AbstractIndividuals vary in their cognitive abilities for a number of reasons, one of which includes the influence of their early environment. Early social experiences and stressors can affect brain development, resulting in cognitive differences later on in life. This study examined whether competitive interactions early in life affected later associative learning in the mangrove rivulus (Kryptolebias marmoratus), a self-fertilizing hermaphroditic fish that essentially produces genetic clones of itself. Genetically identical siblings that hatched on the same day were paired together and grew up in a competitive environment for approximately five months with one fish eventually becoming larger (and presumably dominant) over its smaller, subordinate partner. Siblings were then separated and after five additional months, each individual was trained to associate a particularly patterned wall with a food reward over several days. At the time of these learning trials, subordinate partners had recovered from their initially smaller size and there was no remaining size difference between partners. Despite this compensatory growth, the consequences of the early competitive social environment continued to affect performance. Specifically, the initially larger dominant partner tended to successfully reach the rewarded feeder quicker than their initially smaller subordinate partner. Interestingly, no evidence was found that individuals improved in their performance over time (i.e. no learning), but instead these underlying differences between the initially large and small individuals seemed to be present across all of the trials. Overall, these findings suggest that early social and competitive experiences can have lingering effects on individuals and result in subtle differences on their performance and cognitive abilities.