Ontogenetic emergence of behavioral consistency in a self-fertilizing fish.

Animal personality research assumes that behavioral repeatability indicates personality, yet this assumption has rarely been tested across ontogeny. Using the self-fertilizing fish Kryptolebias marmoratus under standardized conditions, minimizing genetic and environmental variation, we reveal a developmental dissociation between repeatability and personality. Boldness is highly repeatable from 10 days post-hatching (DPH) but fails to be consistent with later life stages. Behavioral stability emerges only at 60 DPH, coinciding with sexual maturation, and persists through 410 DPH. Extreme behavioral types diverge increasingly with age, while intermediate phenotypes remain stable. Our findings reveal that early repeatability reflects transient developmental states rather than behavioral consistency, challenging a core assumption in animal personality research. We demonstrate that repeatability without behavioral consistency emerges during early development with only low genetic variability, revealing personality as a developmental product of physiological maturation rather than a trait present from birth. These results call for ontogenetically explicit frameworks integrating repeated measures with physiological milestones to distinguish behavioral repeatability from true personality.