Bottlenose dolphin (Tursiops truncatus) social structure in the Shannon Estuary, Ireland, is distinguished by age- and area-related associations

Abstract

Social and network analyses that incorporate information on individuals within a population enhance our knowledge of complex species. In this study, the social structure of wild bottlenose dolphins in the Shannon Estuary, Ireland, was analyzed by examining the dynamics of the whole population and then of individuals classed by sex, age, and area. One hundred and twenty-one dolphins were identified during 522 sightings between 2012 and 2015. The mean half-weight association index (HWI) of the population was 0.07 0.02. The highest HWIs for individuals of known sex were for female-male pairs. Mean within-class HWI was significantly higher than mean between-class HWI for both age and area classes. Ordinations and sociograms were used to visualize social networks. Permutation tests revealed nonrandom associations for the population overall and both between and within classes. Temporal analyses showed associations persisting for >1,000 d. The whole population’s best fit model was for two levels of casual acquaintances. Movement analyses demonstrated the use of the inner estuary by only 25% of the population revealing a potential community division by area. The difference between mean HWI when socializing (0.09 ± 0.03) compared to foraging (0.06 ± 0.03) was significant. These results highlight the importance of localized research, reflecting the complexity found in bottlenose dolphin societies globally.