The resident effect dominates over previous winner/loser experience in surface fish of the Mexican tetra

Social structures are widespread across the animal kingdom and play a critical role in future social decision making across mammalian and nonmammalian vertebrates. These social structures often manifest as competitive interactions that result in winner and loser individuals, with these informing potential future winner/loser interactions. Despite the relevance of winner/loser effects in establishing social dominance hierarchies, it remains unclear whether these depend on fixed internal states that may include previous social experiences, opponent-based assessments or residence placement in a social behaviour contest. In this work, we tested whether resident/intruder or winner/loser effects impacted future social contest outcomes in social and aggressive surface fish of the Mexican tetra, Astyanax mexicanus. We designed a resident/intruder tournament in which surface fish engaged in repeated aggressive contests across four consecutive days. Following this sequence, surface fish residents were challenged with an unfamiliar intruder fish that posed no clear advantage (size-matched surface fish), a moderate advantage (smaller surface fish) or an absolute advantage (smaller, nonaggressive, blind conspecific). We found that the resident effect was stronger than previous loser effects, independent of an individual being opposed against an unfamiliar surface fish of the same size or a smaller size or a smaller, nonaggressive cavefish. These results demonstrate that residence has a powerful effect on winner/loser dynamics in A. mexicanus, offering a powerful model for investigating the genetics underlying future social decision making.