Behavioural seasonality of the estuarine dolphin, Sotalia guianensis, on the north-eastern Brazilian coast

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 (Md JN  = 1.524; Md DM  = 0.667; U  = –2.848; P  = 0.004) and forage (Md JN  = 3.268; Md DM  = 1.762; U  = –2.881; P  = 0.004) were significantly higher than from December to May, although the number of individuals (Md JN  = 6.000; Md DM  = 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: Md RS  = 5.000; Md DS  = 6.250; U  = –1.672; P  = 0.095; socialization: Md DS  = 0.947; Md RS  = 1.464; U  = –0.858; P  = 0.391; and forage: Md DS  = 3.017; Md RS  = 2.292; U  = –1.506; P  = 0.132). These results support behavioural seasonality in an equatorial odontocete population.