Differences in the sounds of Guiana dolphin, Sotalia guianensis (Cetacea: Delphinidae) between two areas of southeastern and southern Brazil

Cetacean populations can adjust their sound repertoire depending on the environment they are in, their population structure and the activities they are performing. Our goal was to characterize and compare, qualitatively and quantitatively, the sound repertoire of Sotalia guianensis in two areas of south-eastern and southern Brazil. We expected to find similar sound repertoires between the two regions, as they are geographically close and are part of the same complex estuarine. Acoustical parameters of the whistles, burst pulses and clicks were recorded during both daytime and night-time hours. They were compared between areas through Chi-square and Mann–Whitney tests. The samples resulted in 3,630 recorded whistles, 631 burst pulses and 44 low-frequency narrow-band sounds, with echolocation clicks present in 50.98% of the total minutes analysed. The occurrence rate of all sounds and the acoustic parameters of the whistles and clicks differed between the two areas, so our initial hypothesis was rejected. We highlighted environmental differences, behaviour exhibited by animals, number of individuals and group size and low exchange of individuals between areas as possible explanations that might account for these results, based on our knowledge of the species and areas of study, as well as a substantial literature on the physical and biological characteristics of the sounds.