Unfamiliar conspecifics increase freezing and shift movement towards immobility in zebrafish, most clearly in males

An unfamiliar shoal increases freezing in adult male zebrafish (Danio rerio), raising freezing by +14.5 percentage points and shifting movement toward immobility (8% to 49% of frames).

• Why it matters: It is unclear whether unfamiliar conspecifics act as a stressor and whether males and females respond differently, which matters for interpreting social-housing effects in lab experiments.
• What they did: Adult zebrafish were exposed for at least 75 min to familiar versus unfamiliar shoals, with video tracking over the last 60 min and whole-body cortisol measured afterward.
• The result: Cortisol was 1.6-fold higher after unfamiliar shoal exposure when sexes were pooled, while sex differences appeared for swim bouts and movement states but not for freezing or cortisol.

AI summary of the abstract · may contain errors · not medical advice · read the paper.