Rats are highly sociable animals, living in large packs in the wild, and so are best kept in pairs or groups. Solitary rats can become anxious and lack confidence.
Rats are best introduced at a young age, preferably when they are under 10 weeks of age. When buying two or more rats to live together it is not necessary to get rats from the same litter but it is best if they are roughly the same age and size (and sex if you do not want to breed them) and should have been living in a group community previously.
Although rats are best introduced at a young age, it is possible to introduce older rats although older females are more easily introduced than older males.
When introducing older rats this is best done by dabbing vanilla essence on both rats to disguise their smells and introducing them on neutral territory. With older males it is usually best to introduce the rats gradually, first of all just letting them sniff each other, then introducing them for short periods, working towards putting them together in the same cage.
When introducing rats for the first time they may squabble a little at first - there may be some chasing, squeaking, nipping, boxing, standing and staring at each other or one rat may pin another down while it grooms the its belly or sniffs its rear. This is quite normal and their way of establishing their position within the social structure of the group and unless serious injury or prolonged fighting occurs they are best left to sort this out. If it is necessary to intervene and separate the rats this is best done by throwing a towel over the top of them, separating them with a book or a piece of cardboard, or whilst wearing thick gloves to avoid getting bitten in the process.
Occasionally a very dominant rat may not accept living with another but this is rare.