Selective social interactions and speed-induced leadership in schooling fish

Animals moving together in groups are believed to interact via effective social forces, such as attraction, repulsion and alignment. They can be inferred using `force maps', i.e. by analyzing the dependency of the acceleration of a focal individual on relevant variables. Here we introduce a force map technique suitable for the analysis of velocity alignment interaction. After validating it using an agent-based model, we apply it to experimental data of schooling fish. We observe signatures of an effective alignment force with faster neighbors, and an unexpected anti-alignment with slower neighbors. Instead of an explicit anti-alignment behavior, we suggest that the observed pattern is the result of a selective attention mechanism, where fish pay less attention to slower neighbors. This mechanism implies the existence of temporal leadership interactions based on relative speeds. We present support for this hypothesis both from agent-based modeling, as well as from exploring leader-follower relationships in the experimental data.