Sex-specific weighting of shoal size and movement speed but no evidence of asymmetric dominance effect in zebrafish shoal-size preference

Social decisions often require animals to integrate information across multiple attributes of potential partners. Using biological motion stimuli, point-displays generated from tracked live shoals, we tested how adult zebrafish (Danio rerio) weigh shoal size and movement speed during social preference, and whether these preferences are susceptible to contextual manipulation by an asymmetrically placed alternative. In Experiment 1, we established a multi-attribute indifference point by presenting males and females with dichotomous contrasts in which shoal size and movement speed were traded off. Both sexes showed no preference when a larger, slower shoal (4 fish at 0.75x speed) was pitted against a smaller, faster shoal (2 fish at 1.25x speed), but preferred the smaller, faster shoal when the speed difference was greater (4 fish at 0.5x versus 2 fish at 1.25x), indicating that zebrafish are sensitive to graded differences in movement speed relative to numerical cues. In Experiment 2, unidimensional tests confirmed that both sexes preferred larger shoals when speed was held constant but revealed sex-based differences in speed sensitivity: males preferred faster-moving shoals at both shoal sizes tested, whereas females showed no significant speed preference. Male shoal size preferences were stronger at higher movement speeds, suggesting that speed modulates the strength of size preference. In Experiment 3, we tested the asymmetric dominance effect in males, the only sex sensitive to both dimensions, using the indifferent contrast from Experiment 1 as the primary options and four decoy shoals asymmetrically placed along either the size or speed dimension, under counterbalanced presentation orders. No decoy shifted male preference significantly from chance under any condition. These results indicate that zebrafish weigh social cues in a sex-specific manner, and that asymmetric decoy options do not induce preference biases in males when shoals vary along the dimensions of movement speed and size.