The ecological consequences of consistent inter-individual differences in behaviour (animal personalities) are attracting more and more interest among the scientific community. Yet, the evolutionary causes of animal personality are still a source of debate. Recent hypotheses suggest that animal personalities are associated with differences in key life-history traits such as growth, fecundity, and mortality (“productivity-personality” hypothesis). Nevertheless, it remains unclear whether behavioural and life-history profiles are set early in life or, instead, represent the adaptive outcome of distinct optimal strategies in which bold and active individuals result to be both risk-prone and highly competitive in resource acquisition.

Here, we explore the relationship between personality and life history across the ontogeny of the invasive eastern mosquitofish (Gambusia holbrooki). Since low genetic diversity represents an ideal condition to explore inter-individual variations in phenotypes, we utilize first generation siblings from a highly inbred wild mosquitofish population. Experimental fish have been housed individually since birth and fed abundantly to allow individuals expressing their intrinsic life-history profiles in absence of competition phenomena. We repeatedly measured each fish for body mass, body length, activity, and boldness across three distinct life stages, that is, juvenile, sub-adult, and adult stage. We hypothesize that i) genetically-comparable individuals differ systematically in growth rate under unlimited food conditions; and ii) personality emerges in these contexts as a consequence of cumulative random events that differentiate individuals during their development. Our results show that personality profiles are not manifested in newly born fish and are, instead, observed in sub-adult and adult stages. Such behavioural differences among individuals appear only in relation to risk-taking behaviours and do not reflect variations in fish body mass and body length. Moreover, risk-taking attitudes vary between sexes even before sexual maturation. At the best of our knowledge, this represents the first longitudinal study on vertebrates individually housed under controlled conditions.