Introducing an older rat to new baby rats can be a tense time with the potential for fights or even serious injuries. Fortunately, if you take the right precautions and follow a slow introduction process, most older rats can learn to tolerate and even befriend new babies.
Get Them Accustomed to New Smells
Start out by placing the adult cage near the baby rat cage. This will allow them to get used to each other's scents in a safe way. Make sure the cages are far enough apart so they can't reach each other through the wire. Moving one of the baby rats' toys into the adult's cage and vice versa will also help them get used to each other. Some handlers recommend exchanging cages completely, while others believe this makes the rats nervous and on edge leading up to their first meeting.
Introductions On Neutral Ground
Rats are strongly territorial, so the first meeting should be on neutral ground. Avoid places that the adult rat usually sleeps and plays, even outside of the cage. The bathtub, a tabletop, the top of the bed, or even in your lap if the babies are skittish, can be good neutral locations. Watch their interactions carefully and separate them if things get too tense. When your rats can interact safely in a neutral territory, whether it takes one session or many, they're ready to move on to the next step.
Sharing A Cage
Thoroughly clean and disinfect the cage, so none of the rats will identify it as their own territory. Add a pipe, bottle or hut with an entrance small enough that only the baby rats will fit inside to give them a place to retreat to if they feel overwhelmed.
Once you place the rats together, watch them carefully, but don't interfere with smaller squabbles. These smaller fights establish the order of dominance and will stop once the order is established. Either wear heavy gloves or have a towel ready to use if you have to break up a more serious fight. After a day or so, the rats should start settling in together, and fights should decrease.
Troubleshooting The New Relationships
Some rats will take longer to adjust to newcomers than others. If the older rat is bullying the babies, consider putting them together in the babies' cage. This will give the smaller rats the advantage of being in their own territory. Placing a small dab of vanilla extract on each of the rats will mask their personal scents and may reduce aggression. You can also use a spray bottle to break up fights, since sprayed rats will stop to groom themselves.
Stay With It
If your rats just aren't getting along, you may want to wait until the babies are a little older. That will make them less vulnerable to the older rat. If an older male is continuously aggressive, you may consider getting him neutered, as this reduces aggressive tendencies. With patience and consistency, most rats can learn to get along.