Introductions for rats
Despite what some people say, rats from different age groups or litters can be introduced together. I have successfully introduced new additions to my rat family many times in with my original rats. In general (with the exception of unusually difficult rats) females accept new additions to the cage much more easily than males. It may take anywhere from a single day to a couple of weeks to get them successfully living together but usually it shouldn't take more than a week for most rats. After they are living together in most cases they will quickly become friends and you will often find them sleeping or playing together.
How to do introductions
Firstly, however you go about introducing new rats make sure it is always in neutral territory. You do not want either rat feeling territorial towards the space or they are more likely to react badly to newcomers. Here I will explain how I personally like to do introductions - Feel free to change things to make it more suitable for you and your rats. First make sure the space is neutral and preferably has no areas that rats can get stuck or lost under. I often lay down some fresh towels as it helps make the area smell new even if one of the rats has explored it previously. Make some yummy food for the rats. I usually make cooked pasta, fresh or cooked veggies, treats and lab blocks. If you are overly worried about how they will react together or you know they are not great with intros, dip a cotton bud in vanilla essence and wipe it down the back of both rats. This makes them smell nice and encourages the other rat to sniff them or even clean them. Once you have set the place up it's time to bring in the rats. Kneel down on the floor and put both rats on your lap. This makes them feel like they aren't allowed to misbehave as they know you are the boss. Do not hold the rats together as forcing them to meet causes them to become nervous and may lead to biting. Let them walk around on your lap for a bit and if they react well, put them on the ground in front of you. Watch them closely! You need to observe the way they act together for negative signs. If they are power grooming, standing over another rat to show size or pinning the other rat down this is fine. They may also try and scent each other. If they bite and draw blood or chase the other rat around in what is called a 'ball of fury' where both rats are squeaking loudly this is not good. If you are not sure if what you are witnessing between the rats is all right, email me and ask.
Many people find it hard to know when the rats should go in the cage together. I usually wait until they are completely relaxed around each other as being nervous can cause them to react badly in the new cage. A sign of them being relaxed is when they are eating next to each other. A rat generally will not eat when it is nervous or stressed. Once you are comfortable that the rats are getting on then it is time to set up their cage. You can put them together in a cage that one of them previously lived in however it is more likely to be successful if you can get them living together to start with in a totally new cage. This is inconvenient for a lot of people so there are ways to increase the chance of success within the old cage. If you are using the old cage, make sure you clean it with disinfectant as much as possible to remove any smells. Remove any traces of the old bedding and litter. Put in new hammocks as the material is very good at keeping in the smell. Once you have neutralised the entire cage, put the rats in together. They will explore for a bit and may have some more power grooming or pin each other down. Just watch them and make sure there is no blood. Keep a towel nearby through the introductions as it can be useful to use something other than bare skin to separate them if they fight so that you don't get hurt in the middle of it all.
Many people find it hard to know when the rats should go in the cage together. I usually wait until they are completely relaxed around each other as being nervous can cause them to react badly in the new cage. A sign of them being relaxed is when they are eating next to each other. A rat generally will not eat when it is nervous or stressed. Once you are comfortable that the rats are getting on then it is time to set up their cage. You can put them together in a cage that one of them previously lived in however it is more likely to be successful if you can get them living together to start with in a totally new cage. This is inconvenient for a lot of people so there are ways to increase the chance of success within the old cage. If you are using the old cage, make sure you clean it with disinfectant as much as possible to remove any smells. Remove any traces of the old bedding and litter. Put in new hammocks as the material is very good at keeping in the smell. Once you have neutralised the entire cage, put the rats in together. They will explore for a bit and may have some more power grooming or pin each other down. Just watch them and make sure there is no blood. Keep a towel nearby through the introductions as it can be useful to use something other than bare skin to separate them if they fight so that you don't get hurt in the middle of it all.