To capture prey more efficiently, cetaceans can display a wide range of foraging tactics to separate individual prey. Barrier feeding tactics are performed to restrict prey movements, using natural and non-natural barriers and some species can even create barriers with their own bodies. Mud ring feeding has been observed in bottlenose dolphins in Florida Bay and in Chetumal-Corozal Bay, where ring-maker dolphins create ring-shaped mud plumes to encircle fish schools. Here, we document for the first time Guiana dolphins performing the mud ring feeding behaviour in the Cananéia estuarine system, in the southern portion of the state of São Paulo, Brazil. A total of 11 dolphins were recorded in four expeditions through aerial footage engaging in the behaviour. These findings expand our knowledge about the behavioural plasticity of the species and builds upon existing records of mud ring feeding behaviour in cetaceans.

The present study demonstrates the influence of the time of the day and tide on the frequency of individuals and on the social and foraging behaviour of an equatorial population of Sotalia guianensis. A population of S. guianensis from enseada do Curral (6°13′00″S 35°3′36″W), north-eastern Brazil, was investigated from October 1999 to September 2003. Social and foraging activities oscillated significantly, being less intense at high tide in comparison to other analysed tides. Socialization was more intense between 1000 and 1300 hours and foraging was more intense in between tides, than at high tide. No diurnal or tidal fluctuation was found on the frequency of individuals, which remained constant throughout the day. These results demonstrate the influence of circadian factors on an equatorial population which is highly impacted by human activities and highlights a need for protective measures.

The coloration pattern of the estuarine dolphin, Sotalia guianensis, does not display variations in its distribution and the colour grey, as its common name suggests, is predominant. On the coast of the state of Rio Grande do Norte a dolphin was sighted with atypical coloration in relation to animals from that region. After observing its morphological characteristics, it was verified that the animal was a white specimen of S. guianensis. Although photographic recordings were made, the reddish coloration of the eye cannot be confirmed, a characteristic that would identify the animal as an albino dolphin. Despite not being very frequent, cases of coloration anomalies such as albinism have been previously recorded in 20 species of cetaceans. However, there have been no cases recorded for the estuarine dolphin.

This study tested the association between variation in precipitation and behavioural seasonality in Sotalia guianensis, at the Curral inlet (6º13′00″S 35º3′00″W) in north-eastern Brazilian coast. From October 1999 to September 2003, observations were carried out four days a month. Behaviours were registered instantaneously each five minute interval. The observation period consisted of six hours into the daytime (0600 to 1700 h). Two observers inspected the area simultaneously, from a fixed location at the top of the cliff. During June to November, socialization (MdJN = 1.524; MdDM = 0.667; U = –2.848; P = 0.004) and forage (MdJN = 3.268; MdDM = 1.762; U = –2.881; P = 0.004) were significantly higher than from December to May, although the number of individuals (MdJN = 6.000; MdDM = 5.000; U = –1.395; P = 0.163) did not change over the year. Such behavioural seasonality was not associated with dry/rainy periods (number of individuals: MdRS = 5.000; MdDS = 6.250; U = –1.672; P = 0.095; socialization: MdDS = 0.947; MdRS = 1.464; U = –0.858; P = 0.391; and forage: MdDS = 3.017; MdRS = 2.292; U = –1.506; P = 0.132). These results support behavioural seasonality in an equatorial odontocete population.