Born This Way: Individuality Is Seeded Before Birth and Robust to Ecological Stress

Behavioural individuality is widespread, yet its developmental origins are unclear. We subjected clonal individuals to a critical ecological challenge-the threat of predation-to disentangle the developmental mechanisms of individuality. Predation may erode or amplify among-individual differences, demonstrating that individuality is developmentally plastic. If individuality is instead determined before birth, it should resist environmental influences. We tracked behaviours of genetically identical fish (Amazon mollies), reared with or without predation stress, continuously from birth for 4 weeks. While predation shifted mean-level behaviours, the magnitude of individuality was unaffected, indicating that individuality crystallizes robustly over time under stress and without genetic variation. Surprisingly, maternal identity strongly shaped swimming velocity but not refuge use, suggesting distinct developmental mechanisms for different ecologically relevant behaviours. We show that individuality persists despite ecological stress and is seeded before birth through non-genetic factors. Even in the face of a shared environmental challenge, the behavioural trajectories of individuals are unique.