Acoustic characteristics of whistles associated with the two-voice phenomenon in two oceanic delphinids

Abstract

Dolphins have a sophisticated acoustic communication system capable of producing tonal and pulsed sounds. The simultaneous emission of two distinct acoustic signals can occur due to the presence of two pairs of phonic lips. Although the phenomenon has been reported in some species of delphinids, its biological function remains poorly understood. This study reports the simultaneous production of whistle-click trains by two delphinid species, Stenella attenuata and Steno bredanensis, and compares the modulation and acoustic parameters of whistles emitted simultaneously and nonsimultaneously. Acoustic data were collected during oceanographic surveys along the Brazilian continental margin using passive acoustic monitoring (PAM). Whistle modulation patterns and acoustic parameters were compared between whistles emitted simultaneously with click trains and whistles emitted without simultaneous click trains in Stenella attenuata (277 and 125 whistles, respectively) and Steno bredanensis (743 and 58 whistles, respectively). Differences in whistle modulation and acoustic parameters were evaluated using chi-square, PERMANOVA, and permutation analyses. Whistle modulation did not differ between simultaneous and nonsimultaneous emissions. In contrast, significant differences were observed in acoustic parameters. In Stenella attenuata, maximum frequency, bandwidth, and whistle duration differed significantly between groups. In Steno bredanensis, all evaluated acoustic parameters differed significantly between groups. These findings demonstrate that the two-voice phenomenon is associated with changes in whistle acoustic parameters while preserving whistle modulation patterns, providing new insights into whistle production during simultaneous whistle-click emissions in dolphins.