Rats are social animals and need to live with at least one friend. However, it can sometimes be a bit of a process introducing a new rat to into a group. I have found that, usually, new rats are happily living with the others within a few days, but sometimes it can take a few weeks to get them settled, and I know of some rattie combinations that have taken even longer to become comfortable in the same space.
Here is what I do when adding a new rat to my group.
Step One: Put the cages side by side but not so close that someone could get bitten through the bars. They should be interested, but not super aggressive.
Step Two: Switch the rats into each others cages to let them get a good sniff of their future friend.
Step Three: Allow the rats to meet in a neutral area (out of their cages). Watch for any signs of aggression, particularly from the dominant rat. Some people do this in the bathtub but my rats are a bit scared of being in the bath so I let them all meet on the floor after making sure there’s nowhere they could get lost or stuck. It’s a good idea to have a towel handy in case you need to break up a fight (an aggressive rat might bite you without thinking about what it’s doing, so be careful). Hopefully the rats are interested in each other but not violent. The new rat may squeak when it’s approached by the others, and the others may try to roll the new rat onto it’s back. If the new one starts looking terrified, put everyone back in their cages, let them look at each other through the bars, and try again the next day. After they’ve been together without anyone getting too mean for at least 30 minutes you can move on to the next step.
Step Four: Let all the rats hang out in an area your original rats usually play when out of their cage. Watch for your dominant rat showing sights of aggression – an arched back, raised fur, ‘crab walking’, or chattering. Put the rats on your knee or on your shoulders to get them close to each other and show the others that you like and accept the new rat. If they can go 30 minutes without a problem they are ready for the final step.
Step Five: Give your cage a really good clean and rearrange things a bit so that it feels like a new cage and not your original rats’ territory. Put the rats in the clean cage and watch closely. You’ll probably see the new rat being flipped over and pinned quite a bit, as well as being held down and licked. You may hear squeaks as they sort out a new pecking order. If you see the new rat constantly being chased around the cage, or you think the fights are too rough, move the new rat back to a separate cage and redo step four. Most of my newcomers have picked a ‘safe’ place in the cage and have mostly hung out there for a day or so. I make sure that they have easy access to food and water from their chosen spot and avoid feeding anything too delicious for a few days to lessen the risk of food related arguments.
Usually I can get through step one to four in a day and complete the process the following day. Having a big-ish group of rats and a big cage means no one cares that much when someone new moves in. My dominate girl has her hands full keeping everyone else in line so she doesn’t have a lot of time to terrorize newcomers. However, it can take much longer so be prepared to take it slow if you need to.