Sons pulsantes de botos-cinza (Sotalia guianensis) na costa do Rio de Janeiro: caracterização e variação intraespecífica

Abstract

Most species of the Delphinidae family produce two types of sounds: pulsed sounds, divided into echolocation clicks and burst pulsed sounds, and whistles. Burst pulsed sounds are used in the social communication between individuals, but their specific functions and their spectral structure have been little studied. The Guiana dolphin (Sotalia guianensis) is a species of the Delphinidae family and occurs on the coast of Rio de Janeiro year-round. This work aimed to characterize the acoustic parameters of the burst pulsed sounds emitted by Sotalia guianensis, as well as their intraspecific variations, in Guanabara, Sepetiba and Ilha Grande bays, in Rio de Janeiro’s coast. The recording system used consisted of a Fostex digital recorder (sampling rate of 192 kHz) and a hydrophone C54XRS (-165 dBV, 0.006 to 203 kHz). All the burst pulsed sounds present in the recordings were counted in Raven Pro 1.5 Software so that the emission rate (number of burst pulsed sounds / individuals / minute) of that sound could be calculated. Each recording was filtered with a Butterworth Highpass filter of 1 kHz in Adobe Audition 1.5 Software. Five acoustic parameters were extracted from all the burst pulsed sounds considered of good quality: number of pulses, interpulse interval (ms), duration (ms), peak frequency (kHz) and minimum frequency (kHz). The first four were extracted automatically through the SoundRuler Software while the latter was manually measured in Raven Pro 1.5. In addition, each pulsed sound analyzed was visually classified into four distinct categories based on their apparent spectral structure. Statistical analysis included Kruskal-Wallis tests to investigate the differences in the five acoustic parameters of the burst pulsed sounds between the three bays. In addition, two discriminant functions were performed to analyze the separation of the burst pulsed sounds in different bays and categories based on their parameters. In total, 255 burst pulsed sounds were analyzed, 74 in Guanabara Bay, 79 in Sepetiba and 102 in Ilha Grande. It was observed that 67% of the analyzed sounds were correctly classified into the four categories and the parameters: interval between pulses, duration and minimum frequency were more different between the categories. The comparison between bays showed that the parameters peak frequency and minimum frequency of the burst pulsed sounds produced in Guanabara bay were significantly lower than in the other bays. These results suggest the four categories of burst pulsed sounds described may be related to different functions that this type of sound plays in the communication of this species. The most important parameters for the separation of the categories are different from those that generate the intraspecific variation of this sound. While time parameters are fundamental in separating the structural categories of the burst pulsed sounds, the frequency parameters describe the intraspecific variations of these sounds.