Rats are social small animals and are happiest when living with at least one cage mate. It’s best to get two rats that are already bonded. If you have a single rat and want to get it a cage mate, you need to introduce the two carefully so they learn to get along.
Be sure the rats you pair together are of the same gender so you don’t end up with baby rats. Two females usually get along the best. You can also pair rats that are spayed or neutered with rats that are intact.
Introducing a new rat to another can be tricky and depends mostly on the personalities of the rats involved. Typically, rats are social but some dislike having a cage mate. Others may dislike certain rats but accept others.
You can introduce two rats by keeping them in separate cages next to each other. They can see, smell and hear each other without touching.
After a few days, structure some playtime in a neutral area after a week or so. Don’t be afraid to continue setting up play dates even if the rats squabble a little a first; they may learn to get along over time. When squabbling, some rats rise up on their hind legs and jab with their front paws, and others are known to flip the other over on its back and bite the other’s belly. It’s not unusual for rats to pin one another and aggressively sniff the other’s genitals. Submissive rats may become uneasy with play of this kind but rats rarely hurt each other when playing. Be sure to monitor your rats’ playtime, and don’t be surprised if you notice excessive biting, screeching and bounding around the cage. You can let these fights play out since the rats are establishing hierarchy with these behaviors.
Once the rats seem to be getting along well, try housing them together. Wash the cage thoroughly and add some old bedding from each of the two cages. Leave the cage open and let the rats play in a rat-proofed room. The rats should go in and out of the cage to investigate it together without fighting. It may take two weeks to a month for this to happen, but once it does, it’s safe to house them together.
Never rush or force co-habitation before your pets are ready. Injuries may occur if you proceed too quickly if both rats are adults, particularly if one rat has lived alone for a long time.