Dusky dolphin group dynamics and association patterns in Península Valdés, Argentina

We inspected dusky dolphin (Lagenorhynchus obscurus) group dynamics in Golfo Nuevo and found differences in social organization between cold and warm seasons. Surveys were conducted onboard a research vessel, from which we collected behavioral observations and group fission-fusion data from 2001 to 2008; we also collected photo-identification data from 2004 to 2012. To analyze association patterns, we calculated half-weight association index (HWI) and social differentiation (S). We conducted a Monte Carlo permutation test to determine whether observed association patterns were significantly different from random association using a compiled version of SOCPROG 2.7. Group fission and fusion dynamics depended on group behavior, the main activity after the groups' fusion was feeding, and they never fission before socializing. The social structure of dusky dolphins included long-term preferred companions in the cold season; during the warm season, there were no preferred companions. This seasonal difference in social structure could be related to an accompanying shift in foraging behaviors that appears to be driven by changes in prey availability. If so, then a loosening of bonds among individuals during the warm season, when prey is more available, would reflect these social structure changes.