Evaluating the influence of ecology, sex and kinship on the social structure of resident coastal bottlenose dolphins

Animal social structures are shaped by external environmental factors and individual intrinsic behavioral traits. They represent a balance between the costs and benefits of group-living to maximize individual fitness. Bottlenose dolphin, Tursiops truncatus, societies are fission–fusion with high variations in association strength, grouping patterns and influence of kinship on social bonds throughout the wide range of habitats where they occur. Here, the drivers of social structure in resident coastal bottlenose dolphins of the Normano-Breton Gulf (English Channel) were studied using a multidisciplinary approach combining individual monitoring (photo-identification) information, genetic and ecological data. First, the ecological segregation of the social clusters was tested. Then, the influence of kinship, sex and ecological specializations on association patterns was evaluated. Stable isotopes revealed that the social clusters had relatively distinct ecological niches. Resource partitioning among social clusters may reduce competition and may allow the area to sustain a larger resident bottlenose dolphin population. Individuals did not preferentially associate with related individuals or individuals of the same sex. However, sample size was relatively low for females and, therefore, a role of kinship in shaping association patterns could not be totally ruled out for those individuals. Instead, dolphins preferentially associated with individuals of similar ecology. The study also emphasizes that stable isotope analysis is a promising tool to investigate the link between social structure and ecological specializations, particularly in taxa that are difficult to observe in the wild.