Stimulus context modulates numerosity-guided group association in zebrafish

In many group-living animals, affiliating with larger conspecific aggregations can reduce predation risk through dilution. A central question is whether larger-group affiliation is driven by generic visual magnitude cues, such as greater visual mass, or whether numerosity-guided approach depends on the visual stimulus context in which numerical information is presented. We tested this in zebrafish ( Danio rerio ) by presenting identical numerical layouts as either conspecific-like zebrafish line drawings or predator-like large-fish line drawings. In a spontaneous-choice assay with high-resolution tracking, zebrafish preferentially affiliated with the larger of two groups when stimuli were conspecific-like, and preference strength scaled with the logarithmic ratio between groups, consistent with ratio-sensitive numerical processing of group size. Trial-level model comparison provided modest relative support for an Approximate Number System (ANS) predictor over an object-tracking (OTS) “small-number” account, with no additional small-number advantage under these conditions. When the same numerical contrasts were presented with predator-like stimuli, zebrafish did not preferentially approach the larger group. In single-option trials, fish spent less time near the predator-like display than near the blank side. Together, these results show that the relationship between numerical ratio and approach behaviour differed across stimulus contexts, while the perceptual basis of this difference remains unresolved.