Introducing new mice is quite easy providing you know the right way to do it. The following advice is for female fancy mice or neutered male to female group only. Please do not apply this to any other rodent. Each rodent has different criteria, and it can be extremely dangerous to them to follow the wrong advice. Gerbils for instance may kill each other if you do not introduce safely via the Split-cage method.
Mice must have company, and a group of at least three is recommended. This means that in the sad event of one dying early, you are not left with a lonely mouse.
Male mice can be neutered, and after 6 weeks from their operation date has passed, they can be introduced to girls following these instructions. Although you can cohabit neutered and intact bucks together, this is more risky, and should only be done by experienced mice keepers.
Getting new mice
This is best done from rescue or breeder due to risk of supporting rodent farms – see this external article on Why not pet shops?
If you are mixing the larger show mice, with fancy mice, then your introduction will be more complicated however, it still has a high chance of working.
Be careful when choosing your mice, as if you already have several dominant mice in your cage then you may need to ask your breeder/rescue for a more docile animal.
You may want to quarantine new mice before introducing, for a period of a few days or weeks. This will only be effective if you have a place to keep them that does not share the same airspace as your current mice. You will also need to be very careful about washing your hands and the equipment used whilst doing this.
Unlike gerbils (see our section on split-caging for more details), mice should be introduced on a neutral territory. A small tank or carrier, is ideal, with a minimum of substrate and no toys. Add two water bottles or bowls, and either two food bowls or try scatterfeeding. This means one mouse cannot claim one bottle/bowl and prevent the others from feeding.
You may wish to mark all mice with vanilla essence to help conceal their natural scent and aid the intro.
Add your mice into the tank and watch carefully! It is quite likely the one mouse will do the majority of chasing, so this may well be your alpha female. Chasing, squeaking, and same-sex mounting is all fine. Mice do squabble, even when they've been an established group for years.
If the numbers of the new mice is less than your established group, it can sometimes help to allow them to use the tank first. This means that they can establish their scent across it first which will unbalance the larger group and make them less likely to fight.
You only need to separate if blood is drawn. If this happens, make sure you separate out the bully, and not the victim. Leave them apart for around 30 minutes to cool down and start again later on the neutral territory. Some mouse introductions may never be successful, although this is quite rare.
Monitor them for a good few hours and if things are going positively then add a single item. Rather than a silent spinner wheel, a flying saucer is ideal for groups of mice as several can use it at once so it reduces fighting. Equally, if you do add a nest make sure it is easily large enough to accommodate all.
Scrub and disinfect the cage they will all be in and do not leave any toys or furnishings. Double up on any food/water bowls as above. It might help to transfer some of the bedding from your neutral territory tank.
Move your mice to the new cage. It can be good sometimes to allow your new mice around ten minutes or so in this cage before your established mice, as this gives them a chance to explore and get their scents marked.
You can add items gradually to their cage over the course of the next few days, make sure to keep doubling up on food and water bowls until you are sure that a single mouse does not camp one and prevent the others from feeding. You may also want to weigh all mice on a weekly basis to keep track, that way you will know if one is suddenly being bullied.
After a week or so, depending on their behaviour, you can be back to a fully furnished cage such as the picture to the right.