Sex and Order of Presentation Influence the Decoy Effect in Shoal Size Preference Task in Adult Zebrafish

The decoy effect is a cognitive bias where the introduction of an inferior third option shifts relative preference between two alternatives. Such biases influence foraging, mate choice, and social interactions across species. We investigated whether adult zebrafish (Danio rerio) exhibit the decoy effect in shoal size preferences by comparing their decisions in dichotomous (two-choice) and trichotomous (three-choice) contexts. In Experiment 1, using females as display fish, we compared dichotomous (4 vs. 2 and 6 vs. 3) and trichotomous (4 vs. 2 vs. 1 and 6 vs. 3 vs. 1) sets while counterbalancing the order of presentation. In the trichotomous sets, a single-fish shoal was added as a decoy, representing the most inferior alternative relative to the two primary options. Only females showed a baseline preference for the larger shoal in both the 4 vs. 2 and 6 vs. 3 contrasts. Males preferred the larger shoal in dichotomous trials only when they had been exposed to the decoy in the preceding trial. In contrast, females that initially preferred the larger shoal in the dichotomous condition lost this preference when the decoy was introduced in the subsequent trichotomous condition. Although the conventional attraction-type decoy effect was not observed, prior exposure to the decoy in males elicited a repulsion-like shift to prefer the larger shoal, suggesting cognitive adjustment to the geometry of the choice set rather than random dilution. In Experiment 2, we replicated the trichotomous-first condition for males and the dichotomous-first condition for females, using males as display fish with the 4 vs. 2 contrast. Females showed no baseline preference for the larger shoal, and males did not exhibit a decoy effect. Notably, our findings show that zebrafish evaluate shoal size options contextually, influenced by presentation order, geometry of the choice set, and sex. These results provide new insights into shoaling decisions, with methodological implications for comparative studies of rationality and decision-making across species.