Usually people quarantine new rats for 2 weeks if they aren't able to do a proper quarantine. It is 4 weeks for a proper quarantine with a different air supply, etc. You may want to read up on quarantines and also the thread about the importance of a proper quarantine on jorats forum. You just need to be aware of precautions to take and what the risks are if you do not take them. If a new rat comes from a source you trust or has been in a home where it was basically quarantined, many people take a calculated risk and do not quarantine once the new rat has been found to be healthy and parasite free.
To get ratties used to one another you put cages near each other but far enough apart that they can not reach. You exchange dirty cloths from one cage to another and put rats in one another's cages when the occupant is not home. This is usually done over a
one or two week period, sometimes longer depending on the rats. Sometimes it goes much faster, depending on the ratties.
When they are no longer showing signs of aggression toward the unfamiliar rat, then you do intros in neutral territory. I would suggest you put the plug in the tub, put a fleece blanket in the bottom of the tub and put in a dish of yummy food such as yogurt.
Then add rats and watch.
When rats have been able to be together in a neutral place for 20 to 30 minutes without any signs of aggression on a number of occasions, you repeat in a non neutral area such as the play area.
When rats have been together repeated times in a non neutral area without any sign of aggression they are ready to live together.
You take the large cage they will live in and completely wash it out so it doesn't smell like anyone. You rearrange the items in the cage as well. You don't want ratties thinking they need to defend their territory. Some people suggest having the cage empty except for a plate with yummy food and gradually adding items over the next few hours. You put in a
plate of yummy food. Sit back and keep an eye on them for the next 8 to 12 hours or so.
Each step usually takes a week, but with some rats it can go much faster. There is another step for rats that are very aggressive toward one another but it is not normally needed.
If at any stage ratties show signs of aggression (puffiness, snorting, backing rear end
into other rattie, etc) then you go back a step as they were not ready yet. A bit of aggression is normal and they will pin and power groom but there should be no blood.
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Also, sometimes boys need to be neutered in order to live with other rats
