Julie Thévenet
email: julie.thevenet.phd@gmail.com

Mounia Kehy

Nicolas Boyer

Aurélie Pradeau

Léo Papet

Etienne Gaudrain

Nicolas Grimault

Nicolas Mathevon

Sound categorization by crocodilians

Keywords: Auditory perception, categorization, learning, GO/NOGO operant conditioning, animal communication, crocodiles

Rapidly sorting the information contained in a stream of stimuli is a major challenge for animals. One cognitive mechanism for achieving this goal is categorization, where the receiving individual considers a continuous variation of a stimulus as belonging to discrete categories. Using playback experiments in a naturalistic setup, here we show that crocodiles confronted with an acoustic continuum ranging from a frog call to a crocodile call classify each acoustic variant into one of these two categories, establishing a meaningful boundary where no acoustic boundary exists. With GO/NOGO experiments, we then observe that this boundary is defined along the continuum following learning. We further demonstrate that crocodilians rely on the spectral envelope of sounds to categorize stimuli. This study suggests that sound categorization in crocodilians is a pre-wired faculty allowing rapid decision making, and highlights the learning-dependent plasticity involved in defining the boundary between sound categories.

INTRODUCTION

Detecting and quickly processing information from the massive flow of information emanating from the auditory scene is a major challenge for animals [START_REF] Hulse | Auditory scene analysis in animal communication[END_REF] . Although an auditory stimulus can be accurately described physically, its perception by animals depends primarily on their auditory abilities and perceptual processes [START_REF] Guilford | Receiver psychology and the evolution of animal signals[END_REF][START_REF] Miller | Receiver psychology turns 20: is it time for a broader approach?[END_REF][START_REF] Rowe | Receiver psychology: a receiver's perspective[END_REF][START_REF] Bee | Psychological Mechanisms in Animal Communication Animal Signals and Communication[END_REF] . Being able to categorize stimuli can help process auditory information and make decisions about how to respond. In this process, animals integrate continuous variation in a sound stimulus as belonging to discrete categories carrying different information. This results in a peak in discrimination between stimuli of different categories versus stimuli of the same category [START_REF] Wood | Discriminability, response bias, and phoneme categories in discrimination of voice onset time[END_REF][START_REF] Harnad | Categorical perception: The groundwork of cognition[END_REF][START_REF] Caves | Categorical perception of colour signals in a songbird[END_REF] . However, categorizing sounds, i.e., deciding which stimulus corresponds to which object in the sound scene, such as a conspecific call rather than a call from another species, can be complicated especially when the two signals are acoustically close. As suggested by the signal detection theory [START_REF] Bradbury | Principles of animal communication[END_REF] , the optimal decision in terms of the probability of responding to a conspecific call rather than a heterospecific call depends on the relative probability of each of the two species being present at the location of the sound source, the acoustic cues carried by the sound stimulus, and the cost and benefit of the decision made by the animal. An acoustic feature threshold value -or informational threshold-is identified, determining the point at which animals decide on their categorical classification and behavioral response [START_REF] Weber | De Pulsu, resorptione, auditu et tactu: Annotationes anatomicae et physiologicae[END_REF][START_REF] Akre | Psychophysics and the evolution of behavior[END_REF] .

Previous works have shown that frequency parameters are often used to quickly classify sounds into categories. The cricket Teleogryllus oceani categorizes pulsed sounds into attractive versus repulsive when they cross a frequency threshold of 16 kHz [START_REF] Wyttenbach | Categorical Perception of Sound Frequency by Crickets[END_REF] . The great tits Parus major bases its classification of song notes on their fundamental frequency [START_REF] Weary | Categorization of song notes in great tits: which acoustic features are used and why?[END_REF] . The japanese macaque Macaca fuscata relies on the temporal position of a frequency peak to classify the contact calls of its species into two functional categories [START_REF] May | Categorical perception of conspecific communication sounds by Japanese macaques, Macaca fuscata[END_REF] . Categorical thresholds may also be related to the temporal characteristics of sound stimuli. Thus, the duration of song notes and calls is the criterion used to establish the threshold between two informational categories and guide the behavioral response of the swamp sparrow Melospiza georgiana [START_REF] Nelson | Categorical perception of a natural stimulus continuum: birdsong[END_REF] and the mouse Mus domesticus [START_REF] Ehret | Categorical perception of mouse-pup ultrasounds in the temporal domain[END_REF] respectively. However, our understanding of sound categorization processes in animals remains limited since, among tetrapods, only a few species have been studied [START_REF] Fischer | Categorical Perception in Animals[END_REF][START_REF] Baugh | Categorical perception of a natural, multivariate signal: mating call recognition in túngara frogs[END_REF][START_REF] Green | Categorical perception in animal communication and decision-making[END_REF] . In the present paper, we test whether crocodilians are able to form functional categories from continuous sets of stimuli.

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Crocodilians belong to Archosaurs, a monophyletic group that includes the modern birds as well as past dinosaurs and pterosaurs. As such, they share behaviors with other archosaurs such as forms of parental care, since the crocodilian parents (female, male or both depending on the crocodilian species) often protect the nest and young from predators. While they are more distantly related to mammals, crocodilians exhibit many ecological traits that have evolved convergently with this group, especially with mammalian apex predators [START_REF] Reber | Crocodilians Are Promising Intermediate Model Organisms for Comparative Perception Research[END_REF] . The unique position of crocodilians in the tree of life thus puts these animals in a privileged position for comparative research. Understanding how crocodilians hear the world is key to decipher the evolutionary history of auditory perception in all vertebrates.

Crocodilians use their hearing abilities both to identify potential prey and during their social interactions. The auditory communication channel is essential in their early years where calls are used by the young to communicate with each other and with their parent [START_REF] Vergne | Acoustic communication in crocodilians: from behaviour to brain[END_REF] . Although the sensory organs of crocodilians are quite well known, our knowledge about how they perceive and process the sounds they hear remains scarce. Their position as top predators and their auditory abilities suggest that crocodilians are super-efficient and quick to analyze their sound scene, derive relevant information, and make appropriate decisions in terms of behavioral response [START_REF] Grigg | Biology and evolution of crocodylians[END_REF] . For example, crocodiles are able to accurately localize a sound source [START_REF] Bierman | Sound localization in the alligator[END_REF][START_REF] Papet | Influence of head morphology and natural postures on sound localization cues in crocodilians[END_REF][START_REF] Papet | Crocodiles use both interaural level differences and interaural time differences to locate a sound source[END_REF] and use this ability to effectively perform spatial unmasking to identify a relevant sound despite background noise [START_REF] Thévenet | Spatial release from masking in crocodilians[END_REF] . In this work, we test the hypothesis that crocodilians are able to partition an acoustic continuum into categories and identify the acoustic features used for this categorization. Being able to form meaningful categories from auditory scenes should allow crocodilians to facilitate rapid decision making, and adjust a behavioral response such as an approach to the sound source accordingly.

In a first experiment, we test whether this ability to partition an acoustic continuum into discontinuous categories is innate by observing the response of naive crocodiles to frog and crocodile calls as well as to chimeric signals containing a variable proportion of frog and crocodile acoustic features. Second, we test whether the boundary between these categories can be modulated by learning by training caimans to approach a speaker emitting a crocodile call and ignore the frog call (GO/NOGO procedure) and then testing them with a continuum of acoustic chimeras. Finally, we investigate the acoustic basis of this categorization using acoustic chimeras based on only certain orthogonal dimensions of the signal (f 0 , aperiodicity, spectral envelope). Our results establish that crocodilians follow behavioral decision rules J o u r n a l P r e -p r o o f based on the establishment of sound categories constructed from the spectral envelope of signals.

RESULTS

Experiment 1: Naïve crocodilians respond discontinuously to continuous stimuli.

We tested juvenile Nile crocodiles Crocodylus niloticus (n = 14) with an acoustic continuum between two signals that, while sharing some acoustic similarities, came from two different animal species and thus had different biological meanings: a Saharan frog call Pelophylax saharicus, a species routinely participating in the spring night sound scene of the zoo where we did the experiments, and a contact call of a young Nile crocodile, which previous studies have shown to be attractive to crocodiles [START_REF] Vergne | Acoustic communication in crocodilians: information encoding and species specificity of juvenile calls[END_REF] . The frog call and the crocodile call are harmonic series in the same frequency range (1000-3500 Hz and 250-4000 Hz respectively) and are of comparable duration (175 ms and 200 ms respectively). To create the acoustic continuum (acoustic morphing), we first decomposed the frog and crocodile calls into three orthogonal acoustic dimensions: the pitch (f 0 ), the aperiodicity (ap) and the spectral envelope (env). Each acoustic chimera was then re-synthesized by combining the three dimensions of a frog call and those of a crocodile call in different proportions (see Methods for details). Finally, the acoustic continuum consisted of 11 calls, ranging from a 100% frog call to a 100% crocodile call, through 9 chimeric calls where the relative proportion of the three dimensions between the two species varied from 90% frog / 10% crocodile to 10% frog / 90% crocodile, with a 10% step between each signal (Figure 1A).

The experiment took place in an experimental basin (6 X 7 meters), with four loudspeakers placed near the water on the edge of the basin at four opposite locations (Figure 1B). From sunset until the end of the experiment, the 4 speakers emitted frog calls, in an unsynchronized manner (i.e. every 9, 10, 11 and 13 seconds for speakers n°1 to n°4 respectively), to mimic a frog chorus to which crocodiles are used to. As the experiment took place during fall, no natural frog calls interfered with the artificial chorus. The tested crocodile was then exposed to a succession of experimental tests using each acoustic chimera of the continuum (see Figure 1B for a schematic of the experimental setup).

As illustrated in Figure 1C, the crocodiles were attracted to the speaker when the stimulus had a very high proportion of crocodile features. Specifically, their response score increased J o u r n a l P r e -p r o o f sharply for the 10% frog / 90% crocodile stimulus whereas it was low for stimuli with a lower proportion of crocodile features (response score 1.2 point higher for the 90% crocodile signal compared to the 80% crocodile signal, 95% CI [0.1, 2.3], 98.5% of the posterior distribution is positive).

Experiments 2 and 3: Crocodilians trained to distinguish two sounds establish a sharp boundary along their acoustic continuum.

The ability of crocodilians to categorize sound signals forming an acoustic continuum between a "frog call" and a "crocodile call" was further tested in the laboratory via a GO-NOGO experiment (Experiment 2) where young dwarf Cuvier's caiman Paleosuchus palpebrosus (n = 5) were trained to respond positively (i.e. by approaching the speaker) to a crocodile call. The stimuli used were identical to those used in Experiment 1 except that we reduced the step between each signal: the continuum consisted of 6 calls, from a 100% frog call to a 100% crocodile call, through 4 chimeric calls where the relative proportion of the three dimensions between the two species varied from 80% frog / 20% crocodile to 20% frog / 80% crocodile, with a 20% step between each signal.

As shown in Figure 2A, the caimans learned to systematically move toward the speaker emitting the 100% crocodile signal (GO signal). At the beginning of training, the response rate was around 67.4%, while it was only 38.3% for the 100% frog signal. After 2 weeks of Go/No-Go training, this response rate settled above 75% and the caimans were 76.2% more likely to respond to the GO stimulus than to the NOGO stimulus (95% CI [66. 5, 84.1]).

The results of Experiment 2 are shown in Figure 2B. Caimans did not respond to the 100% frog / 0% crocodile signal, and their response to the 80% frog / 20% crocodile signal was not significantly higher (0.6% higher, 95% CI [-14.7, 23.4]). In contrast, their response increased sharply from the 60% frog / 40% crocodile chimeric signal (69.6% more likely to respond with the 40% crocodile signal compared to the 20% crocodile signal, 95% CI [41.4, 87.8]). This high level of response was maintained for all the following signals on the acoustic continuum, with no marked difference from the response to the 60% frog / 40% crocodile signal (probability in obtaining a stronger response: with the 40% frog / 60% crocodile signal = -6.3%, 95% CI [-32.1, 17.0]; with the 20% frog / 80% crocodile signal = -0.6, 95% CI [-24.1, 21.8]); with the 0% frog / 100% crocodile signal = 4.7%, 95% CI [-5.6, 25.4]).

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The response of the caimans thus appears clearly dichotomous, with a boundary between two signal categories located between the 80% frog / 20% crocodile signal and the 60% frog / 40% crocodile signal (probability of response 69.2% higher for the second category, 95% CI [55.0, 79.5]). This boundary was remarkably stable across individuals: all animals tested reached a maximum response rate of 33% for the 80% frog / 20% crocodile signal, whereas they all reached a response rate of at least 75% for the 60% frog / 40% crocodile signal. These results were confirmed when comparing the latency to respond to the stimulus between the frog category (100% and 80% frog stimuli, mean latency of 12.8 ± 12.3 s) and the crocodile category (40% crocodile and above stimuli, mean latency of 5.3 ± 6.4 s): the latency decreased by 8.9 s (95% CI [-25.5, -1.3]), while there was no difference of latency for stimuli within the frog category (difference of -2.0 s, 95% CI [-14.2, 23.9] between the 100% and 80% frog stimuli) and for stimuli within the crocodile category (difference of -1.6 s between the 40% and 60% crocodile stimuli, 95% CI [-16.1, 12.0]; -2.9 s between the 40% and 80% crocodile stimuli, 95% CI [-17.9, 6.8]; and 0.5 s between the 40% and 100% crocodile stimuli, 95% CI [-14.7, 5.6]).

After completing Experiment 2, the same caimans were tested in another experimental paradigm, also based on the GO-NOGO approach, but in a two-choice testing procedure (alternative choice test). The goal was to test the ability of crocodilians to categorize acoustic signals when confronted with two sound sources alternately emitting a pair of neighboring acoustic chimeras within the continuum. 5 pairs of stimuli were formed: 100% frog / 0% crocodile with 80% frog / 20% crocodile, 80% frog / 20% crocodile with 60% frog / 40% crocodile, 60% frog / 40% crocodile with 40% frog / 60% crocodile, 40% frog / 60% crocodile with 20% frog / 80% crocodile, and 20% frog / 80% crocodile with 0% frog / 100% crocodile.

The results obtained in Experiment 3 (Figure 2C) strengthen the conclusions drawn from Experiment 2. Indeed, when presented with a choice between the 80% frog / 20% crocodile signal and the 60% frog / 40% crocodile signal, the caimans showed a clear preference for the second signal: they were 40.5% more likely to choose the 40% crocodile signal (95% CI [18.5, 58.7]). Conversely, when confronted with the 100% frog / 0% crocodile and 80% frog / 20% crocodile signal pair, the caimans responded only weakly and equally to both signals CI [-25.3, 22.2]. The latencies to respond were however not different from one pair of stimuli to another (minor difference of 2.8 s, 95% CI [-1.4, 8.8] between the between category pair [20%, 40%] and the frog category pair [0%, 20%]; -1.5 s, 95% CI [-4.4, 0.6] of difference between the pair [20%, 40%] and the crocodile category pairs).

Experiment 4: Sound categorization is based on the spectral envelope.

To investigate which signal dimensions (f 0 , ap, and/or env) are used by crocodilians to categorize calls as "frog" or "crocodile", we first tested the caimans with acoustic chimeras bearing on only one or two dimensions. In this first step, each chimera was constructed based on a 100% frog / 0% crocodile call, and one or two dimensions were raised to its value in the 100% crocodile call. In addition to the two control calls 100% frog / 0% crocodile and 0% frog / 100% crocodile, the following 6 acoustic chimeras were tested:

-0% f 0 , 100% ap, 100% env frog / 100% f 0 , 0% ap, 0% env crocodile; -100% f 0 , 0% ap, 100% env frog / 0% f 0 , 100% ape, 0% env crocodile; -100% f 0 , 100% ap, 0% env frog / 0% f 0 , 0% ap, 100% env crocodile; -0% f 0 , 0% ap, 100% env frog / 100% f 0 , 100% ap, 0% env crocodile; -0% f 0 , 100% ap, 0% env frog / 100% f 0 , 0% ap, 100% env crocodile; -100% f 0 , 0% ap, 0% env frog / 100% f 0 , 100% ap, 100% env crocodile.

Only the acoustic chimeras containing the spectral envelope of the crocodile call induced a significant behavioral response from the caimans (Table 1, Figure 3A). The presence of the crocodile spectral envelope alone was sufficient to induce a response identical to that obtained with the 0% frog / 100% crocodile signal (negligible difference of response probability of -8.3%, 95% CI [32, 3.9]). We tend to obtain concordant results if we compare the latency times (Table 2).

Comparison of labelling between the control acoustic continuum and the acoustic continuum involving only the spectral envelope shows that caimans establish the boundary between the two categories "frog" and "crocodile" at the same location on both continua (67.7% frog / J o u r n a l P r e -p r o o f 32.3% crocodile and 67% frog / 33% crocodile, respectively; Figure 3B). Furthermore, there is no significant difference in behavioral response between full chimeras and chimeras involving only the spectral envelope throughout the acoustic continua (median difference of 5.1 %, 95% CI [-4.6, 15.8], Table 3). Consistent results are obtained if latency times are compared (Table 3).

DISCUSSION

In this study, we demonstrate through playback experiments that young crocodiles confronted with a continuum of acoustic chimeras ranging from a frog call to a crocodile call classify each acoustic variant into one or the other of these two categories, thus establishing a clear meaningful boundary where no acoustic boundary yet exists. By conducting GO/NOGO experiments, we suggest that this boundary between two categories can be established along the acoustic continuum by learning. Finally, we show that crocodilians' categorization of acoustic stimuli is based primarily on the spectral envelope rather than the pitch or aperiodic dimension of the sound signals.

Confronted with chimeric signals consisting of a mixture of "frog" and "crocodile" characteristics, the tested individuals made the choice to approach or not approach the speaker. One would have expected variations in behavioural response to be linearly correlated with the percentage of "crocodile" signal in the chimeras. This was not the case. While our animals had previously heard frog calls, which are naturally present in the zoo ponds, as well as calls from conspecifics, they had never experienced acoustic chimeras. Our results thus suggest that naive crocodilians practice sound categorization innately.

It is likely that crocodilians can form categories under other circumstances. Because crocodilians have developed a vocal repertoire, it is possible that this ability to categorize is also useful during acoustic communication between individuals. For example, young crocodilians emit cries with "graded" acoustic characteristics by playing on intensity, energy distribution in the frequency spectrum and frequency modulation. A study conducted with the black caiman Melanosuchus niger showed that the repertoire of young caimans extends along an acoustic continuum from contact calls allowing the cohesion of the group of young to distress calls inducing maternal protection [START_REF] Vergne | Acoustic signals of baby black caimans[END_REF] . It is possible -although not yet demonstratedthat young caiman and their mothers establish two distinct categories along the contact-J o u r n a l P r e -p r o o f distress acoustic continuum, which would facilitate decision making. However, it remains to be established where the boundary between contact and distress lies. This boundary is likely to vary according to the context. If the level of risk experienced by the young (in terms of predation pressure for example) is high, it is likely that the boundary between the contact and distress categories may be at a different level than if the level of risk is lower. This hypothesis remains to be tested.

The labelling experiment (Experiment 2) may suggest a role of learning in defining the boundary between two sound categories. It may seem surprising that crocodilians trained to respond to a 0% frog-100% crocodile signal have a low categorization threshold (when the signal is "33% crocodile"). This result contrasts with that obtained in the first experiment, where the crocodiles received no prior training and were never rewarded. Although it should be kept in mind that Experiments 1 and 2 are hardly comparable (different crocodilian species, different experimental conditions), one might still have expected trained individuals to have a high response threshold, since only the "100% crocodile" signal led to a reward. In our laboratory conditioning protocol, animals were not confronted with any aversive reinforcer, meaning that there is no risky choice [START_REF] Bradbury | Principles of animal communication[END_REF] . This may have led individuals to never take the risk of missing a potentially rewarded signal, and thus be more likely to respond to signals with only a small proportion of crocodile features. In animals, context-dependent decisions are indeed strongly related to estimates of the probabilities of making a good choice [START_REF] Degroot | Optimal statistical decisions[END_REF][START_REF] Akre | Communication Through a Window of Error: Proportional Processing and Signal Categorization[END_REF] .

The alternative choice experiment (Experiment 3) confirms the results obtained in the labelling experiment (Experiment 2). Tested individuals expressed a particularly distinct choice when offered the 80% frog / 20% crocodile and 60% frog / 40% crocodile signals. However, for all other pairs of signals, they chose randomly between the two acoustic chimera.

What are the mechanisms underlying this sound categorization process? In particular, can it be considered as categorical perception? Categorical perception is defined as the process by which stimuli are considered similar or different depending on whether or not they fall into the same perceptual category [START_REF] Harnad | Categorical perception: The groundwork of cognition[END_REF][START_REF] Green | Categorical perception in animal communication and decision-making[END_REF] . Strictly speaking, two features characterize a categorical perception: 1) the labelling of stimuli in one or the other of the categories considered, which corresponds to what we observed in our experiments with crocodilians; 2) a "category boundary effect" in which two stimuli belonging to the same category are differentiated less rapidly than two stimuli located on either side of the category boundary. We could not rigorously test this second condition with our crocodiles. Testing the category boundary effect J o u r n a l P r e -p r o o f would require numerous GO/NOGO tests (aiming to discriminate the two stimuli in each pair of stimuli) that are not reasonably feasible with these animals due to the small number of individuals available in the lab combined with a long learning time (unlike what is possible with humans, birds or monkeys) [START_REF] Weary | Categorization of song notes in great tits: which acoustic features are used and why?[END_REF][START_REF] May | Categorical perception of conspecific communication sounds by Japanese macaques, Macaca fuscata[END_REF][START_REF] Morse | An investigation of categorical speech discrimination by rhesus monkeys[END_REF][START_REF] Sinnott | Speech sound discrimination by monkeys and humans[END_REF][START_REF] Kuhl | Speech perception by the chinchilla: Identification functions for synthetic VOT stimuli[END_REF][START_REF] Kluender | Japanese quail can learn phonetic categories[END_REF][START_REF] Wetzel | Categorical discrimination of direction in frequency-modulated tones by Mongolian gerbils[END_REF][START_REF] Furuyama | Acoustic characteristics used by Japanese macaques for individual discrimination[END_REF][START_REF] Burgering | Zebra finches (Taeniopygia guttata) can categorize vowel-like sounds on both the fundamental frequency ("pitch") and spectral envelope[END_REF] . In our protocol, it was also difficult to use a habituation/dishabituation approach [START_REF] Nelson | Categorical perception of a natural stimulus continuum: birdsong[END_REF][START_REF] Wyttenbach | Categorical Perception of Sound Frequency by Crickets[END_REF][START_REF] Fischer | Barbary macaques categorize shrill barks into two call types[END_REF] because crocodilians become satiated with food rewards rather quickly and quickly lose motivation to respond. Despite these experimental limitations, our observations strongly suggest the presence of a categorical perception of sound stimuli in the crocodilians tested. Indeed, while caimans were conditioned to respond to the 0% frog -100% crocodile stimulus in the choice experiment (Experiment 3), they only showed a preference for the signal containing the most crocodile trait only when the pair "80% frog / 20% crocodile and 60% frog / 40% crocodile" was emitted. For the other combinations (40/60, 60/80, and 80/100 crocodile traits), test subjects showed no preference for either of the two signals in each pair (Figure 2C). In other words, the crocodilians tested reacted as if they were truly unable to perceptually differentiate between these two stimuli.

Our final experiment questions the acoustic basis for the categorization performed by crocodilians. It turns out that crocodilians rely on the spectral envelope of sound signals to categorize stimuli while they seem to ignore the pitch and aperiodic component of the signals.

Using the spectral envelope to identify a complex sound made up of a series of harmonics is a widespread and reliable strategy [START_REF] Burgering | Zebra finches (Taeniopygia guttata) can categorize vowel-like sounds on both the fundamental frequency ("pitch") and spectral envelope[END_REF] . In human perception, the spectral envelope of the voice (i.e the voice timbre) is related to the vocal tract length of the speaker which is related to the speaker height, weight, age and gender [START_REF] Fitch | Morphology and development of the human vocal tract: a study using magnetic resonance imaging[END_REF] . As such, this feature is often used to encode speaker identity [START_REF] Smith | The processing and perception of size information in speech sounds[END_REF] . The spectral envelope, corresponding to the timbre of the emitter, is indeed an invariant acoustic parameter, while the f 0 contour and the aperiodicity might vary with the context [START_REF] Chabert | Size does matter: crocodile mothers react more to the voice of smaller offspring[END_REF] . The spectral envelope would be a more stable and reliable cue to categorize crocodile calls, all types of calls combined.

In conclusion, our study shows that crocodilians exhibit an ability to categorize sound stimuli. This ability certainly contributes to an awareness of auditory scenes in their everyday life. It complements other processes such as their ability to locate precisely the position of a sound source [START_REF] Bierman | Sound localization in the alligator[END_REF][START_REF] Carr | Sound Localization Strategies in Three Predators[END_REF][START_REF] Papet | Influence of head morphology and natural postures on sound localization cues in crocodilians[END_REF][START_REF] Papet | Crocodiles use both interaural level differences and interaural time differences to locate a sound source[END_REF] and their ability to identify a meaningful sound despite background noise by spatial unmasking [START_REF] Thévenet | Spatial release from masking in crocodilians[END_REF] . Given their lifestyle, crocodilians are confronted with complex sound environments, and must be able to make quick decisions. Being able to quickly categorize sounds according to their functional relevance is certainly extremely helpful.

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LIMITATIONS OF STUDY

One limitation of this study is that we had to work with two different species of crocodilians. This makes it difficult to compare results between experiment 1 (performed with Nile crocodiles) and the other experiments (performed with Cuvier's caimans).

Another point is that we did not have the possibility to rigorously test the "category boundary effect". While our study clearly demonstrates that crocodilians respond in a categorized manner to an acoustic continuum, we cannot prove that the mechanism of this categorization is strictly a "categorical perception" as defined in the literature. As mentioned above, testing "categorical perception" would have required the implementation of numerous GO/NOGO tests, which is impractical given the complexity of working with these animals.

MAIN FIGURES Figure 1. Categorization tests with naive crocodilians reveal that they switch their behavioral response across a specific acoustic boundary along a signal continuum (Experiment 1).

(A) Acoustic continuum of frog/crocodile chimeric signals, ranging from a 'frog' signal (100% frog / 0% croc) to a 'crocodile' signal (0% frog / 100% croc). The 'frog' signal was used to create a frog chorus. Only 5 of the 10 signals tested are shown here (the entire continuum spanned from a 100% frog / 0% crocodile signal to a 0% frog / 100% crocodile signal, with 10% increments; see Methods for details of the acoustic morphing procedure).

(B) Experimental set-up. Throughout each experiment (average duration = 2.5 hours), all four speakers were emitting 100% frog calls (1 call every 9 to 13 seconds, different rhythm for each speaker, reproducing a frog chorus). During each test, one of the speakers switched to one of the chimeric signals (duration of a test = 4 minutes; tests separated by at least 10 minutes; different test speaker from one test to another; see Methods for details). The crocodile's response to the chimeric signal was assessed for 10 minutes after the start of each test. Each individual (N = 14 Nile crocodiles) was tested successively with all chimeric signals, presented in random order during the experiment.

(C) Behavioral response of crocodiles to chimeric signals. Solid circles show the fitted probabilities of getting a response (the crocodile approached the test speaker) expressed as the median of posterior distribution with 95% CI. Violin plots show the distribution of the fitted values for each signal. The abrupt increase in approaches to the test speaker from the 10% frog / 90% croc signal (marked by the black arrow) suggests sound categorization. This result was obtained with naïve, untrained crocodiles, which had therefore never heard the test signals before the experiment.

Figure 2. Crocodilians trained to respond to a meaningful signal highlight their ability to form a sensory boundary along an acoustic continuum (Experiments 2 and 3).

(A) Training procedure: Young caimans (N = 5) were trained using food reward to approach the speaker only when it emits a 100% croc signal (GO-NOGO procedure). As the graph on the right shows, training reinforces the natural propensity of animals to respond to that signal.

(B) Experiment 2: Categorization tests along the acoustic continuum suggest a sensory boundary between the 20% and 40% croc signals (black arrow). Solid circles show the fitted probabilities of getting a response (the crocodile approached the test speaker) expressed as the median of posterior distribution with 95% CI. Violin plots show the distribution of the fitted values for each signal.

(C) Experiment 3: Fitted probability of choosing between two chimeric signals in an alternative-choice task (upper graph), and difference between the probabilities measured for each pair of signals (below). Choice between neighboring chimeric signals is effective only between the 20 and 40% croc signals (black arrow).

Figure 3. The categorization of frog/croc chimeric signals by crocodilians is based on the spectral envelope of the sound stimulus, excluding other acoustic dimensions (Experiment 4).

(A) Results of playback experiments with animals trained to respond to crocodile call. Solid circles show the fitted probabilities of getting a response (the crocodile approached the test speaker) expressed as the median of posterior distribution with 95% CI. Violin plots show the distribution of the fitted values for each signal. One-dimensional and two-dimensional chimeric stimuli were created from a 100% frog J o u r n a l P r e -p r o o f call by transforming into 100% croc three acoustic dimensions separately or in pairs: pitch (f 0 ), aperiodic dimension (ap), and spectral envelope (env), and their combinations f 0 + ap, f 0 + env, and env + ap. Only stimuli with 100% crocodile spectral envelope elicited a behavioral response. The subjects did not respond to stimuli with only 100% f 0 croc or 100% ap croc.

(B) When the spectral envelope is the only dimension to change in the acoustic continuum (in pink), the sensory boundary remains close to that identified for the original continuum where all three dimensions of the signal change together (boundaries between the 32 and 33% croc signals: blue and pink arrows). Medians of posterior distribution (%) and 95% CI.

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STAR METHODS

RESOURCE AVAILABILITY

Lead contact

Further information and requests for resources should be directed to and will be fulfilled by the lead contact, Julie Thévenet (julie.thevenet.phd@gmail.com).

Materials availability

This study did not generate new unique reagents.

Data and code availability

• Dataset have been deposited at Zenodo and are publicly available as of the date of publication [START_REF] Thévenet | Data from: Sound categorization by crocodilians[END_REF] . The DOI is listed in the key resources table.

• All original statistical code has been deposited at Zenodo and is publicly available as of the date of publication [START_REF] Thévenet | Data from: Sound categorization by crocodilians[END_REF] . The DOI is listed in the key resources table.

• Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

EXPERIMENTAL MODEL AND SUBJECT DETAILS Experiment 1: Labelling experiment with untrained naïve crocodilians

This experiment was performed at Crocoparc zoo (Agadir, Morocco). The animals (n = 14 Nile crocodiles Crocodylus niloticus, 1-2 month old, sex unknown) were housed together in an outdoor enclosure, not visible to the public. They had never been included in any experiment before. All studies were conducted under the ethical approval of the ENES laboratory (n° D 42-218-0901) and complied with Moroccan and French laws.

Experiment 2: Labelling experiments with trained crocodilians

The experiment was conducted at the ENES laboratory. We worked with young dwarf Cuvier's caiman Paleosuchus palpebrosus, born in captivity at the "Zoo de Paris" (N = 5, including 2 females and 3 males; age: 2.5 years; size: 59 ± 2 cm). These animals had never been included in an experimental protocol prior to the experiment. Individuals were housed all together in a J o u r n a l P r e -p r o o f large enclosure (16 m 2 ). The room was maintained at 28 ± 1 °C, with a light on from 9:00 am to 9:00 pm.

Experiment 3: Alternative-choice test with trained crocodilians

We performed this experiment at the ENES laboratory, with the same dwarf Cuvier's caimans Paleosuchus palpebrosus as in experiment 2 (n = 5 individuals).

Experiment 4: Identification of acoustic cues supporting category labelling

This experiment was conducted at the ENES laboratory, and involved the same dwarf Cuvier's caimans (n = 5 individuals), after they had performed experiments 2 and 3.

METHOD DETAILS Experiment 1: Labelling experiment with untrained naïve crocodilians

To limit pseudo-replication, we used 4 frog calls (recorded by Léo Papet in the zoo) and 4 juvenile crocodile calls (recorded from free-ranging young crocodiles during a field expedition At the time of a test, a loudspeaker was randomly selected among the four, eliminating the one or ones from which the crocodile was closest. For 2 minutes, this test speaker continued to emit frog calls, but with a rhythm of 1 every 12 seconds. The choice of a fixed rate of emission, the same for all tests of all crocodiles, eliminated the risk of obtaining different behavioral responses following different rates of emission. At the end of the first 2 minutes, the test speaker started to emit for the next 2 minutes one of the acoustic chimeras (chosen at random between the 90% frog / 10% crocodile and the 0% frog / 100% crocodile) at the same rate of one call every 12 seconds. At the end of the 2 minutes, the test speaker resumed emitting frog calls at its original rate. The next test was initiated at least 10 minutes later, from a different speaker (stimulus chosen at random). The behavior of the crocodile was observed and evaluated blindly and independently by 4 observers for 10 minutes from the start of the test. The entire experiment was filmed by an infrared camera (ABUS TVCC34010).

The intensity of the crocodiles' behavioral response to sound stimuli was scored from 0 to 5 according to the following scale: 0 = no response; 1 = set in motion without swimming; 2 = swimming without preferred direction; 3 = swimming in the direction of the speaker but not exceeding 1/3 of the distance to the speaker; 4 = swimming in the direction of the speaker without approaching the speaker within 50 centimeters; 5 = swimming in the direction of the speaker and approaching within 50 centimeters.

Experiment 2: Labelling experiments with trained crocodilians

The experiment was conducted in a square-shaped pool (1.75 m wide), placed in a soundproof cabin, with a water depth of 12 cm allowing the animals to swim freely (water temperature:

J o u r n a l P r e -p r o o f 29°C). Four speakers (Audiopro Bravo Allroom Sat, see Figure S1E and Figure S1F for technical specifications) were placed just above the water surface at the four corners of the pool (Figure 2A). Sound playback was controlled by the experimenter from outside the booth using a computer delivering the signals to an amplifier (Yamaha AX-397) connected to the speakers.

A remote-controlled reward system was associated with each speaker, which allowed for multiple delivery of food rewards during a single test session. All tests were performed in low light conditions to limit the stress of the animals and increase their motivation to respond (these animals are more active at night). The whole experiment was filmed (ABUS TVCC34010 camera), which allowed real time and delayed observations.

Training took place in weekly sessions via operant conditioning using a food reward. The subject was first placed in the pool and did not receive any stimulation for an acclimatization period of at least 3 min (we always waited until the animal had stopped exploring the pool).

Then, the GO stimulus (sequence of crocodile calls) and the NOGO stimulus (sequence of frog calls) were presented three to seven times each in a random order (10 calls per sequence at a rate of one call every 2.5 ± 0.4 sec, total sequence duration = 35 sec; 10 ± 3 sequences in total per training session). The animal was systematically rewarded if it came within 35 cm of the active speaker when a sequence of crocodile calls (GO stimulus) was presented. This 35 cm distance corresponded to the location where the reward (food) was delivered. There was a minimum delay of 1 min 30 seconds between two trials. The acoustic stimuli were emitted at a sound level of 60 dB SPL at 1m from the speaker.

Once trained, i.e., when the subject responded correctly in more than 75% of the tests, the subject moved on to weekly test sessions, during which the animal was confronted with three to five sequences of calls, randomly selected from the acoustic continuum. No rewards were given during the tests when the crocodile response was measured, regardless of the signal and the animal's response. However, in order to maintain motivation over the test sessions, the animal's response was regularly reinforced by repeating the go/no-go protocol, i.e., by associating a positive response to the GO signal with a food reward. These reinforcements were randomly distributed during the sessions. During each test session, the caiman heard 12 The response of the animals to the acoustic stimuli was measured blindly from the videos using Kinovea software, v0.8.24 Beta 49 , during the 45 seconds following the emission of the first call of the tested sequence. For each trial, the response was scored as "0" (no approach)

or "1" (reach the food reward system within 35 centimeters of the speaker). We also measured the latency between the first call in the sequence and the onset of the behavioral response (first movement), if any.

Experiment 3: Alternative-choice test with trained crocodilians

The two alternative stimuli were presented antiphonally during 35 seconds through two speakers located at equal distance from the animal. The overall rhythm of emission was the same as in the second experiment, which means that only 5 calls were emitted per speaker instead of 10 calls. A test session always began with a training pair combining a 100% frog / 0% crocodile signal (NOGO) with a 0% frog / 100% crocodile signal (GO). Discrimination tests only began if the animal moved to the speaker emitting the GO signal twice in a row. If the animal made the wrong choice, it was trained again with the GO stimulus. Then, the animals were tested with all the five pairs of stimuli beginning once by one stimuli of the pair and then the other, in a random order (i.e. 10 pairs of test stimuli were presented by session, total duration of a test session = between 20 and 40 minutes). In total, each caiman was tested with 87 ± 31 pairs of stimuli throughout the experiment.

Experiment 4: Identification of acoustic cues supporting category labelling

As in Experiment 2, the stimuli were delivered in a sequence of 10 successive calls (one signal every 2.5 ± 0.4; stimuli presented in a random order). The assessment of caiman behavioral responses and the statistical approach were identical to those used in Experiment 2. In total, the caimans were tested with 18 ± 3 test stimuli throughout the experiments.

Since the results of this first step showed that the spectral envelope was a decisive acoustic criterion for categorization, we tested whether the boundary between the two categories "frog" and "crocodile" was identical between a control continuum where the 3 acoustic dimensions varied and a continuum where only the spectral envelope varied. Based on the results of experiments 2 and 3 which indicated that the acoustic boundary should be for a J o u r n a l P r e -p r o o f signal containing around 30% crocodile features, we created a series of 6 acoustic chimeras where the 3 dimensions of the signal varied together, from a 85% frog / 15% crocodile signal to a 60% frog / 40% crocodile signal, with a 5% step. We also created another series of 6 acoustic chimeras, where only the spectral envelope was involved, from a 100% f 0 , 100% ap, 85% env frog / 0% f 0 , 0% ap, 15% env crocodile signal to a 100% f 0 , 100% ap, 60% env frog / 0% f 0 , 0% ap, 40% env crocodile signal.

QUANTIFICATION AND STATISTICAL ANALYSIS Experiment 1: Labelling experiment with untrained naïve crocodilians

Statistical analyses were performed in R (v3.6.2) using Bayesian mixed models fitted with the brms R package [START_REF] Bürkner | brms: An R Package for Bayesian Multilevel Models Using Stan[END_REF] . The advantages of using the Bayesian approach are multiple, including its high flexibility, quantification of uncertainty in estimates, and intuitive interpretation of confidence intervals [START_REF] Kruschke | The Bayesian New Statistics: Hypothesis testing, estimation, meta-analysis, and power analysis from a Bayesian perspective[END_REF]48 . Response scores were modeled using a gaussian function. The model included subject-specific random intercepts. 5000 iterations were run over four MCMC chains with the first 500 iterations of each chain used to adjust the algorithm. Because the behavior of the tested animals was difficult to predict, flat priors were kept for this model. Results were summarized as medians of the posterior distributions and 95% credible intervals (CIs).

Credible intervals for estimates that do not include zero indicate a credible effect given the observed data and model structure 48 . Similarly, when contrasting two conditions, CIs excluding the null value can be inferred to indicate a credible difference between the conditions.

Experiment 2: Labelling experiments with trained crocodilians

Response scores were modeled using a logistic function (Bernoulli family) and reaction times using a shifted-lognormal distribution. All models included subject-specific random intercepts. 5000 iterations were run over four MCMC chains, with the 500 first iterations of each chains used to tune the algorithm. To improve convergence when modeling response scores, we specified mildly informative priors for all factor levels (nature of stimulus, normal distribution of (0, 3)), except for the GO stimulus (set with an informative normal distribution of (2, 2)) and the NOGO stimulus (normal distribution (-2, 2)). Results were summarized as medians of posterior distributions and 95% credible intervals (CIs).

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Experiment 3: Alternative-choice test with trained crocodilians

For each trial, the caiman was considered to have made its choice between the two speakers when it approached one of them within 45 seconds of the call. The response was scored blindly. Choices were modeled using a multinomial function (categorical family), with random intercepts per animal.

Experiment 4: Identification of acoustic cues supporting category labelling

The experimental and behavioral response analysis procedures were identical to those in Experiment 2. In total, the caimans were tested with 54 ± 2 test stimuli throughout the experiments.

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(

  probability of 65.0 to get no response, 95% CI [53.3, 75.2], difference of only 2.3%, 95% CI [-12.3, 16.4] to choose one or another stimulus). For all pairs involving signals containing at least 40% crocodile acoustic features, the caimans showed no preference, responding strongly to either signal in the pair (pair [40%, 60%]: 87.2% of response, 95% CI [78.5, 93.3], probability J o u r n a l P r e -p r o o f of a different response between the two signals = 13.0%, 95% CI [-12. 4, 37.2]; pair [60%, 80%]: 83.0% (95% CI [73.7, 90.2], probability of a different response = 4. 3%, 95% CI [-20.2, 28.1]; pair [80%, 100%]: 80.9%, 95% CI [71.2, 88.6], probability of a different response = -1.4%, 95%

  in the Okavango Delta by N. Mathevon and T. Aubin). Each of the 4 acoustic continua used during the experiment was made from one of these frog calls and one of these crocodile calls.Signal decomposition in three orthogonal dimensions (f 0 , aperiodicity, spectral envelope) was performed with the open source Matlab toolbox STRAIGHT[START_REF] Kawahara | Restructuring speech representations using a pitch-adaptive time-frequency smoothing and an instantaneousfrequency-based F0 extraction: Possible role of a repetitive structure in sounds1Speech files available[END_REF][START_REF] Kawahara | STRAIGHT, exploitation of the other aspect of VOCODER: Perceptually isomorphic decomposition of speech sounds[END_REF] . The f 0 contour represents the pitch variations as a function of time. The spectral envelope shape and scale are related to the shape and size of the vocal tract, or in general, of the resonating cavity used for vocal production. The aperiodicity matrix indicates where in time and frequency the extracted f 0 applies, i.e. it is used to represent breathiness in the vocalization. Due to some limitation in the parameters' variation, STRAIGHT failed to extract the f 0 contour of the frog calls. To do so, we used an auto-correlation to estimate the modulation frequency of the temporal envelope in successive temporal windows. This homemade code is provided as an open source resource.The acoustic chimera and original calls were then re-synthesized using STRAIGHT by combining the three orthogonal dimensions of a frog call and those of a crocodile call in different proportions. The aperiodicity and the spectral envelope dimensions from frog and crocodile calls were linearly combined as provided by STRAIGHT, while their f 0 value were first J o u r n a l P r e -p r o o f log-transformed before being linearly combined, and then transformed back to a linear Hertz scale.To avoid habituation, each individual was tested in only one experimental session (one night), with a single set of acoustic chimeras. Three hours before the start of the experiment, the crocodile was released into the experimental basin. The experiments, conducted in the fall, started one hour after sunset as it is during the night that the frogs sing the most during spring and the crocodiles are the most active. All sound signals used in the experiment were emitted at an intensity of 67 dBA at 1 m from the speaker (FoxPro Fusion© with Visaton SL 87 ND internal speaker; see FigureS1Band FigureS1Cfor technical specifications).

± 3

 3 sequences of calls, including 40% test stimuli (without reward), 40% GO stimuli, 20% NOGO stimuli (total duration of a test session = between 20 and 40 minutes). Testing sessions were repeated until each caiman had been tested three times with each of the test stimuli of the J o u r n a l P r e -p r o o f continuum (i.e. total of 12 ± 4 test stimuli per caiman throughout the experiments, with at least 3 repetitions for each of the 4 test stimuli).

Table 1 . Effect of the stimulus type on the caiman behavioral reaction

 1

	MAIN TABLES
		Fitted	Median [95% CI]	Median [95% CI]
	Stimuli	response	difference with the	difference with the
		probability (%)	NOGO stimulus (%)	GO stimulus (%)
	100% frog / 0% crocodile	15.8		-77.8 [-85.1, -68.7]
	0% frog / 100% crocodile	93.9	77.8 [68.7, 85.1]
	100% f 0 crocodile	5.5	-9.7 [-20.5, 8.1]	-87.8 [-94.8, -69.8]
	100% env crocodile	85.2	68.8 [45.6, 82.8]	-8.6 [-32, 3.9]
	100% ap crocodile	5.5	-9.6 [-20.5, 8.0]	-87.8 [-94.7, -70.0]
	100% f 0 + env crocodile	85.4	69.0 [45.4, 83.1]	-8.3 [-31.5, 4.0]
	100% f 0 + ap crocodile	5.5	-9.6 [-20.4, 8.5]	-87.8 [-94.7, -69.5]
	100% env + ap crocodile	78.7	62.4 [37.3, 79.1]	-15.0 [-40.3, 0.7]

(Experiment 4). Medians of posterior distribution (%) and 95% CI.

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Table 2 . Effect of the stimulus type on the caiman latency time to react (Experiment 4

 2

		Fitted response probability (%)	Fitted latency (sec)
	Stimuli	Full morphing	Partial morphing	Full morphing	Partial morphing
	15% crocodile	9.5 [2.0, 28.3]	6.3 [1.3, 21]	12.3 [6.0, 26.5] 17.5 [8.4, 37.1]
	20% crocodile	3.9 [0.2, 24.4]	2.6 [0.1, 17.5]	11.9 [5.9, 24.7] 16.9 [8.5, 34.5]
	25% crocodile	7.1 [0.2, 46.6]	4.7 [0.1, 37.0]	9.9 [4.9, 19.9]	14.1 [6.8, 29.4]
	30% crocodile 19.8 [2.4, 57.5] 13.6 [1.6, 47.2] 10.9 [5.3, 22.5] 15.5 [7.6, 31.5]
	35% crocodile 60.6 [25.1, 86.4] 49.9 [17.9, 80.3] 10.6 [5.3, 21.5] 15.0 [7.6, 30.7]
	40% crocodile 80.6 [41.7, 95.6] 73.0 [31.6, 93.1] 8.0 [4.0, 16.1]	11.4 [5.8, 22.8]

). Medians of posterior distribution and 95% CI.

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Table 3 . Effect of stimulus type on the caiman behavioral reaction and latency time (Experiment 4).

 3

ACKNOWLEDGEMENTS

The authors are grateful to "Crocoparc" zoo in Agadir, Morocco (Luc Fougeirol, Ariane Marinetti, Leila Sdigui, Philippe and Christine Alleon, and the staff), to Olivier Marquis for providing us with the caimans and to David Reby for his advice. This research has been funded by the Institut universitaire de France (NM), the Labex CeLyA (Lyon Center of Acoustics ANR-10-LABX-60, PhD funding to Julie Thévenet), the CNRS and the University of Saint-Etienne.

AUTHORS CONTRIBUTIONS

JT: conceptualization, data acquisition, analysis, writing; MK: data acquisition, analysis; EG: synthesis of chimeric sound signals; NB: data acquisition, manipulation and well-being of the animals; AP: well-being of the animals; LP: conceptualization, data acquisition; NG: conceptualization, data acquisition, writing; NM: conceptualization, data acquisition, writing.

DECLARATION OF INTERESTS

The authors declare no competing interests.

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