Abstract: The study of the nocturnal activity of the estuarine dolphin (Sotalia guianensis) was conducted in Cananéia on the southern coast of São Paulo state. The observations were made between March and October of 2003, which included 22 field outings resulting in a total of 66 h of field work. Two procedures were used: (1) visual observation of behaviours, using the focal-animal method; and (2) recording and acoustic monitoring of the animals. Observations were made during the nocturnal period from a medium-sized boat, illuminated by a crescent or full moon, which allowed easy observation of the behaviours. In view of the need to learn more about the biology and ecology of S. guianensis, the objectives of this study were to follow the behaviours of dolphins at night using only moonlight, describe the behavioural patterns displayed, and determine the activities of the dolphins using acoustic monitoring. Observation of dolphins with the help of moonlight was demonstrated to be an effective means of observing the animals, but at the same time, it did present some difficulties. Behaviours were described using a combination of empirical and functional methods, and then they were compared to those described by other investigators. All the behaviours observed for S. guianensis during the nocturnal period— diving, hunting, and leaping—were the same as those seen, named, and described by other authors for the diurnal period.

Key Words: estuarine dolphin, Sotalia guianensis, nocturnal activity, behaviour, Brazil

Document Type: Research article

DOI: 10.1578/AM.32.2.2006.236

Page Numbers: 236-241