Social structure of Atlantic spotted dolphins (Stenella frontalis) off Bimini, The Bahamas (2003–2016): alternate reasons for preferential association in delphinids

Abstract

Social systems are characterized by the associations and interactions between individuals. For highly social groups such as delphinids, understanding the demographics and long-term association patterns of a population is the first step in interpreting its overall social structure and specific relationships between individuals. This study investigated the social patterns of a population of Atlantic spotted dolphins (Stenella frontalis) off Bimini, The Bahamas, during a 14-year period (2003–2016). We generated association indices and conducted cluster, network, and temporal analyses using SOCPROG 2.8. Dolphins in this population display long-term, year-round residency around Bimini, as well as long-term preferences in association with a combination of rapid disassociation, constant companions, and casual acquaintances. We found no evidence of distinct social clusters within the study group; however, we discovered at least one subgroup with higher association than the rest of the population. This group was composed of male spotted dolphins, which overall tended to associate together more than females and chose companions with similar levels of association to themselves. Despite strong relationships between males, we found no evidence for the existence of alliances or coalitions. This study provides a baseline understanding of the social structure of this population that will allow us to investigate the variable relationships between individuals, in addition to exploring the effects of demographic changes within the resident population.