Abstract:

Sotalia fluviatilis was the only cetacean species observed in all 16 boat surveys (82 h) conducted in the Sepetiba Bay (22(degrees) 58', 44(degrees) 02'W), between December 1993 and August 1995. During the boat survey of 5 January, 1995---a very calm day (Beaufort = 1)---it was possible to make good subaquatic sound recordings (30 min) of a group of ca. 20 young and subadult dolphins. As there is little bioacoustic information on tucuxi, sonogram analysis was used to characterize the tucuxi's whistles of ascendent frequency. The qualitative analysis resulted in 866 whistles divided in 48 types. Some of these sonograms shapes match the ones presented by other cetacean species sonograms (Tursiops truncatus, Delphinus delphis, and Globicephala macrorhynchus). The quantitative analysis was based on: number of harmonics; number of inflexions; beginning, final, maximum, and minimum frequecies; duration; and frequency modulation. Minimum, maximum, and average values, standard deviation, and coefficient of variation were calculated for each type of whistle. The S. fluviatilis whistles are extremely diverse (high coefficient of variation values for each type) and frequent (38.5 whistles/min), seeming to have an important role in social relations among these animals. Such whistle diversity and quantities could be explained by the composition of the dolphin group, i.e, it was exclusively formed by young and subadult animals.