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.