Day and night sounds of the Guiana dolphin, <Emphasis Type="Italic">Sotalia guianensis</Emphasis> (Cetacea: Delphinidae) in southeastern Brazil

Many cetaceans are known to be acoustically active at night. However, for most dolphin species, there is little information about their nocturnal acoustic activities. To study the acoustic repertoire of Sotalia guianensis, diurnal and nocturnal sounds (whistles, burst pulses, low-frequency narrowband (LFN) sounds, and clicks) were identified in the Cananéia estuary (25° 01′ S–25° 13′ S/47° 52′ W–48° 06′ W), south of the state of São Paulo, southeastern Brazil, during April, June, and November of 2012. The emission rate of these sounds was compared between daytime and nighttime using the chi-squared statistical test. The mean values of the acoustic parameters of whistles, burst pulses, LFN sounds, and clicks were compared using the t test. Whistles, burst pulses, and LFN sounds were more frequent at night, as these individuals require greater acoustic communication in the absence of light, mainly for social communication. Echolocation emission rates were similar in both day and nighttime. Dolphin sound structure also varied throughout the day, with dolphins emitting lower-frequency sounds at night. Low-frequency sounds, with longer wavelengths, provide many advantages for dolphins active at night because such sounds propagate greater distances. This study demonstrates that the sounds produced by S. guianensis are dependent on the time of day, with social communication sounds being more influenced by the presence of light.