Emergence and development of personality over the ontogeny of fish in absence of environmental stress factors

Abstract: Animals typically display among-individual differences in behavior that are consistent over time (i.e., personality). These differences are often triggered by variable individual responses to environmental stress factors experienced during life, such as competition for resources and risk of predation. While the causes underlying animal personality are considered to be an issue of prime importance, it is still unknown whether personality emerges and develops over ontogeny if the main sources of behavioral differentiation are absent. Here, we tested whether personality emerged and was strengthened during the lifetime of Eastern mosquitofish (Gambusia holbrooki), once intraspecific competition and risk of predation were completely removed and genetic and maternal differences minimized. We found that individual differences in behavior were overall repeatable over ontogeny (i.e., personality was manifested). Personality was, however, not detectable in juvenile individuals but emerged during and after their sexual maturation. The emergence of personality was triggered by the decline in behavioral plasticity of individuals over ontogeny, while differences in behavior among individuals did not vary with age. Our results suggest that animal personality might be inevitable and emerge in fish under laboratory-controlled conditions even in absence of extrinsic factors that typically lead to behavioral differentiation. The decline of behavioral plasticity over lifetime might be a relevant mechanism for the development of personality in animals. Significance statement: Increasing evidence suggests that animals have personality, that is, individuals consistently differ in behavior among each other (e.g., bold and shy or social and non-social individuals). Personality differences among animals should be, by definition, consistent over time and often caused by environmental challenges experienced early in life. In this study, we observed that personality differences were not present at juvenile age in social fish but emerged later in their life, despite the fact that environmental challenges (i.e., predation risk and competition for space, food, and mates) were absent. Personality differences strengthened over lifetime, resulting from declines in individual behavioral plasticity. Our results suggest that the decline in behavioral plasticity with age may represent a relevant mechanism for behavioral differentiation in animals, in agreement with evidences from the human literature on age-related loss in behavioral plasticity.