Effects of an Increase in Group Size on the Social Behavior of a Group of Rough-Toothed Dolphins (Steno bredanensis)

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Little is known about the social structure of rough-toothed dolphins (Steno bredanensis), but general knowledge of dolphin social structure suggests that individual dolphins often engage in multiple social relationships. In the wild, roughtoothed dolphins often swim in tight subgroups, consistent with the notion that social bonds and proximity to others are important for this species. Such behavior may also facilitate physical contact, which has been demonstrated to play important roles in social interactions in other dolphin species. In this study, the social behavior of captive rough-toothed dolphins was examined before and after the unification of two separate animal groups (both subgroups, n = 3) to investigate the effects of an increase in group size and a change in social structure on social behavior. The strongest new social partnership was between two juvenile males. Prior to the merger of subgroups, one of the juvenile males spent more time with a juvenile female. This dyad maintained a high interaction rate; however, after the move, this male significantly increased interaction rates with another juvenile male. The results of this study demonstrate that the integration of two small social groups provided opportunities for new social relationships to form but did not affect stable social partnerships that already existed. The formation of new social partnerships appeared to be influenced by age and gender. Although these results are based on a captive population, they provide insight into social behaviors that may be relevant to understanding the dynamic social structure of dolphin societies in the wild as well as in captive settings.