Collective Decision-making in Response to Visual Looming Stimuli in Medaka Fish

Abstract

Cohabitation within groups promotes behavioural synchrony and facilitates information transfer. Whether it shapes collective decision-making under predator threat is unknown. Here groups of six medaka (Oryzias latipes) cohabited for one month were used to test whether cohabitation promotes instantaneous collective decision-making in response to a looming stimulus (LS) mimicking a predator attack. Firstly, we analysed behavioural transitions before, during, and after LS in groups composed of six individuals. Individuals showing high-speed swimming in response to LS typically tended to shift to freezing-like behaviour afterwards, whereas non-responders were more likely to maintain normal swimming. Group-level analysis revealed a bimodal distribution in the number of individuals exhibiting freezing-like behaviour, with peaks at zero and six individuals, corresponding to all non-freezing and all-freezing. Importantly, clustering analysis revealed that experimental groups showed consistent collective behavioural characteristics that could be categorised into three profiles: all-freezing, all non-freezing, and partial freezing. In non-cohabited groups assembled immediately before testing, the all-freezing profile was absent, and the distribution in the number of individuals exhibiting freezing-like behaviour shifted to unimodal. In these groups, even at the individual level, responses more often showed a transition from high-speed swimming to normal swimming rather than freezing. The results indicate that cohabitation promotes synchronous freezing-like behaviour and consensus decisions under looming threat. Our study presents a behavioural assay for predator-evoked collective decision-making in a genetic model fish, providing a framework laying the groundwork for future efforts to link behavioural ethology with neuroscience.