This study aims to investigate the association patterns of a small and resident

population of bottlenose dolphins (Tursiops truncatus) in Bahıa San Antonio

(Argentina), and assess any seasonal variation in view of the reported seasonal variation in the population’s grouping behavior. The estimation of social differentiation

(S) was 0.29 (SE 5 0.08), suggesting a homogenous society. The half-weight index

(HWI) averaged 0.23 (6 0.06), with evidence of long-term preferred associations.

However, data showed seasonal variation in the association patterns. In winter,

when large groups were reported in this population, HWI values averaged 0.30

(6 0.09) with no indication of preferred or avoided associations. However, during

summer, when group size was generally small, HWI values averaged 0.14 (6

0.07), with an indication of preferred/avoided associations. This study indicates

that the social structure of the bottlenose dolphins in Bahıa San Antonio seems relatively

homogenous and flexible over time, with the formation of a random social

network at times when large aggregations are formed, and a more disconnected

network made up of strongly connected components when the cost of grouping is

high.