Very briefly, here is a little advice on introducing baby rats (kittens) to resident adult rats:
For years I dreaded introducing rats to one another. Or rather, I didn’t mind too much introducing does to new babies but I was scared to death of introducing male rats. The babies look so SMALL compared to the adults and even the most mellow human-loving buck can be difficult to introduce to new babies.
Adult-adult introductions are quite tricky sometimes, and unfettered adult male rats do not take well to strangers. A perfectly peaceable buck will turn into a a puffed up, adrenaline-fed hellraiser the moment he sees an intruder and new rats can inflict serious injury to one another, even death. Luckily, introducing babies to adult bucks is usually okay.
Timing is important. Let’s start with the easiest: babies. Kitten-to-kitten introductions are generally very easy. You can introduce babies from different litters at any time up until ten or twelve weeks without much trouble. Breeders typically mix eight week old rats from one litter with a similar age from another litter without any need for special introductions. The same may be true with 10-14 weeks rats. Though the 14-week old is likely going to pull rank with younger kittens, you won’t usually see a problem.
However, by the time they are juveniles, you should be a little more respectful of the need for measured introductions. Some people will introduce young rats on neutral ground — a sofa or dry bathtub on which a towel has been laid. The idea is that such a space is nobody’s territory and therefore the rats may simply get used to each other without feeling any need to defend.
I don’t do it this way. I go straight to the “carrier method” which I’ll detail shortly. But first, let me tell you what you will hear from the days before we figured out the carrier method. It’s advice you’ll receive and you should largely ignore which involves “neutral territory” or bath tubs:
The famous “rat lady”, Debbie Ducommun writes that “with time and patience almost any rat will accept a newcomer” with only the occasion rat requiring spaying or neutering in order to become more accepting. The reason for the neutering is to bring the level of hormones down. Apparently, territorial aggression is often transformed with a sudden knock to the gonads.” While the world of rats owes a lot to Debbie Ducommun, a renowned expert, I am not as big a fan of the advice that introductions should be gradual, starting on neutral territory, then followed by introductions in a relatively large cage. Ms. Decommun recommends “cleaning out the larger cage completely and rearrange the furnishings so it appears to be a new cage. Trim the back toenails of the rats to minimize scratching in a scuffle. Put vanilla extract or perfume on all the rats to make them smell the same. Then put the rats in this cage.”
This may work for some rats or very young rats, and she is right that the best time to start introductions is when the rats are sleepy (so the middle of the day). I also agree with her that if fighting is taking place over a period of weeks you may have to give up with two rats ever being friends. However, my preferred method of introductions is one called the “Carrier Method”. I didn’t make it up — I’m not sure who did — but it hasn’t failed me with any of the doe-to-doe introductions I’ve tried, nor with introducing adult bucks to baby bucks (10 weeks old). I haven’t tied adult buck to adult buck, except on reintroductions, which is a slightly different situation.
So what do you do with the carrier method? You put the rats to be introduced, whether it is a single individual to a single individual or a group to a group, into the smallest cage or carrier that will fit all of them. When I say “fit”, the rats need to be in as small a space as you can find that allows them to be shoulder to shoulder and still lie down. In the case of 2-3 rats, a small carrier will do. For 5 does, you may need a large carrier. For 10 rats, you’ll need a small hamster cage.
It should look crowded. A little like this:
Once you’ve put them all in, walk around with the carrier. Rats that are unsure tend not to find, so usually they won’t do much while in “transit”. Keep them moving until they seem more relaxed. If you want to extend this time, put them in a car. The motion of the car is likely to keep them quiet as well.
Later, you can just sit down with the carrier near you. Hear squeaking and fussing? Walk around with the carrier to distract them from this. You may see a rat pinning down another rat, or forcibly grooming another so that the poor victim is squeaking lightly. That is to be expected. But as long as the one being pounced upon submits to the other, they will soon be fine. If a rat is really being aggressive with another, you may need to intervene. Do not do this with your bare hands! Get a water spray or a towel or oven gloves or anything, but do not ever break up a rat fight without protection. It is a sure way to get bitten.
Once it seems they are fine in the carrier you can add a mouse-sized water bottle (I say mouse-sized as it is the only thing that is going to stay in the holes of the carrier, most likely).
Add food scattered on floor. Do not give any treats or anything that too exciting to them that they will fight over but give them much more of their regular food than normal. You don’t want them hungry.
Some people keep them in a carrier for a few hours or overnight. I’ve had mine in a carrier for as little as 5 hours to as long as 12 hours before moving them up. I’ve been known to sleep with a carrier next to my bed. In fact, I have many carriers in different sizes and I put them into an identical carrier every 12 hours so they are clean. Only once I’ve seen they are bored as hell (approximately 1 day ) do I move to the next level, a really small hamster cage. From there it might be another day (or two or three) before they get moved to a single story rat cage like a Savic Ruffy. No hammocks. A day later, I try a hammock, then another, then maybe a little house. Once they are sleeping together and look like nothing much is stirring I move to a larger cage ( my SRS, but only half of it. I am making it sound like I do this every week. I do it about 3-4 times a year, when I breed. You can go much faster if it is a totally nothing event , but some people take a week to get from carrier to normal rat cage with no hammocks.
Rats will play fight and will have occasional grumbles but it is natural for them to live in groups and they usually get on. If you have a particularly difficult situation, you may need to create alternative groupings or even neuter/spay a rat or rats. However, generally speaking, peace reigns.
Be warned that you may have to “reintroduce” rats on occasion. For example, if you’ve had one out of the group for days or weeks at a time or when the “alpha” rat dies. You also need to remember that not all rats will get along. Imagine living in a cage with someone you were afraid of! That isn’t a sustainable situation and sometimes introductions fail. If so, keep two cages. You may be able to try again later.