Dominance relations of Norway rats in groups versus pairs

Several experimental studies assessing the hierarchy of a group in nonhuman animals have tested pairs of subjects in isolation. However, mounting evidence suggests that the dominance hierarchy of individuals tested in groups differs from that of individuals tested in pairs. In the present study, we compared the hierarchy of the same wild-type Norway rats, Rattus norvegicus , when tested in groups in the so-called Visible Burrow System, and pairs in the classic dyadic tube test. We show that the dominance order derived in groups differed from that inferred in pairs and was less stable in groups. This difference may be due to the presence of bystanders when testing in groups and their absence in isolated pairs. This suggests that the dominance hierarchy in a group is not reliably inferred when testing isolated pairs, and the classic tube test should be reconsidered as a standard protocol for dominance assessments in rats. • The dominance order of rats differs when studied in groups and in isolated pairs. • The dominance rank tested in groups of rats was less stable than in isolated pairs. • Lower stability in groups may be due to interference of bystanders.