacements because once a

tested animal moved towards the speaker, it never moved

away from it during our recordings. These two scores (head

or bodymovement and displacements) were then summed to

obtain a general score representing the behavioural response

to playback, varying from 0 to 6, with 0 corresponding to the

weakest behavioural response and 6 to the strongest.

Using the behavioural response scores, we first per-

formed a Friedman two-way repeated measures analysis of

variance by ranks to detect differences in Nile crocodiles’

responses throughout the different playback tests. Second,

we did Wilcoxon tests to compare responses to the Nile

crocodile calls with responses to the two other signals.

Statistical tests were conducted using Statistica package.

Results

Acoustic differences between the contact calls of the three

species

Although the acoustic structure of calls of the three species

show strong similarities, as shown by spectrographic rep-

resentations (Fig. 1), the analysis sheds light on significant

differences between the measured parameters from one

species to another (Table 1, MANOVA, F(14,132) = 21.2,

P\ 0.001). The one-way ANOVAs and the Fisher’s LSD

tests reveal that all measured acoustic parameters except the

slopes of the frequency modulation allow differentiating a

Nile crocodile contact call from a Black or a spectacled

caiman’s call (ANOVAs: df = 2,72; P\ 0.001 for each of

the seven acoustic variables, ‘‘DT’’: F = 178, ‘‘F0max’’:

F = 21.2, ‘‘Fend’’: F = 9.29, ‘‘Bandpass’’: F = 31.2,

‘‘Slope 1’’: F = 7.65, ‘‘Slope 2’’: F = 8.30, ‘‘PicF’’:

F = 22.9; homogeneous groups: see Table 2). Nile croco-

dile calls are higher pitched (F0max, Fend, Bandpass),

possess more energy towards high frequencies (PicF) and

have a longer duration (DT) than calls of the other tested

species. Differences were also measured between contact

calls of Spectacled and Black caimans. For instance, the

maximum of the fundamental frequency (F0max) and the

1098 Anim Cogn (2012) 15:1095–1109

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frequency bandpass (Bandpass) are higher pitched in

Spectacled caiman calls than in Black caimans’ ones. The

frequency modulation slopes (Slope 1 and 2) also differ-

entiate Spectacled caiman contact calls from the two others,

the frequency modulation of the calls from this species

being more pronounced (Table 2).

In summary, though the basic acoustic structure of

crocodilian calls (a complex sound with multiple harmonic,

modulated in frequency and amplitude) appears similar

between different species, differences exist and are sub-

stantial enough to assign each call in its proper category

with confidence. A cross-validated discriminant function

analysis correctly classified 96 % of the Nile crocodile

contact calls, 88 % of the Black Caiman contact calls and

80 % of the Spectacled caiman calls (Fig. 3).

Behavioural response to contact calls

The Friedman analysis found significant differences

between Nile crocodiles’ responses to the broadcast signals

(P\ 0.02, Fig. 4): four individuals out of the five tested

did not react with the noise stimulus; conversely, the three

types of crocodilians’ calls elicited strong behavioural

responses from all juvenile Nile crocodiles. There was no

significant difference between responses to Nile crocodile,

Spectacled caiman and Black caiman calls.

Because acoustic analysis had shown significant struc-

tural differences between the calls of the three species, one

hypothesis that could explain why Nile crocodiles indi-

viduals respond similarly to calls of their own species and

to heterospecific calls is that all these signals encode the

same information. Investigation of the acoustic parameters

that are responsible for the crocodiles’ behavioural

responses may shed some light on this matter.

Experiment 2: identification of calls’ salient acoustic

parameters

Methods

From playback experiments 1, it appeared that Nile croc-

odile juveniles react similarly to calls of their own species

and heterospecific calls. However, they differentiate croc-

odilian calls from noise, meaning that calls convey some

‘‘crocodilian’’ information. In order to identify the relevant

parameters of calls (in the frequency and/or temporal

domains), we ran playback experiments using acoustic

lures (modified signals). Each acoustic parameter was

tested independently. The original sounds used to prepare

experimental signals were selected from the bank of

recorded contact calls previously described.

Adult crocodiles (especially females) are also known to

react to juvenile calls (Vergne and Mathevon 2008). Thus,

we decided to also test adult crocodilians in order to see

whether the coding rules might change with age.

Experiments on juveniles

Twenty-two naıve individuals (6 Spectacled caimans—

2 months old, 16 Nile crocodiles—1 month old, sex

unknown) were available for our experiments. To avoid

habituation due to multiple testing, we performed playback

experiments on two different sets of individuals:

The first experimental set was composed of 6 Spectacled

caimans and 6 Nile crocodiles. We tested them individually

with a series of 7 experimental signals (Fig. 5). Each

individual was tested with natural and modified signals

from its own species. To avoid pseudo-replication, we used

a different series of experimental signals for each indi-

vidual—each series being built from a different natural

contact call (NAT). Spectacled caiman natural calls came

from 2- to 3-week-old individuals while Nile crocodile

natural calls came from 4- to 6-days old individuals (see

details in Experiment 1 above). We worked with synthetic

copies of calls so that we could modify all parameters of

the signal. Modified versions of natural contact calls were

created with Avisoft-SAS Lab-Pro software. The six fol-

lowing signals were built from a NAT signal (Fig. 5): (1) a

synthetic copy of the natural call (Scontrol; control signal),

(2) a signal without amplitude modulation (noAM; the

frequency modulation of the NAT was retained), (3) a

signal without frequency modulation (FM1; the amplitude

modulation as well as the energy distribution between the

different harmonics was identical to the NAT), (4) a signal

without any harmonic structure (1H, only the first

Table 1 Mean ± SD of acoustic variables from contact calls of three species of crocodilians—Nile crocodiles, Spectacled caimans and Black

caimans (5 individuals, 5 calls/individual)

DT(s) F0max (Hz) Fend (Hz) Bandpass (Hz) Slope 1 (Hz/s) Slope 2 (Hz/s) PicF (Hz)

Nile crocodiles (N = 25) 0.195 ± 0.029 506 ± 116 211 ± 61 3,925 ± 760 -2,225 ± 1,536 -1,291 ± 429 721 ± 222

Black caimans (N = 25) 0.085 ± 0.014 319 ± 65 169 ± 46 2,573 ± 193 -2,784 ± 1,607 -1,437 ± 595 651 ± 242

Spectacled caimans (N = 25) 0.09 ± 0.024 379 ± 122 149 ± 49 3,289 ± 694 -4,005 ± 1,784 -2,091 ± 1,048 599 ± 123

DT (s): total duration, F0max (Hz): maximal frequency of the fundamental, Fend (Hz): final frequency of the fundamental, slopes 1 and 2 (Hz/s):

frequency modulation slopes, PicF (Hz): frequency at the maximum power amplitude of the spectrum

Anim Cogn (2012) 15:1095–1109 1099

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harmonic, which is also the one of highest energy, was

retained), (5) a temporally reversed natural call (Srev; a

temporally mirror of the frequency modulation slope of the

original call), and (6) a temporally reversed version of 1H

(1Hrev).

The second experimental set was composed of 10 naıve

Nile crocodiles (2 weeks old). Besides the control NAT

signal, they were challenged with 6 synthetic signals

(Fig. 5): (1) a synthetic copy of the natural call (synthetic

control signal, Scontrol); (2) a signal with a frequency

modulation slope reduced by one-third compared to the

natural slope (SLOPE1): in the Nile crocodile SLO-

PE1 = 1,017 ± 384 Hz s-1, in the Black caiman: SLO-

PE1 = 1,182 ± 417 Hz s-1 and in the Spectacled caiman:

SLOPE1 = 1,713 ± 557 Hz s-1; (3) a signal with a fre-

quency modulation slope reduced by a half compared to the

natural slope (SLOPE2): in the Nile crocodile SLO-

PE2 = 763 ± 288 Hz s-1, in the Black caiman:

SLOPE2 = 887 ± 313 Hz s-1 and in the Spectacled cai-

man: SLOPE2 = 1,284 ± 418 Hz s-1; (4) a signal with-

out any frequency modulation or harmonics series (FM2);

(5) a signal with a modified energy distribution across the

spectrum (ENER, with 80 % of energy between 2,500 and

5,000 Hz instead of between 200 and 2,500 Hz as in the

control signal); (6) a noise having the same duration, fre-

quency bandpass and amplitude modulation as the control

signal (NOISE). The first four and the NOISE experimental

signals were built using the Avisoft-SAS Lab-Pro software.

The ENER signal was built using PRAAT software.

The experimental procedures and the assessment of

behavioural responses were identical to those described in

the experiments testing the species specificity of calls

(Experiment 1; air and water temperatures were 28–30 and

30 �C, respectively). All statistical tests were conducted

using Statistica package. We performed a Friedman two-

way repeated measures analysis of variance by ranks to

detect differences in crocodilian responses between the

different playback tests, and Wilcoxon two-tailed tests for

comparisons of behavioural responses to the Natural call

with responses to other signals tested.

Experiments on adults

Adult experiments were performed at the zoo ‘‘La Ferme

aux Crocodiles’’ (hosting 350 adult Nile crocodiles in a

8,000 m2 tropical greenhouse; 80 % of the crocodiles are

mature females of 3–8 years old; the air and water tem-

peratures are 28–30 and 30 �C, respectively). Although it

was possible to approach animals conveniently, it was

impossible to test them individually. We thus played back

experimental signals to clusters of 7–24 adults and moni-

tored the response of all individuals together. Five clusters

were carefully chosen within the greenhouse to avoid

possible interferences between them: they were located as

far away as possible from each other (more than 30 m

apart). To limit the beam size of propagated sounds, we

used a directional loudspeaker (Audax) and only the tested

cluster was situated in front of the loudspeaker (the closest

Table 2 Results of Fisher LSD tests to identify statistically homo-

geneous groups of calls on the basis of 7 measured acoustic

parameters

Acoustic parameter Homogeneous groups

F0max (Hz) (Nile) (Black) (Spec)

Bandpass (Hz) (Nile) (Black) (Spec)

Slope 1 (Hz/s) (Nile, Black) (Spec)

Slope 2 (Hz/s) (Nile, Black) (Spec)

DT (s) (Nile) (Black, Spec)

Fend (Hz) (Nile) (Black, Spec)

PicF (Hz) (Nile) (Black, Spec)

‘‘Nile’’: Nile crocodile calls; ‘‘Black’’: black caimans calls; ‘‘Spec’’:

spectacled caiman calls. Calls within the same brackets cannot be

distinguished on the basis of the parameter considered

Fig. 3 Classification of crocodilian calls by discriminant function

analysis. The DFA correctly classified 96 % of the Nile crocodile

contact calls, 88 % of the Black Caiman contact calls and 80 % of the

spectacled caiman calls

Fig. 4 Behavioural reaction of Nile crocodile juveniles to contact

calls from different species and to white noise (each of the 5 tested

individuals is represented by a different colour) (colour figure online)

1100 Anim Cogn (2012) 15:1095–1109

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individual of the cluster was at least 10 m from the loud-

speaker). The amplitude of emitted sounds was of

61 ± 4 dBSPL at 1 m from the loudspeaker. The maximum

range of propagated signals from the loudspeaker was

approximately 20 m (at this distance, the intensity level

was below the background noise). Given the minimum

distance between clusters (30 m), we assumed that indi-

viduals within a cluster were not able to hear the signals

emitted during another cluster’s test. The experiments took

place in April, a month when most adults are sexually

active. Females had already started to lay their eggs; males

were defending their harems and thus tended to stay around

the same area in the greenhouse (±10 m). In addition, all

the experiments only lasted 2 days, so it is likely that the

composition of clusters remained stable during this time

and that no animal was tested twice.

To avoid habituation, each cluster was challenged once,

with only one experimental signal. Due to the limited

number of clusters that could be tested independently

(N = 5), we thus only used 5 experimental signals chosen

from those used for Nile crocodile juvenile tests: (1) a

natural contact call (control signal, NAT, number of adult

crocodiles within the tested cluster n = 24), (2) a signal

without amplitude modulation (noAM, n = 7), (3) a signal

without any harmonic structure (1H, n = 20), (4) a

signal without frequency modulation (FM1, n = 7), (5) a

signal without any frequency modulation nor harmonic

series (FM2, n = 20).

Behavioural responses were assessed according to a

4-level scale: 0 was scored if the playback did not provoke

any observable response during the 90 s of playback, 1 if

there was a head and/or body orientation towards the

loudspeaker but no displacement, 2 if the individual moved

towards the loudspeaker but stayed at more than 5 m from

it, and 3 if it came at less than 5 m from the loudspeaker.

The behaviour of each animal within a cluster was assessed

independently, allowing calculation of the proportion of

females with scores of 0, 1, 2 and 3 (expressed in % to the

total number of observed females) for each tested cluster

and thus each experimental signal. Due to the limited

number of tests, no statistical tests were performed on these

adult data and only raw results are presented.

Results

The frequency modulation is a key parameter

for both juveniles and adults

Playback experiments run on juvenile Spectacled caimans

and Nile crocodiles showed that the shape of the frequency

modulation seems to be a biologically relevant call

parameter: the responses to FM1, 1Hrev and Srev were

significantly weaker than the responses to the synthetic

copy of the natural call (Scontrol) for both species (Fig. 6a;

Wilcoxon tests, in Spectacled caimans, N = 6, T = 0.0,

P = 0.028; N = 6, T = 1, P = 0.046; N = 6, T = 0.0,

Fig. 5 Experimental signals used for playback experiments (NAT:

natural contact call; Scontrol: synthetic copy of the natural

call = control signal; noAM: signal without amplitude modulation;

FM1: signal without frequency modulation; 1H: signal without any

harmonic structure; Srev: temporally reversed natural call; 1Hrev:

temporally reversed 1H signal; SLOPE1: signal with a frequency

modulation slope one-third reduced compared to the natural slope;

SLOPE2: signal with a frequency modulation slope half reduced

compared to the natural slope; FM2: signal without any frequency

modulation nor harmonics series; ENER: signal with a modified

energy distribution among the spectrum; NOISE: noise having the

same duration, frequency bandpass and amplitude modulation as the

control signal)

Anim Cogn (2012) 15:1095–1109 1101

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P = 0.028, respectively; in Nile crocodiles, N = 6,

T = 0.00, P = 0.028 for the three signals). This observa-

tion was verified by the second run of experiments on

juvenile Nile crocodiles alone: individuals responded sig-

nificantly less to FM2 compared to the Scontrol signal

(Wilcoxon test, N = 10, T = 0.0, P = 0.005; Fig. 6b). In

addition, an interesting point is that, while the juveniles

continued to respond to a signal whose frequency modu-

lation slope was slightly modified (SLOPE1, Wilcoxon

test: N = 10, T = 4, P = 0.091), a stronger modification

significantly reduced their responses (SLOPE2, Wilcoxon

test: N = 10, T = 0.0, P = 0.008; Fig. 6b).

Figure 7 illustrates the proportion of juveniles and

adults who expressed a strong behavioural response to

signals (score C4 for juveniles, C3 for adults, see

‘‘Methods’’). Only signals with no or weak modifications of

the frequency modulation elicited such strong responses.

Conversely, FM1 and FM2 (no frequency modulation),

Srev and 1Hrev (temporally reversed frequency modula-

tion), as well as SLOPE2 (half reduced frequency modu-

lation slope) rarely elicited strong responses. These results

emphasize the importance of frequency modulation as an

acoustic parameter for the ‘‘crocodilian’’ information in

both juveniles and adults.

Conversely, our experiments demonstrated that ampli-

tude modulation is not a determinant parameter: more than

80 % of both juveniles and adults expressed strong

responses to the noAM signal (no significant difference

between juveniles’ responses to noAM compared to Scon-

trol: in juvenile Spectacled caimans, N = 6, T = 5, P = 1;

in juvenile Nile crocodiles, N = 6, T = 0.0, P = 0.109;

Figs. 6, 7). Moreover, the presence of harmonics did not

seem necessary: all tested adults strongly responded to 1H

and more than 70 % of the juveniles also did (Fig. 7; no

significant difference from Scontrol, Wilcoxon tests, in

Spectacled caimans, N = 6, T = 8, P = 0.60; in Nile

crocodiles, N = 6, T = 2, P = 0.075, Fig. 6a). We also did

not detect any differences in juveniles’ responses when the

energy of the calls was moved towards high frequencies

(responses to ENER were not significantly different to

Scontrol responses, Wilcoxon test: N = 10, T = 1,

P = 0.285; Figs. 6b, 7).

Thus, our experiments showed that the presence of the

frequency modulation of the call is necessary to elicit a

behavioural response in crocodilians from the youngest to

adult age. However, amplitude modulation, the presence of

harmonics and the energy distribution across the frequency

spectrum do not seem to be key parameters.

Experiment 3: test of juveniles’ discrimination abilities

using conditioning experiments

Methods

From the playback experiments, it appears that signals that

differ in their acoustic structure elicit a similar behavioural

response. It is worth asking whether that could arise from a

lack of discriminatory abilities (i.e. the tested animals are

not able to differentiate between the signals) or whether it

Fig. 6 Juveniles’ responses to

playback of natural and

modified calls. a Tests

performed on 6 Nile crocodile

juveniles and 6 Spectacled

caiman juveniles. b Tests

performed on 10 naive Nile

crocodile juveniles. The stimuli

eliciting the weakest

behavioural responses are those

with no frequency modulation

(NOISE) or a modified one

(1Hrev, FM1, Srev, SLOPE2,

FM2). Black stars highlightbehavioural responses that are

significantly different from the

control ones (i.e. Scontrol

signal)

1102 Anim Cogn (2012) 15:1095–1109

123

could be the result of a true behavioural choice (i.e. the

tested animals perceive the difference between calls of

different acoustic structures but do not show any differ-

ential responses to them). This situation (similar responses

to structurally different calls) occurred during our playback

experiments on juvenile Nile Crocodiles when, for

instance, we compared the behavioural responses to the

control synthetic Scontrol signals and to the ENER signals

(Scontrol signals = synthetic copies of natural calls;

ENER = signals with a modified energy distribution

among the spectrum, that is, 80 % of energy between 2,500

and 5,000 Hz instead of between 200 and 2,500 Hz in the

Scontrol signals). With the setup of experiment 2, behav-

ioural responses elicited by Scontrol and ENER signals

were not different (see ‘‘Results’’ of experiment 2 above);

however, this result does not prove that the tested animals

are not able to distinguish between both the two. It is

possible that both signals have similar biological relevance.

Conditioning experiments are a way to test whether this

undifferentiated response is linked to a something other

than discriminatory ability. Although conditioning experi-

ments have already been used in several situations with

crocodiles (Davidson 1966; Williams 1968; Burghardt

1977, to our best knowledge, this method had never been

used on crocodiles to test their discrimination abilities with

acoustic signals. To test whether young crocodiles can

learn to respond to a sound stimulus, we first started with a

simple GO/NOGO task. Second, we used a GO/NOGO

procedure to test the crocodile’s ability to discriminate

between Scontrol and ENER signals.

Animals and experimental conditions

Three naıve young Nile crocodiles (1 month old) partici-

pated in the experiments. The home tank in which the

individuals were kept had the same characteristic as in the

previous experiments. Before the beginning of condition-

ing experiments, we stopped feeding the young crocodiles

in their home tank. Feeding was used as a reinforcer during

the conditioning tasks and thus occurred only during

experimental periods when the individual was isolated

from the two others in the test tank. As the crocodiles were

used to being fed once every 3 days before the experi-

ments, we maintained this feeding rhythm and tested each

of the three crocodiles every 3 days. All experiments

occurred during the night between 8 p.m. and 5 a.m. (water

temperature: 30 �C; air temperature: 28–30 �C). All trialswere video-recorded. The tested individual was isolated in

the test tank the morning before the test. We used 4

‘‘speaking’’ feeders (CAT Gato) located in the four corners

of the test tank (Fig. 8a). The feeders were put on bricks at

about 5 cm above the water level, not only to protect them

from getting wet but also so that it would require a physical

effort for the crocodile to access them and check for the

presence of food. Each speaking feeder was composed of

four compartments of exactly the same size (Fig. 8b). The

opening of each compartment was electronically pro-

grammable. It took 15 s for each compartment to open

completely, and during that time, a soft and regular

mechanical noise could be heard. Once the compartment

was open, we could choose to programme the feeder to

play 20 s of a sound sequence or to remain silent. Each

feeder possessed an integrated microphone and a loud-

speaker (Fig. 8b).

Experimental signals and conditioning protocol

The first conditioning protocol (pre-training) aimed at

testing whether it was possible to condition a crocodile to

respond to a feeder opening associated with a sound

Fig. 7 Proportion of

responding individuals to

experimental signals (score C 4

for juveniles, C3 for adults).

The behavioural scores were

established according to a scale

(see text). Only few individuals

showed a strong behavioural

response to the signals with no

or disrupted frequency

modulation (Srev, 1Hrev, Noise,

SLOPE2, FM1 and FM2).

n = total number of tested

individuals (Nil = juvenile Nile

crocodiles; Spec = juvenile

Spectacled caimans;

Ad = adult Nile crocodile)

Anim Cogn (2012) 15:1095–1109 1103

123

stimulus. On the morning before any nocturnal test and

before we brought the crocodile in the experimental room,

the test tank was organized with two ‘‘food’’ feeders filled

with food (the four compartments of each feeder were each

filled with a piece of food, for example, fish, frog’s legs,

shrimps, chicken, meat…) and programmed to play 20 s of

a pre-recorded sound sequence (3 repetitions of the same

series; one series = 7 successive Scontrol signals from

different individuals; interval between Scontrol sig-

nals = 1 s; interval between series = 1.5 s). The pre-

recorded sound sequences differed between the 2 ‘‘food’’

feeders (different Scontrol signals), and we also made sure

not to use the same sequences from one night to another for

the same individual. The two other ‘‘no-food’’ feeders were

soaked with the smell of food (the same as the one placed

in the other feeders) but contained no food, and their

compartment opening was not programmed to be followed

by sounds. The position of the 4 feeders in each corner of

the tank was chosen at random and changed for every test.

We programmed the opening times in alternation between

the feeders so that every 30 min one opening occurred

from a given feeder. The order of openings between feeders

was randomly determined and changed for every test. The

experiment was run during 45 consecutive nights. Each of

the three individuals was tested every three nights (16

openings per night 9 15 nights/individual = 240 learning

trials/individual).

The second conditioning protocol (training) involved the

same three pre-trained crocodiles. There was no rest night

between the two protocols. The presence of food was still

associated with the Scontrol signal. However, we replaced

the silent signal associated with no food by a sound

sequence built with the ENER signal previously used

during the coding-decoding tests. The pre-recorded sound

sequences were different for each feeder (new Scontrol and

ENER signals), and we also made sure not to use the same

sequences from one night to another one on the same

individual. As in the first experiment, the two ‘‘no-food’’

feeders were soaked with the smell of the same kinds of

food as those placed in the other feeders, and the position

of the 4 feeders in each corner of the tank was chosen at

random and changed every day.

Measurements and analysis of behavioural responses

As for experiments 1 and 2, all conditioning tests were

controlled by a computer located outside the experimental

room. The audio and video recordings were set up to start

5 min before each feeder’s opening and to stay on for a

total of 15 min. We quantified the responses of the croc-

odiles using the video recordings by blindly assessing the

following 3 parameters (blind evaluation was achieved by

analyzing the behavioural responses without knowing what

was the sound stimuli that triggered them): (1) first, reac-

tion (head or body movement) of the tested individual

following the beginning of the feeder’s opening. We

expressed the number of positive responses (i.e. the event

‘‘crocodile reaction’’ occurred) as a percentage of the total

number of tests per category (i.e. no-food feeder or food

feeder). Every night, a crocodile was exposed to 16 fee-

der’s openings per category. In addition, we measured the

reaction latency T0 (i.e. the time in seconds between the

beginning of a feeder opening and the animal’s first reac-

tion). A 3-level scale was set up with a score of 3 assigned

when T0 B 10 s, 2 when 30 s B T0 B 11 s, 1 when

60 s B T0 B 31 s or 0 when T0[ 60 s (or no response).

(2) approach: the crocodile displacements towards the

feeder. A displacement was considered as an approach if

the crocodile came within 5 cm of the active feeder. As for

the reaction parameter, we expressed the number of

approaches as a percentage of the total number of tests per

feeder’s category. We also measured the time to approach

the feeder T1 (i.e. the time in seconds from the first reac-

tion to come within the 5 cm around a feeder). We used the

same scale as for the reaction parameter with 3 if

T0 B 10 s, 2 if 30 s B T0 B 11 s, 1 if 60 s B T0 B 31 s,

0 if T0[ 60 s. (3) climbing behaviour: once the crocodile

had reached the feeder, it had to decide whether or not to

Fig. 8 Test tank and speaking

feeders used during

conditioning experiments.

a Position of the 4 speaking

feeders at each corner of the test

tank. b Detail of a speaking

feeder. It is composed of 4

compartments whose openings

are programmable. The

integrated microphone was used

to record a 20 s sound sequence

which could be played via the

integrated loudspeaker

1104 Anim Cogn (2012) 15:1095–1109

123

climb on the feeder’s platform to look for the presence of

food. Note that every time a crocodile climbed onto a

feeder, it ate the food if it was available. We expressed the

number of climbing behaviours as a percentage of the total

number of tests per feeder category. We also measured the

climbing time T3 (i.e. the time in seconds from the end of

the approach to the feeder to come up onto the feeder to

check for the presence of food). The same scale as for the

two previous parameters was used.

For data analysis and statistical tests, we grouped each

three consecutive days of test (i.e. from day 1 to day 3

together, day 4 to day 6…until day 13 to day 15). For each

of these groups, we used Wilcoxon tests to compare

crocodiles’ responses to ‘‘food feeders’’ (associated to the

Scontrol signal) with their responses to ‘‘no-food feeders’’

(associated with Silence in the pre-training protocol or

ENER signals in the training protocol).

Results

Discrimination between calls with different energy

distribution over the frequency spectrum

The pre-training tests showed that it was possible to teach

the crocodiles to respond to a sound stimulation versus

silence. Results of the pre-training conditioning protocol

are shown Fig. 9. For the reaction parameter (Fig. 9a),

statistical tests show significant differences between

crocodiles’ reaction to ‘‘Scontrol feeders’’ compared to

‘‘Silent feeders’’ from the first day of test until the last:

crocodiles reactions were stronger in response to Scontrol

feeders. However, during the whole experiment, the tested

individuals were highly motivated and we have observed a

response in more than 80 % of all tests for both silent and

Scontrol feeders triggers. Times to react did not improve

with time or depend on the type of feeder (graphs not

shown). Conversely, the approach parameter (Fig. 9b)

shows that the crocodiles quickly learnt to associate the

presence of food with the Scontrol signal. This learning

was apparent in a significant decrease in approach behav-

iours towards the silent feeders. Until day 9, we still

observed a high percentage of errors (crocodiles approa-

ched both types of feeders in more than 80 % of the time:

no significant differences between responses to Scontrol

compared to Silence, D7–D9: P = 0.14, z = 2.38). How-

ever, from day 10, the number of approaches in response to

silent feeders decreased significantly (from 68 % of

approaches from day 10 to 12 to 55 % during the last

3 days of tests) while responses to Scontrol feeders remain

constant and close to 100 %. The speed of approaching the

active feeder did not change with time or depend on the

type of feeder. The climbing parameter was even more

discriminatory. Crocodiles climbed significantly less on

silent feeders compared to Scontrol feeders (Fig. 9c).

Indeed, while crocodiles climbed on Scontrol feeders more

than 90 % of the time, we observed a decrease from above

Fig. 9 Results of the pre-training tests: crocodiles learnt to associate

the presence of food with the Scontrol signal compared to silence.

a Reaction parameter: crocodiles answered during all the tests period

([80 % of reaction for both conditions) but analyses showed a

significantly stronger response to the Scontrol feeders compared to the

Silence feeders from the first days of experiment. b Approach

parameter: from day 10 the number of approaches in response to

Silence feeders decreased significantly (from 83 % of approaches at

day 7–9 to 55 % at day 13–15 of tests) while the number of

approaches to Scontrol feeders remained constant and close to 100 %.

c Climbing parameter: crocodiles climbed significantly less on

Silence feeders compared to Scontrol feeders and got better with

time (while crocodiles climbed on Scontrol feeders in more than 90 %

of the time, we observed a decrease from more than 60 % of climbing

behaviours at days 1–3 to less than 20 % at days 13–15 of tests in

response to Silence feeders). Results for Wilcoxon tests are presented

on each chart, *P\ 0.05, **P\ 0.02, ***P\ 0.01

Anim Cogn (2012) 15:1095–1109 1105

123

60 % of climbing behaviours the first 3 days to below

20 % the last 3 days of tests in response to silent feeders.

Also, times to climb did not change with time or depend on

the type of feeder. It is thus likely that crocodiles learnt

quickly to associate the presence of food with the Scontrol

sound sequence.

Second and interestingly, the training protocol showed

that crocodiles are able to discriminate between Scontrol

and ENER signals (Fig. 10). For the reaction parameter,

there is no significant difference between crocodiles’

responses to ‘‘Scontrol feeders’’ compared to ‘‘ENER

feeders’’ from the beginning to the end of our conditioning

tests (Fig. 10a). Furthermore, reaction times comparisons

still did not bring any information about the learning pro-

cess. The reaction parameter was thus not the most sensi-

tive for detecting learning. The crocodiles also seemed to

approach Scontrol feeders as much as ENER feeders (no

significant differences, from the beginning to the end;

Fig. 10b). Thus, the ‘‘approach’’ parameter did not help to

demonstrate any associative learning nor did the time to

make these displacements towards the two types of feeders.

However, the climbing parameter did show that crocodiles

learnt to differentiate between Scontrol and ENER signals

(Fig. 10c). From day 7, the number of climbing behaviours

in response to ENER feeders decreased significantly

compared to those in response to Scontrol feeders (between

70 and 92 % of climbing behaviours in response to Scon-

trol feeders during the 15 days versus a decrease from

85 % the first 3 days to about 50 % on the last 3 days of

tests in response to ENER feeders). Hence, crocodiles

learnt that the presence of food was associated with

Scontrol and not ENER and are thus able to differentiate

between these acoustic signals.

From a methodological point of view, note that playing

the sound after the feeder was opened gave us another clue

as to whether the crocodiles were able to learn that the

discrimination was based on sounds. The opening of the

feeder was relatively silent but crocodiles did react to it.

However, after a while we could see that the crocodiles

merely reacted to the sound and stayed motionless during

the feeder operation. This process confirmed that croco-

diles were attending to the broadcast sounds and not to the

opening of the feeders.

General discussion

The aim of this study was to investigate the acoustic

structure and information coding in the contact calls of

juvenile crocodilians. On the basis of acoustic analyses and

playback experiments, the following points have been

demonstrated:

First, in spite of common acoustic structure between

the contact calls of juvenile Nile crocodiles, Spectacled

caimans and Black caimans, most of the measured acoustic

parameters showed significant inter-specific differences

and it is straightforward to classify calls according to their

species origin using a multivariate analysis. Although calls

of all crocodilian species share the same overall acoustic

features, previous studies had already shown inter-specific

Fig. 10 Results of the training test: crocodiles were able to learn

discriminating between Scontrol and ENER signals. a Reaction

parameter: crocodiles did not express any significant differences in

their responses to Scontrol versus ENER feeders. They reacted to both

feeders in more than 80 % of the tests. b Approach parameter: they

approached both feeders and did not seem to learn with time not to

approach ENER feeders. c Climbing parameter: conversely, from day

7, crocodiles started climbing significantly less on ENER feeders

compared to Scontrol ones (between 70 and 92 % of climbing

behaviours in response to Scontrol feeders during the 15 days versus a

decrease from 85 % at days 1–3 to about 50 % at days 13–15 in

response to ENER feeders). Results for Wilcoxon tests are presented

on each chart, *P\ 0.05, **P\ 0.02, ***P\ 0.01

1106 Anim Cogn (2012) 15:1095–1109

123

variations (reviewed in Vergne et al. 2009). However, these

inter-specific differences may be biased in the present

study due to the relative age heterogeneity of our animals:

we showed in a previous study with newborn Nile croco-

diles’ distress calls that acoustic variables related to call

duration and to the fundamental frequency vary with the

individual’s age (Vergne et al. 2007). Specifically, the

youngest/smallest individuals produce the highest-pitched

calls. This may explain why the contact calls of the Nile

crocodile we studied possessed more energy towards high

frequencies and a broader frequency bandwidth than those

of the spectacled or black caimans. Nevertheless, the slope

of the frequency modulation has been found to remain

stable with the age in Nile crocodile’s distress calls (Ver-

gne et al. 2007), and in the present study, this parameter

was one of those that distinguished most strongly between

the contact calls of the different species. Hence, the vari-

ability in age between the animals recorded is unlikely to

explain all the observed differences and it is not unrea-

sonable to assume that inter-specific differences in the fine

structure of juvenile calls do exist.

Second, playback experiments showed that these inter-

specific structural differences between calls seemed not to

be relevant to the animals. We observed no significant

behavioural differences in juvenile Nile crocodile’s

responses to calls of their own species compared to calls of

other species. We have to be cautious with this result

because of the small sample size and unknown power of

the experimental system: in a more subtle paradigm and a

fortiori in the wild, discrimination between species calls

remains conceivable. Nevertheless, this result supports a

previously established hypothesis about a possible inter-

specific repertoire of crocodilian calls with behavioural

responses not restricted to the conspecific calls (Britton

2001; Campbell 1973) and is in accordance with several

preliminary experiments we have made in the wild. That

gives us several reasons to think that species-specific rec-

ognition based on juvenile calls is extremely weak or

nonexistent in crocodiles. The species specificity of juve-

nile calls may be irrelevant information for these animals.

One main point is that effective sympatry is rare among

crocodiles. The Nile crocodile lives in Africa while Black

and Spectacled caimans are American species. This geo-

graphical distribution might explain why the Nile croco-

diles have not developed the ability to discriminate their

calls from those of other species. The Black and the

Spectacled caimans are sympatric but do not usually meet

in the field, their preferential habitat being slightly different

(and observations have been made of Black caimans

chasing the other species, P. Taylor pers.com.). Based on

our own observations in the field, it is likely that acoustic

exchanges between juveniles and between adults and

juveniles occur only within family groups (i.e. a female and

her young; for example, Black caiman family clusters stay

away from other individuals, Vergne et al. 2011).

Third, our study shed light on the key acoustic param-

eters responsible for the biological relevance of the juve-

niles’ contact calls and yielded information on the process

of coding and decoding crocodilian information by juve-

niles and adults. The importance of the slope of the fre-

quency modulation and the tolerance of tested individuals

to slight modifications of this parameter is in accordance

with the inter-specific responses we observed during

Experiments 1, as FM slopes differed slightly between

species. An information encoding process using frequency

modulation is widespread in animals using acoustic to

communicate (Becker 1982). A coding based on a slow

frequency modulation has the advantage of being robust,

particularly in the face of propagation, because modulation

characteristics are only slightly damaged during transmis-

sion over long ranges (Wiley and Richards 1982). Also, as

the crocodilian contact call’s frequency modulation

extends over a large frequency band, its characteristics are

useful for improving the localization of the sound source

(Aubin and Jouventin 2002b). In addition, playback

experiments using modified signals showed that amplitude

modulation does not play a major role in inducing a

behavioural response. It is known that this parameter is

quickly modified during signal transmission throughout the

environment (Aubin et al. 2000; Mathevon and Dabelsteen

2002; Wiley and Richards 1982). Previous propagation

experiments using crocodilian calls have confirmed this

result (Vergne et al. pers. obs.). In birds (Jouventin et al.

1999) and mammals (Charrier et al. 2002), amplitude

modulation is never a parameter encoding specific or

individual identity. Nevertheless, this parameter can play a

crucial role during sound localization as has been demon-

strated for instance in the barn owl Tyto alba (Konishi

1973; Shalter and Schleidt 1977) and in the King Penguin

Aptenodytes patagonicus (Aubin and Jouventin 2002b).

Further experiments would be necessary to determine

whether this is also the case in crocodilians. Playback

experiments also showed that the entire frequency spec-

trum is not necessary to induce a behavioural response.

One unique harmonic signal seems to be enough to main-

tain the biological effectiveness of the signal. Experiments

with birds and mammals have shown that tolerance

towards such a modification is extremely variable from

one species to another. For instance in Adelie Penguin

Pygoscelis adeliae, parent-offspring recognition needs the

entire frequency spectrum while the same type of recog-

nition in Subantarctic Fur Seal Arctocephalus tropicalis or

in King Penguins is effective with a reduced number of

harmonics (Aubin and Jouventin 2002a; Charrier et al.

2002; Searby et al. 2004). Young crocodilians seem to be

quite tolerant towards modifications of this parameter. Just

Anim Cogn (2012) 15:1095–1109 1107

123

as for amplitude modulation, it does not mean that this

parameter could not contribute to the communication

process. In particular, the fact that harmonic series extend

over a large spectral bandwith is also likely to considerably

improve the localization of the emitter (Wiley and Richards

1982). We have to remember that calls are often emitted in

a context when the emitter is soliciting parental care or in

order to gather siblings: being easily localized by

receiver(s) thus makes sense. In the field, it is known that

adult crocodiles can localize juvenile calls accurately (Hunt

and Watanabe 1982; Passek and Gillingham 1999). How-

ever, research on sound localization in crocodilians is still

in its infancy. Only one study (Carr et al. 2009) has

investigated neurophysiological processes involved in

localization tasks. Future experiments are necessary to test

whether sound energy distribution across a wide spectrum

effectively reinforces the reliability of localization of the

emitter in crocodiles. Finally, we observed that shifting the

signal’s energy towards high frequencies (ENER signals)

did not seem to modify the level of response of the animals.

In crocodilians, the youngest and smallest individuals are

the ones who produce the highest-pitched sounds with

more energy towards high frequencies, especially in

stressful conditions (emission of high-pitched distress calls,

Vergne et al. 2007). Playback experiments would be nec-

essary to discover whether age information or stress level

could be carried in the calls and could trigger different

behavioural responses on the receiver’s side. However,

testing crocodilians’ abilities to discriminate between high-

pitched calls and natural calls via conditioning experiments

was a first step and further investigations are now needed.

Indeed, fourth and finally, the conditioning experiments

showed that crocodilians are able to learn an associative

task based on acoustic stimuli. In our experiment, indi-

viduals learnt to discriminate between a control contact call

and a modified call with more energy towards high fre-

quencies. Lack of discriminatory abilities thus cannot

explain why crocodilians did not express different behav-

ioural responses during playback experiments using these

two signals. It is likely that we observed the results of true

behavioural choices by the individuals tested. However,

this conditioning experiment remains quite preliminary and

requires further investigations. For instance, if tested ani-

mals could discriminate between both control and modified

calls, we must wonder why they would approach both

loudspeakers equally. We of course took great care to give

the same odour to all feeders, and thus, a crocodile that has

approached a feeder was unlikely to get olfactory cues to

tell whether there was food available. Also, from a com-

parison between climbing behaviours during the pre-

training and training experiments (Figs. 9c, 10c), it appears

that there were far more errors (i.e. animals climbing on the

wrong no-food feeder) when no-food feeders were

associated with the modified sound stimulus than with

silent no-food feeders. If crocodiles had cues by which they

could identify feeders with food, they should have suc-

ceeded equally in the two tests. So, a reasonable hypothesis

is that crocodiles had some difficulty in distinguishing

between the two acoustic stimuli whereas they had of

course no difficulty in distinguishing between the control

sound and silence during the pre-training conditioning

experiment. In spite of possible unknown biases in the

experimental setup, and although field experiments are

irreplaceable, this first investigation using conditioning to

assess discrimination between acoustic signals enhances

the interest of testing crocodiles in the laboratory.

This study has given new insights on acoustic com-

munication in crocodilians. In addition to previous work

that has demonstrated the biological roles of sounds in

crocodilians, especially in the context of parent-offspring

interactions (Vergne et al. 2009, 2011), here, by studying

the features (acoustic structure and information coding) of

a crocodile sound, we have found evidence that crocodiles

calls are true communication signals. A crocodile sound

carries information in the form of a code decipherable by

crocodilian receivers and which includes several acoustic

characteristics likely to make acoustic communication in

crocodilians particularly effective. Crocodiles appear to

be tolerant of substantial modifications of the acoustic

structure of juvenile calls. A previous study has shown

that adults are also tolerant to variations of adult signals

(Wang et al. 2009). Taken together, these results under-

line that information coding is resistant to signal degra-

dation. Crocodilians together with birds are the modern

representatives of the Archosauria phylum that includes

the extinct dinosaurs and pterosaurs (Hopson 1975;

Walker 1972). Most species of these modern archosaurs

provide parental care in terms of food provisioning (birds)

and protection (birds and crocodiles), and sound signals

play a major role in these parent-offspring interactions.

Birds and crocodiles share several important traits

regarding acoustic communication, and this supports the

hypothesis that one of the shared behavioural features of

past and present Archosaurs is the use of acoustic com-

munication, especially in the context of parental care and

sibling interactions.

Acknowledgments We thank Luc Fougeirol and the staff of ‘‘La

Ferme aux Crocodiles’’, Pierrelatte, France, for their great help. We

thank Peter Taylor and his staff for help with logistics during the field

study in Guyana; and Toshao Isaac Rogers for permission to conduct

work at Yupukari. This study was funded by the Institut universitaire

de France (NM), the Centre National de la Recherche Scientifique

(TA) and the Ministere de l’Education Nationale et de la Recherche

(AV). Experiments were performed under the authorization no

42–218-0901–38 SV 09, (ENES Lab, Direction Departementale des

Services Veterinaires de la Loire) and were in agreement with the

French legislation regarding experiments on animals.

1108 Anim Cogn (2012) 15:1095–1109

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