The willingness of rats to accept a new roommate depends on their personality and age. It will take more time--sometimes up to several weeks--to introduce a newcomer to adult rats, but it can be done. With time and patience, almost any rat will accept a newcomer.
The introduction process has several steps and the time required for each may vary. The easiest pairing will be two young rats, while the most difficult introduction is between two adult males, especially if they aren't neutered. On the other hand, as rats start to age they can mellow out and be more accepting of new roommates. A young rat is the best choice for a new roommate for an adult male, but the newcomer should be at least six weeks old. This is because adult males will sometimes kill strange baby rats younger than this.
When introducing rats, you should have two cages so the new rat can have his own cage at first. (The second cage can be the resident rat's travel cage.) Never just plop a new rat in the resident rats' cage because the residents will always defend their territory.
During the introduction process, you should only advance to the next step if there is no sign of aggression within 30 minutes. A resident rat will express aggression by puffing up his fur, arching his back, and swinging his body sideways at the newcomer. If you see this type of behavior, remove the new rat quickly to prevent an attack. Use care when picking up an aggressive rat as they will sometimes bite. Pick them up with a towel. Or if need to quickly break up a fight, you can pick up the aggressor by the base of his tail. If an introductory session results in aggression, try again later, or go back a step.
When introducing a new rat to a group of rats, usually only the dominant resident rat will be aggressive toward the newcomer at first. This is the dominant rat's "job". Once the new rat is accepted by the dominant rat, the others may show some aggression in turn, but it usually won't be as severe.
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