Social cohesion in a hierarchically structured embayment population of Indo-Pacific bottlenose dolphins

We investigated community structure and association patterns for a small population of Indo-Pacific bottlenose dolphins, Tursiops aduncus, inhabiting the Port Stephens embayment in southeastern Australia. Association data for 120 regularly sighted individuals were obtained from seasonal photoidentification surveys collected over 7 years (1998-2007). Using a combined cluster and social network analysis approach, we found association patterns between dolphins were hierarchically structured, where two mixed-sex communities were subdivided into smaller, temporarily dynamic social groups. Community membership corresponded to differences in individual ranging patterns and habitat occupation. The larger eastern community inhabits a typically marine environment, while individuals of the western community range over a larger area that is dominated by estuarine processes. Both communities were composed of long-term preferred companions; however, the degree of social cohesion differed considerably between the two communities. Associations between individuals were considerably stronger and temporally more stable in the western community. Western individuals also had significantly fewer preferred associates despite living in similar-sized schools. Finally, in direct contrast to associations within each community, intercommunity associations were highly variable and resulted primarily from aggregative behaviour. We propose the segregation of communities resulted from individual adaptation to local environmental conditions, facilitated by individual variability in association preferences. The disparity in association patterns between communities may have resulted from a combination of ecological, population density, kinship and anthropogenic factors.