Violation of rationality in shoaling decision making in zebrafish (Danio rerio)

The decoy effect, a bias in choice between two options when a third, inferior option is introduced, has been observed across various organisms, from slime molds to humans. In this study, we investigated whether zebrafish (Danio rerio), a widely used biological model organism, exhibit the decoy effect in their shoaling group choices, specifically examining this effect by varying only shoal size. Using spatial trajectory analysis of freely swimming zebrafish interacting with conspecifics in adjacent display tanks, we tested how shoaling decisions varied between dichotomous (two-option) and trichotomous (three-option) choice sets. Our experiments compared preferences for 4 versus 2 and 6 versus 3 shoal sizes, with a single fish serving as a decoy in the trichotomous sets. The results revealed sex-specific differences in the decoy effect: males exhibited a shift in preference only in the Trichotomous-first condition, where prior exposure to the decoy led to a significant preference for the larger shoal. In contrast, females displayed the decoy effect exclusively in the Dichotomous-first condition, shifting from a significant preference for the larger shoal to showing no clear preference in the presence of the decoy. Notably, our findings demonstrate that the decoy effect can occur even when studied unidimensionally, with both sex and the order of presentation influencing zebrafish shoaling preferences. These results offer new insights into decision-making processes and highlight the importance of considering order effects in studies of choice behavior, with potential methodological implications for research in other species.