Experimental Evidence of Stress-Induced Critical State in Schooling Fish

How animal groups dynamically adjust their collective behavior in response to environmental changes remains an open and challenging question. Here we investigate the mechanisms that allow fish schools to tune their collective state under stress, testing the hypothesis that these systems operate near criticality, a state that maximizes sensitivity, responsiveness, and adaptability. We combine experiments and data-driven computational modeling to study how group size and stress influence the collective behavior of rummy-nose tetras (). We quantify the collective state of fish schools using polarization, milling, and cohesion metrics and use a burst-and-coast model to infer the social interaction parameters that drive these behaviors. Our results indicate that group size modulates stress levels, with smaller groups experiencing higher baseline stress, likely due to a reduced social buffering effect. Under stress, fish adjust the strength of their social interactions in a way that leads the group into a critical state, thus enhancing its sensitivity to perturbations and facilitating rapid adaptation. However, while large groups require an external stressor to enter the critical regime, small groups are already near this state. Our findings suggest that adjusting interaction strength is a simple yet effective mechanism that drives collective state transitions while minimizing individual cognitive load. By revealing how stress and group size drive self-organization toward criticality, our study provides fundamental insights into the adaptability of collective biological systems and the emergent properties in animal groups.