Rats are social creatures, living in colonies that are housed in a complex network of underground burrows similar to the warrens dug by wild rabbits. To protect the colony from predators, the entrances to the burrows are well-hidden among rocks, the roots of shrubs, or under other thick vegetation. In temperate regions, most of the burrow is below the frost line, ranging from a few inches to several feet below the surface. Inside, the rats build nests of shredded vegetation, feathers, paper, and various other materials and huddle together for warmth.
One colony may consist of hundreds of rats of both sexes and all ages. According to observations made by zoologist S.A. Barnett, the colony is a relatively peaceful place. Due to an established social hierarchy among the males, there is little infighting for the right to mate with the females. Among rats, familiarity breeds content: seldom do rats that have grown up together in the colony fight with each other, although they may play in a rough-and-tumble fashion.
Conflict usually occurs when a new rat, especially an adult male, appears and tries to join the colony. The newcomer's status, and sometimes its fate, is determined by the first few encounters it has with the colony residents. Fights that occur are seldom intense or bloody. Dominance is quickly established, and once the newcomer adapts to its new place in the colony the issue is settled. Male newcomers that lose the fight seldom remain for long; soon after the fight they either leave the colony or die, although they are uninjured. Some zoologists hypothesize that they die of social stress.