The social structure of bottlenose dolphins, Tursiops truncatus, in the Bahamas

The social structure of coastal ecotype bottlenose dolphins, Tursiops truncatus, is largely unknown as they inhabit regions far from shore. This study reports on a community of bottlenose dolphins greater than or equal to 27 km from Grand Bahama Island (May-September, 1993-2002). Resident and non-resident dolphins occurred in the area. Some dolphins traveled over 320 km between communities; others showed long-term site fidelity up to 17 yr. Average group size was 3-5, and was significantly larger with calves present and significantly smaller when traveling. The half-weight index was used to determine coefficients of association (COA) for individuals of known sex annually and for pooled years. Permutation tests revealed non-random associations and presence of preferred/avoided companions in all data sets. Annual COAs were low: female-female x = 0.31, male-male x = 0.30, and mixed-sex x = 0.26. Mother-calf associations showed the highest values. Some males formed strong, long-term bonds. Female COAs fluctuated with reproductive status. Using pooled data, COAs were lower and the same basic trends were evident. However, strong associations seen in the annual data were not evident in pooled data. Bottlenose dolphins that inhabit offshore, shallow water show many of the same social structure characteristics as in well-studied coastal populations.