Identifying influential neighbors in animal flocking

Schools of fish and flocks of birds can move together in synchrony and decide on new directions of movement in a seamless way. This is possible because group members constantly share directional information with their neighbors. Although detecting the directionality of other group members is known to be important to maintain cohesion, it is not clear how many neighbors each individual can simultaneously track and pay attention to, and what the spatial distribution of these influential neighbors is. Here, we address these questions on shoals of Hemigrammus rhodostomus, a species of fish exhibiting strong schooling behavior. We adopt a data-driven analysis technique based on the study of short-term directional correlations to identify which neighbors have the strongest influence over the participation of an individual in a collective U-turn event. We find that fish mainly react to one or two neighbors at a time. Moreover, we find no correlation between the distance rank of a neighbor and its likelihood to be influential. We interpret our results in terms of fish allocating sequential and selective attention to their neighbors.Author summary: Schooling fish exhibit impressive group-level coordination in which multiple individuals move together in a seamless way. This is possible because each individual in the group responds to the movement of other group members. But how many individuals does each fish pay attention to? Which are the influential neighbors? It is necessary to answer these questions in order to understand how directional information propagates across a group. Our research shows that in the rummy-nose tetra species there is a limited number of influential neighbors which are not necessarily the closest ones.