How do animal groups adjust their collective behaviors in response to a sudden change in their environment?
In a study published in the journal
PRX Life, scientists show that groups of fish subjected to environmental stress modulate their social interactions in a way that self-organizes toward critical states that enhance their adaptability.
Fish organization in response to disturbances
A central challenge in the study of fish schools is understanding how they effectively respond to disturbances, whether environmental in origin or related to predator attacks. To explore these mechanisms, scientists conducted laboratory experiments with a tropical fish species well known for its schooling behavior:
Hemigrammus rhodostomus.
Groups of fish were exposed to sudden light changes, simulating moderate environmental stress (see Figure 1 and video). The scientists combined experimental data with a mathematical model reproducing the intermittent swimming of fish and their social interactions. This allowed them to represent different collective behaviors in a "phase diagram" distinguishing various collective movement regimes: polarized swimming in the same direction, vortex formation, or disordered swarming (see Figure 2).
Collective behavior of a group of 25 fish subjected to light stress. Stress, group size and collective intelligence
The results, published in the journal
PRX Life, show that in large groups (25 individuals), fish subjected to stress adjust the intensity of their social interactions to bring their school closer to a critical state (see Figure 2). This state, well known in physics, corresponds to a zone where a system can change states and also becomes highly sensitive to disturbances. This allows the school to remain responsive to dangers while avoiding disorganization.
Surprisingly, small groups (10 individuals) remain almost always close to this critical state, even in the absence of external stress. This observation suggests that in these small groups, stress is permanent, probably due to the low effectiveness of the buffering effect induced by the presence of conspecifics that reduces individual stress.
Figure 1: Collective behavior of a group of fish swimming in a circular tank under stress conditions.
© CRCA, CBI, Toulouse
The scientists also showed that this adaptation is achieved through a simple mechanism: fish adjust only the intensity of their attraction and alignment interactions with typically two of their most influential neighbors, rather than modifying all social relationships. This reduces cognitive load while enabling an effective collective response.
This work illuminates the positive role of stress in the collective intelligence of social animals. It suggests that stress is not only a disruptive factor but also plays a driving role in the collective adaptation of these organisms. Thus, in fish, stress and group size are two fundamental levers that allow schools to self-organize toward critical states that promote their adaptability.
Figure 2: Within a school, attraction and alignment interactions between fish produce different forms of collective movement.
Beyond fish, these results open perspectives for understanding how other animal collectives (from social insects to gregarious mammals) optimize their interactions according to context. They could even inspire robotic systems such as autonomous drone swarms, capable of automatically adjusting their organization in the face of the unexpected.