Materials and Methods
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Study site: The Brazilian Pantanal Matogrossense is a large continental savanna wetland covering an area of 147 574km² (Alho et al. 1988, Alho 2008). This study took place in the Nhecolândia subregion of Brazilian Pantanal wetland. The Nhecolândia landscape is characterized by a mosaic of semi-deciduous and xeric vegetation, seasonally flooded grasslands and ]]> ca 4 350ha of area), on February and April 1992.

Vocalization recording: The vocalizations of female caimans were recorded in February, the time of egg incubation and nest guarding. The distress calls of young caimans ]]> C. yacare calls were statistically described and quantitatively compared intra-specifically. The comparisons with calls from other crocodilian species were qualitatively made based on literature. All records are digitally preserved in the Laboratory of Bioacoustics, Instituto de Biologia, Universidade Federal do Rio de Janeiro, Brazil.

Because adult female ]]> Bromelia balansae), which surrounds the freshwater ponds locally known as “baías” (i.e. “bays”). The mere approaching was enough to stimulate the aggressive behavior and vocalization by nest-guarding females; otherwise, they usually ]]>

During the second visit to the study area, searchlights were used to locate young caimans on the flooded grassland along the bay shores, one hour after the sunset (from 19:00hr to 00:00hr). Young caimans were captured along eight bays in the study area (Fig. 1), had their vocalizations recorded and, then, they were immediately released. Each bay was visited twice (total=16 ]]>

Due to the practical ]]>

During the field activities, we reproduced playbacks of recorded distress calls nearby the bays during ]]> i.e. duration time, distance, and the individual performing the ]]>

The structure of the vocalizations were then quantified and statistically described and compared. The different samples were tested for distribution Normality, kurtosis, skewness and homoscedasticity (Shapiro-Wilk’s and Levene’s Tests respectively). Non-parametric univariate tests (Kruskal-Wallis and ]]>

The calls referent to individuals of different bays were also compared by Canonical Discriminant Analysis (CDA) in order to characterize the acoustic space of each group and to establish the Mahalanobis multivariate distance between them. Canonical discriminant function analysis was performed using the pods as a priori ]]> a priori group that it ]]> a priori groups is an indication of how well the groups can be separated according the variables used (Manly 1994). The significance of the overall discrimination is given by the Wilks’ Lambda. The post-hoc test of significance used to assess the statistical multivariate difference between groups was based on the Mahalanobis distance between the centroid of each group (Manly 1994). The samples had their normality and homo-cedasticity tested, as well as the skewness and kurtosis of distributions. The geographic distances ]]> et al. 2007).


Vocalization and behavior of adult Paraguayan caiman females: We found 18 caiman nests, but adult C. yacare females were present only in 11 of them. The adult females were ]]>

Aggressive vocalizations emitted by ]]> C. yacare adult females start with a deep inhalation (causing an apparent increasing in the caiman body), followed by air exhalation and a prolonged growl associated with an acute hiss with an average duration of 400ms (SD=18, n=11). The vocalization analysis indicated that this hiss seems like a white noise without a melodic structure (i.e., no evident harmonic formation; Fig. 2). In 57.1% of these calls, pulsed sounds of low frequency accompany the hiss. Despite the variant structure of this kind of vocalization, three marked patterns were ]]>

]]> When playbacks of the distress calls were played, the adult caimans responded with approximations toward the source of the calls. This behavior could be observed in eight out of 10 times when this experiment was performed. These adult caimans rushed warily toward the source of the playbacks regardless the fact of the distress calls belonged to pods of different bays of origin. Responses to ours imitations of the distress call were also observed. Nevertheless, none adult caiman displayed the full aggressive behavior usually described for these situations, limiting their ]]>

Vocalization and behavior of young Paraguayan caimans: A total of 196 calls were recorded from several individuals from different pods, all of them with less than two months old. We observed that when a young C. ]]> is disturbed, it runs/swims away vocalizing, while the other members of the pod follows the same behavior despite not have been directly stimulated.

The distress call of young caimans, according the sonographic analysis, consists of short notes with defined harmonic structure (Fig. 4). Sonograms revealed the formation of up to 12 harmonics, which swept downward with declining modulation. ]]> C. yacare of less than two months old was 141.63ms (SD=38.50). The coefficient of variation is relatively high, 27.18%, denoting the large individual variation of duration of juvenile distress calls (Fig. 5). The upsweep modulation was found in 67.35% of the calls and, in such cases, it corresponds to 15.46% of total duration of the call (SD=8.87). The average dominant harmonic frequency was 1 456.01Hz (SD=279.04) and in 84.18% of the calls, the dominant harmonic was the ]]>

Comparing the total vocalizations collected by night with searchlights and manipulation of the individuals (n=154), and by day with simulated attempts of capture (n=42), we found differences in the calls’ structure in six out of nine ]]>

The structure of the calls of the young caimans from three out of eight bays (Bays 21, 46, and 47) where ]]> [night]=78, n_[day]=8, U=23.5, p<0.001; Bay 46, n_[night]=22, n_[day]=9, U=2.5, p<0.001; and Bay 47, n_[night]=13, n_[day]=17, U=0.0, p<0.001).

]]> Following the trend observed on the pooled-within analysis, the ratio the duration of the upsweep modulation (when it was present in the calls) and total duration of the call, was significantly higher in the nocturnal experiment than in the diurnal experiment, in the bays 46 (U=0.0, p<0.001) and 47 (U=6.5, p<0.001). No significant difference was observed in the ratio of the upsweep modulation by the total duration of the calls of the young caimans of the Bay 21 (U=70.0, p>0.17) regarding the time of the day and style of sampling.
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A slight difference between the average values of the dominant harmonic frequency of the distress calls was observed in the Bay 47, regarding the time of the day (nocturnal=1 505.39Hz e diurnal=1 251.18Hz, U=46.5, p<0.01). No other difference was detected in the call samples along the other bays.

Since significant ]]>

We observed significant differences in the vocalization structure according to bay of origin in both experiments. It means that geographic differences exist among the calls ]]>

Discusion

Aggressive response of adult C. yacare females compared to ]]> The nest guarding behavior was present in 61.1% of the Paraguayan caiman nests in this study. Ayarzagüena (1983), based on 30 observations in Venezuelan llanos, reported that 70% of C. crocodilus females presented the guarding nests behavior after laying. However, Crawshaw & Schaller (1980) pointed out that C. yacare females are rarely present at a nest in daytime, but often can be seen in adjacent water bodies. Therefore, it remains unclear if the nest guarding in C. yacare females is significantly different from the C. crocodilus. The full aggressive behavior, characterized by the initial intimidating hiss/growl followed by rapid movements towards the researchers, exhibited by two C. yacare was similar to that reported for C. crocodilus against natural egg predators by Ayarzagüena (1983).
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The aggressive vocalizations emitted by Alligator mississipiensis females in similar circumstances of nest defense, described by Garrick et al. (1978) as a simple hiss, showed frequencies under 1kHz between 1-3s of duration. Thus, nest guarding vocalization of the adult C. yacare females observed was ]]> A. mississipiensis, showing three different structural patterns.

Distress call of young C. yacare compared to other crocodilians: Vocalizations of young C. yacare (less than two months old) recorded in Pantanal of Nhecolândia presented the acoustic structure consistent with Vergne’s et al. (2009) description of the distress call of young crocodilians. Qualitative comparisons between sonograms of distress calls of C. yacare, A. mississipiensis, Crocodylus acutus, Melanosuchus niger, ]]> C. fuscus (treated by Campbell (1973) as C. crocodilus in Panama) revealed differences among them. We found differences on harmonic number, duration of call, and presence or absence of upsweep modulations at the beginning of the note. These comparisons were based on Campbell’s (1973) descriptions, although in his paper, he made no references to individual variations in the distress call of those other crocodilian species. Moreover, Campbell (1973) presented vocalizations of juvenile individuals of different size and age, thus the comparisons made here ]]> A. mississipiensis shows a simpler acoustic structure; the fundamental begins at about 0.6kHz and sweeps downward to about 0.3kHz in approximately 0.1s. Two main harmonics spring clearly at 0.8kHz and 1.2kHz. The remaining crocodilians examined by Campbell (1973) shows a more complex acoustic structure in their calls. In C. fuscus the call structure has the fundamental beginning at approximately 0.7kHz and sweeps downward to about 0.2kHz in 0.1s, with several harmonics ]]> C. acutus call structure has a fundamental beginning at 0.6-0.5kHz and sweeping down to 0.3-0.2kHz; several weak harmonics are presents in the juveniles but the harmonic present in the hatchlings are stronger (Campbell 1973). Despite some structural similarities, the distress call of M. niger is shorter than that of C. yacare and the lower frequency lies at 0.2kHz.
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Nevertheless, our results are quite congruent with Vergne et al. (2007) results for young Crocodylus niloticus that present a marked individual variation. However, the average total duration of C. niloticus juvenile distress calls is longer (206.00ms±39ms) than that of C. yacare ]]> C. yacare ranges from the pronounced frequency modulation with simple downsweep slopes, to complex initial upsweep modulations followed by the downsweep modulation described by Britton (2001) as “circumflex” shape. The circumflex shaped calls are reported as featured for C. niloticus and Crocodylus johnsoni (Britton 2001, Vergne et al. 2009). ]]> et al. (2009) pointed out that there is a high change in the individual acoustic structure of the calls along the days following hatching. Among other Caimaninae, the distress call of C. yacare seems to be longer than that of M. niger (100ms±20ms) as indicated by Vergne et al. (2011).

Intraspecific vocal variations among juvenile Paraguayan caimans: Our quantitative analysis also indicates that a high variability in the structure of the distress call exists among different pods of young Paraguayan caiman. We, however, regarded the differences in the call structure during the nocturnal and diurnal experiments to an effect of the juveniles’ manipulation during the capture, rather than a biological circadian variation. On the other ]]>

Nevertheless, the degree of similarity or difference in the call structure between pods, measured by the squared Mahalanobis distance (and associated p-value), is not correlated to the geographic distance between the bays where the calls ]]>

During nocturnal activities, adult caimans could be observed swimming nearby the source of the distress calls. We observed adult individuals of C. yacare (possibly the parent female of those pods) reacting to calls of the juveniles, usually by ]]> A. mississipiensis, which reacted to distress calls from other species. The adult caimans seem to display the parental care to anything similar to the young caimans’ distress call. However, the typical aggressive behavior of the adult caimans against predators of juveniles described ]]>

Distress call of young C. yacare: does the variability favored by natural selection?: During our field ]]> C. yacare is granted even to high heterogeneous calls of juveniles in distress situations. Thus, based on these field experiments and in the high variability of the distress call structure assessed, we hypothesize that adult caiman females present low specificity in the recognition of the juvenile distress call. Consequently, this might reduce the stabilizing selection of this character. According to Passek & Gillingham (1999), young alligators usually get assembled in pods ]]>

On the other hand, ]]> C. yacare, the absence of social connection between pods of different bays, and the low ]]>

Acknowledgments

We are indebt with the curator of ]]>

Received 30-VII-2012. Corrected 10-I-2013. Accepted 24-I-2013.