Irrational groups: decoy placement, not group size, shapes collective shoal-size preference in adult zebrafish

Group living often improves decision accuracy in animals, yet whether increasing group size buffers against context-dependent biases remains unexplored. One such bias is the decoy effect, where the presence of a third option shifts preferences between two alternatives. Here, we tested whether introducing a decoy shoal influences collective preference for the larger female shoal in adult male zebrafish (Danio rerio), and whether the strength of this effect depends on group size. Groups of two, three, or four males were presented with female shoals under two choice contexts: a dichotomous contrast (four vs. two fish) and a trichotomous contrast including an additional alternative of one, three, or five fish. The order of presentation (dichotomous-first or trichotomous-first) was counterbalanced, and multi-animal tracking was used to quantify group-level shoal-size preference (time spent near shoals), inter-individual distance (IID), polarization, and swimming speed. In the dichotomous-first order, across all group sizes, subject shoals consistently showed a baseline preference for the larger shoal. Adding a third decoy option altered this preference only when the decoy was extreme (one or five fish relative to the 4 vs. 2 alternatives), reducing relative preference toward indifference. Group IIDs were associated with context-dependent shifts in relative preference in the dichotomous-first order, whereas polarization and swimming speed were not. In the trichotomous-first order, groups showed no preference for the larger shoal, with or without a decoy shoal. Our results demonstrate that context-dependent biases, shape collective shoal choice, with effects driven by choice structure and order of presentation of options than by group size.