Rats are unlikely to keep their costumes on for long, but it sure makes an adorable Christmas card. If you start out with one rat and decide to introduce a companion or two, there’s a right and a wrong way to do so.
Start by choosing a neutral location, like a cat carrier or an empty terrarium, for the rats to meet. Put some bedding on the bottom of it, along with some food and a water dish. Add all of the rats at the same time, and watch carefully to see how they interact. After some initial hustle and bustle, and possibly some coat fluffing and squeaking, they’ll likely settle into a pecking order and relax.
Keep them in this neutral location for at least 24 hours before moving them to their larger habitat. Add in enrichment items, like hammocks and tunnels, once activity has calmed down and they’re content to sleep near each other.