Description of the agonistic calls of the broad-snouted caiman (Caiman latirostris)

Abstract

Bioacoustics is the area of knowledge responsible for the study of sounds emitted by animals, integrating in many areas such as natural history, population ecology, taxonomy and monitoring applied to conservation. However, for crocodilians, bioacoustics is underexplored, with the majority of studies restricted to a few species, mainly with captive animals. This gap of knowledge is evident in the broad-snouted caiman (Caiman latirostris), species with broad distribution across Brazil, but lacking scientific reports of its vocalization behavior or acoustic repertoire. Therefore, the objective of the present study is to register agonistic calls of C. latirostris. Nocturnal surveys (5-7 days) were conducted in the months of December 2024 and January, February, April, July, September-December 2025, at Rio Doce State Park, in the lakes Anibal, Carioca, Dom Helvécio and Lagoa dos Patos. Individuals were located with the spotlight method and captured manually (<1m) or with a steel cable snare (>1m), the agonistic calls emitted during the capture proceedings were recorded with a TASCAM DR-40X and a microphone SENNHEISER ME66. Snout-vent length (SVL) were registered to determine individuals size class (I, II, III or IV). Our data is composed by 289 acoustic signals, belonging to individuals of size classes I (n=4), II (n=10) and III (n=11). Recordings were analyzed visually and aurally in RavenPro 1.6 software and, for each acoustic signal, we extracted de parameters: Duration (s), Minimum Frequency (Hz), Maximum Frequency (Hz) and Peak Frequency (Hz). Signals with good quality were selected to produce images in the SEEWAV package, in R 4.6.0 software. We identified two types of agonistic calls: distress calls (n=239) and pulsated calls (n=50). Distress calls were characterized by the presence of complex harmonic structures, short duration (average: 0,44 seconds) and variable modulation in its frequency bands. Pulsated calls were distinguished by absence of harmonics and the presence of pulsated notes, referred here as pulses, along its duration (average: 0,66 seconds). In distress calls, size class I showed lower duration but higher values of Max, Min and Peak frequencies, whereas size class showed higher duration but lower values from Max, Min and Peak frequencies in acoustic signals. Furthermore, three variations in distress calls were observed in different proportions between size classes. Pulsated calls were only observed in size classes II and III, with duration values being higher in size class III. Our study represents the first description of C. latirostris agonistic calls, and our results suggest differences associated with individual size in the acoustic parameters, possibly reflecting a well-defined biological role, related with social contexts that broad-snouted caimans are submitted in its life stages, such as defensive behaviors in hatchlings and juveniles and territorial defense in adults. In conclusion, we suggest that the effect of environmental factors and seasonality in acoustic parameters should be investigated, as well the expansion of the species acoustic repertoire, integrating areas of conservation and population monitoring.