The social ecology of Pacific coast bottlenose dolphins (Tursiops truncatus) in San Diego, California, was assessed with boat-based photo-identification surveys. A total of 146 surveys were conducted in the San Diego study area between January 1984 and December 1989. Approximately 200 hours were spent in direct observation and photography of 2,869 dolphins distributed across 145 separate schools. Dolphin schools were encountered on 79% of all surveys and 373 individual dolphins were identified. School size varied within and between years, as well as between winter and spring seasons. Schools containing calves were considerably larger than schools without calves, and dolphins identified as "probable mothers" showed an affinity for calf-schools throughout their resight histories. Social affiliation between individual dolphins was determined using a coefficient of association index. Observed patterns of social affiliation indicate that bottlenose dolphins tend to associate with a large percentage of all individuals in the population as well as forming relatively stable associations with some of their affiliates over short periods of time. Bottlenose dolphins in the San Diego study area appear to exist in highly dynamic social units with individuals forming complex social networks. The patchy distribution of prey species within the dolphin' s home range combined with their extensive coastal movement was hypothesized to underlie the flexible and dynamic social structure observed.
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