Development of dominance in domestic rats in laboratory and seminatural environments

Dominance hierarchies were studied in mixed-sex colonies of albino rats Rattus norvegicus reared and housed in three different types of physical environments. Colonies were observed. In either an outdoor pen, an indoor pen, or in laboratory cases for a period of 24 weeks initiated at 35 days of age. In both pen colonies, male dominance relationships were based on the asymmetric display of biting attacks by dominants and the unequivocal display of submissive postures by subordinates. These behaviours did not fully develop, nor did dominance emerge until males were 140-150 days of age. No asymmetric display of attacks and submission, nor dominance relationships were observed in two different types of laboratory cage settings; these males continued to play fight throughout adulthood. However, males of all colony types attacked male intruders. The nature of the physical environment appeared to have a powerful influence on the development of dominance, but did not affect agonistic behaviour directed toward intruders.