How to introduce new rats to pre-existing rats is one of the number one question I am asked. While Google does have a lot of links that have decent information, it's difficult to trust what you read on the internet. I'll detail what has worked well for me in the past and talk a bit about body language.
Please note, NO introductions of ANY kind should take place until after the full quarantine period has elapsed. Reminder that failure to comply to our quarantine requirements nullifies our health guarantees.
In general rats are social creatures. The only time there are typically issues is if one or more of the rats has hormonal aggression issues (more often post puberty) or is territorial or dominant. Here at WCR, I do not draw out introductions. I am comfortable moving rats between cages and putting breeding pairs together with minimal fuss, but that is not always the case - especially if you are nervous, and one or more of the rats pick up on it.
For new home introductions, what I personally recommend is the following. I will put it into "steps" as some of these can be skipped if you are comfortable or experienced.
(1) Clean and sanitize both cages, let them live in it for a day or two, and then swap either a few items, or the rats themselves between the cages a couple of times over the next few days.**
(2) Find a neutral location in your home that is both secure and spacious. I recommend using a bathroom for this, however if you have no safe, neutral space, a tub can work. Allow the rats to interact under direct supervision. You can add a bunch of toys, toilet paper rolls, and yummy high value treats to the area to give them ways to hide from each other, and to offer the opportunity to bond over good food. If ANY of the rats are food dominant, avoid this step for obvious reasons. If there are no signs of aggression (see below on how to differentiate between "working it out" and "aggression"), you can either do this again over a couple of days, or skip right to the next step. If there is aggression, see 3b.
(3a) If there's been no issues or major scuffles during play-time, clean the cage (yes, again) and make sure there are no traces of the rats that lived there previously. Wash and scrub all toys and hammocks, or replace them. This makes the cage "Brand New" for the new family unit moving in - can't get jealous over your space pod if it smells different and is in a different spot! Put them together, but ONLY when you have a day or so to watch/listen. Remember to watch for the body language on the chart/in the paragraphs below.
(3b) If there are issues or aggression during the previous step of introductions, you will simply need to continue doing what you are doing. Some methods that have worked but I cannot/will not personally vouch for their effectiveness, are to do one of the following at a time - dab real vanilla extract at the base of each rats tail. If they smell the same, theoretically they're less likely to make a stink about each other. Alternatively, a LIGHT coating/dab of coconut oil on each rat (body OR tail) will make them groom like mad. Groom each other, groom themselves, etc. It will make them greasy, but it's good for their coat and great for them when ingested, so it's really a win-win... As long as you go ahead and clean that cage afterward or make sure they're relatively oil free before they are returned to it! I've also heard of people successfully bathing their rats all together and they bond over the stress... I don't particularly like the sound of that, but realistically if you've tried everything, it may just work. Remember I'm not vouching for or endorsing 3b's suggestions!!! They're said to work, from other breeders and pet owners anecdotal experiences.
If they manage to function together in a cage with no blood being drawn for 48 hours straight, I'm apt to say they're safe toegether. Some positive signs that integration is going well would be:
Grooming each other
Sharing sleeping spots, or sleeping together
Eating out of the same bowl without scrapping at all
Playing, even squeaking and wrestling, as long as it is not aggressive - even if it sounds bad, they may just be working out who's boss
Negative signs that integration is not going well would be:
Always sleeping alone
Wounds, normally around the neck/shoulders
Aggressive posturing around the food dish, or around sleeping areas
A stark change in personality, especially in the new addition (jumpier, skittish)
Some rats are simply too aggressive genetically - a sign of poor, or thoughtless breeding - and they can't live with other rats. This is very sad if you think about it, that we humans can mess up a social species on a genetic level to the point where they can't live with each other at all. Sometimes it is hormonal, sometimes it is simply an inborn aggression. If it is hormonal, a neuter/spay may help, in my experience neuters are probably 75% effective in altering a rats personality, whereas spays are probably 50/50. I don't have statistics or proof on this one, it's all anecdotal. However, if the aggression is truly a hormonal aggression, altering the rat will very very likely have a positive effect on their overall personality.
We do not approve of or condone the solo housing of rats, unless in very specific and very special circumstances. Anyone found to be keeping rats solo for any reason other than extreme aggression, or for a health reason, will be automatically black-listed from ever purchasing animals from us. It's inappropriate and cruel to the animal, unless absolutely necessary.
During introductions, one thing to watch for (in the cage) is if one of your rats is rubbing their body against the bars, repeatedly. It will sort of resemble a cat rubbing its face on something - like the rat is trying to rub their scent off on the bars. That's exactly what they're doing. This is a sign of being territorial and aggressive. If your rat is doing this, please monitor it closely. You'll want to watch for puffed hair coat (piloerection), bruxing and crab-walking during neutral place introductions. Puffed hair is again, like a cat, showing visibly its annoyance. Bruxing is typically considered a happy thing, and most times it is. But rats also use it to show annoyance/anger, and it's a precursor to a bite. If you see negative body language with bruxing attached, re-direct if possible or remove the other rat(s). Crab-walking is a sideways walk, with their body in a sort of "C shape" in the direction of their 'opponent.' MAKE SURE IF YOU UNCERTAIN HOW THE RAT(S) WILL REACT, THAT YOU HAVE AN OVEN MITT, OR THICK LEATHER GLOVE, OR SIMILAR HANDY, IN CASE YOU NEED TO BREAK UP A FIGHT. DO NOT USE YOUR HANDS. RAT BITES CAN CAUSE PERMANENT AND SERIOUS DAMAGE INCLUDING NERVE DAMAGE IF THEY BITE YOUR HAND. BE SMART, BE SAFE. Never, ever push your rats. I recommend adopting in pairs (or more) because of introductions and integration, primarily. It is important - no, pertinent - that your rats not be alone during quarantine, and when QT is over and you go to introduce them, they have a much easier time integrating when they have an ally by their side. If they are alone, a juvenile may become so frightened that they develop anxieties. Many do not fully recover from the fear they feel as a baby being thrown into a strange environment with scary (mean!) adults they've never met before. I've seen it personally, which is one of the reasons why I refuse to sell single rats now. The fearful behaviour did not dissipate, and was not present in the rat at all until after a couple of failed introductions with a very dominant and pushy adult. If you have any questions or ideas for information to be added to this page, please don't hesitate to contact us on Facebook or through the website.