Ecological context gates numerosity-based affiliation decision in zebrafish

In many group-living animals, affiliating with larger conspecific aggregations can reduce predation risk through dilution. A central question is what simple mechanisms can support such affiliation: are responses driven by generic visual magnitude cues (e.g., "more visual mass" results in stronger attraction), or is affiliation conditioned on perceptual cues about what is being viewed, such that numerical group-size information is used for conspecific-like stimuli but suppressed for predator-like stimuli. Here we test this in zebrafish (Danio rerio) by orthogonally manipulating number and stimulus regime (conspecific zebrafish line drawings versus predator-like large-fish line drawings, differing in both apparent body size and body morphology/identity). 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 favoured an Approximate Number System (ANS) predictor over an object-tracking (OTS) "small-number" account, with no additional small-number advantage under these conditions. When stimuli were predator-like, numerosity-based affiliation was abolished and sometimes weakly reversed, and neither ANS nor OTS predictors explained systematic variance, consistent with threat-like appearance suppressing affiliation even when a larger group is available. These findings show that zebrafish display ANS-like sensitivity to conspecific group size, but that its behavioural influence is selectively deployed: a predator-like stimulus regime (larger body size with correlated changes in morphology/identity) gates whether numerical information guides social affiliation.