Novel Competition test for food rewards reveals stable dominance status in adult male rats

Abstract

Social hierarchy is a potent modulator of behavior in many species, including humans, that is typically established through overt agonistic interactions between individuals in the group. Once established, social ranks are maintained through subtler interactions allowing the redirection of energy away from agonistic interactions towards other needs. Most of the available tasks for assessing social rank in rodents allow the study of the mechanisms by which social hierarches are formed but fail to assess the maintenance of established hierarchies between stable pairs of animals, which might rely on distinct neurobiological mechanisms. Here we present and validate a novel trial-based dominancy assay, the modified Food Competition test, where established social hierarchies can be identified in the home cage of non-food deprived pairs of male rats. In this task, we introduce a small conflict in the home cage, where access to a new feeder containing palatable pellets can only be gained by one animal at a time. We found that this subtle conflict triggered asymmetric social interactions and resulted in higher consumption of food by one of the animals in the pair. To assess the reliability of the observed asymmetries as reflecting dominance relationships we investigated the behavior of same rat dyads in multiple social tasks adapted from social hierarchy studies. We found a positive correlation in dominance indices across most tests used. Our findings reveal stable dominance status in pair housed rats and provide a novel tool for the evaluation of established social hierarchies, the modified Food Competition test, that is robust and easy to implement, thus expanding the set of tasks available to study dominance in the lab.