Collective decision-making in medaka elucidated — Shared living experience produces synchronous behavior

Professor Hideaki Takeuchi from the Graduate School of Life Sciences at Tohoku University and Part-time Researcher Ryohei Nakahata from the National Cerebral and Cardiovascular Center (a graduate student at Tohoku University at the time of the research) announced on January 9 that they have clarified that shared living experience among medaka fish is essential for producing synchronous danger avoidance behavior. This is expected to lead to the elucidation of the neural basis and genetic control of collective decision-making. Their findings were published in Scientific Reports.

One of the main reasons animals form groups is to gain survival advantages, such as more efficient foraging and escape from predators. In this context, the behavior that a group instantaneously chooses in response to an approaching predator has a significant impact on survival. Previous research has shown that in vertebrates, including fish, living together for a certain period enhances group "cohesion" and information transmission.

On the other hand, little was known about which behavior groups with "cohesion" would choose at the moment of crisis when a predator approaches, or how experience spent with companions influences this.

Therefore, this study made use of medaka fish, a molecular genetics model organism widely used in group behavior research. Visual stimuli mimicking the approach of a predator were presented to groups that had lived together for a long period and to groups meeting for the first time. The researchers compared the behavioral choices made during danger.

In this experiment, which studied groups of six fish, the researchers developed a system that can quantitatively analyze what crisis avoidance behavior groups choose. The system presents visual stimuli—a quickly expanding black circle mimicking the rapid approach of a predator—on a liquid crystal display.

At the individual level, a response pattern showing crisis avoidance behavior (freeze) was observed, in which fish rapidly accelerated in response to visual stimuli and then stopped as if frozen. There were also a certain number of individuals that did not respond to the visual stimuli and continued swimming as usual.

Next, when examining how many fish froze after visual stimuli in groups of six medaka, the researchers found that the behavior was biased toward either "all six fish freeze" or "all six fish continue swimming," with post-stimulus behavioral choices polarized. This revealed that unique behavioral characteristics are formed for each group.

Furthermore, in groups composed of individuals who had assembled just before the experiment and were meeting for the first time, the synchronized response of "everyone freezing" did not appear at all. Although many individuals rapidly accelerated in response to visual stimuli, they did not transition to freezing and merely returned to normal swimming.

From these results, it became clear that the experience of living together with companions enables "synchronized decision-making," where all group members instantly choose the same behavior in emergency situations such as predator approach.

Takeuchi commented: "In this research, we struggled to establish an experimental behavioral system reproducible in the laboratory and a method for quantitatively analyzing group behavior. This method has made it possible to analyze at the neural and genetic levels how group synchronization arises in the brain, using small fish as research subjects. Humans also sometimes align their judgments as a group in emergency situations, and using fish to explore the evolutionary origins shared with humans is an important challenge for the future."

Journal Information
Publication: Scientific Reports
Title: Social familiarity shapes collective decision-making in response to looming stimuli in Medaka fish
DOI: 10.1038/s41598-025-30656-4