Group size varies with climate and oceanographic conditions in bottlenose dolphins

An understanding of animal grouping patterns is essential to increase knowledge about their social systems. We used a 5-year continuous dataset to investigate grouping behavior in a free-ranging highly social mammal, the bottlenose dolphin. Our objective was achieved by examining the relationships between a set of oceanographic, climatic, topographic, anthropogenic and social variables and bottlenose dolphin group size. Modeling analysis revealed that bottlenose dolphin group dynamics was linked to both small-scale oceanographic variation and large-scale climatic variation. Larger groups of bottlenose dolphins were predicted with an elevated concentration of dissolved oxygen in the water column, a moderate concentration of chlorophyll-a 60 days before each encounter, and a neutral NAO index. The observed link between environmental variables and dolphin group size is likely associated to changes in dolphin prey availability. Bottlenose dolphin group dynamics are likely driven by multiple factors, and social variables may act synergistically with environmental parameters. Larger groups of bottlenose dolphins were indeed also predicted with a greater number of dependent calves, likely because of enhanced care towards the calves. This study illustrates the value of using multiple variables at different scales to explore the factors that shape animal societies. The current study therefore contributes to the growing body of literature on how environmental change, occurring at different spatio-temporal scales, is indirectly related to the social behavior of a marine top predator.