Introducing new mice

Some notes in advance: No method can fit for all species and individuals best. If you didn't succeed with one just try another. But sometimes the two (or more) individuals simply don't fit together. Then you have to find another partner or make two groups. That's why you shouldn't try for endless weeks again and again. Sometimes you have to accept that the individuals don't like each other. Another one might then be the right one.
Some pet owners are afraid that their mouse won't accept a cage buddy because it has been alone for a longer time. Usually that's wrong. In general there are 3 possibilities how such animals can react:

a) It is really enthusiastic and accepts the other as a friend from the first moment on.
b) It is afraid of the other(s). So you should give it a chance to discover that the new one(s) is/are no threat. This may take some days or even a week or two.
c) It is highly aggressive. But this is not a sign that you pet wants to be alone it's rather a sign of fear. But you should fit the way to introduce new friends to this fact. Maybe you will also need another partner.

A social animal will never be happy alone! You will find the right partner for everyone, even if it might last a little longer!

The older a mouse is the more sensitive is it for the stress of an introduction. This doesn't mean you should keep old mice alone if they lost their partner. You should simply be a bit more careful concerning method and time span of the introduction. Is your mouse very old you should always use the method causing least stress to it and choose an uncomplicated buddy.

To neutralise the smell of the used cage and furniture a solution of vinegar essence and water has proven best. Put it on for an hour and wash it off well with clear water afterwards. Not to irritate the sensitive noses of your pets you should dry everything in a well aired place for 24 hours.

You should begin an introduction preferably on Friday or weekends because then you should have enough time to watch the animals. Especially at the beginning this is very important.

Natal rats in a pet carrier

The step by step method usually covers a longer period of about one or two weeks and it is very successful compared with other methods.
The mice meet in your bath tub or another really huge tank the first time. Except for the mice there should be only two dishes in there, one for food and one for water. If the mice get in touch with each other or they tussle in an inoffensive way you should only watch them, but take care that no bloody fight occurs. If that happens anyway, separate the mice immediately und try again later. If the second try doesn't succeed another method might be the right solution. Sometimes it can be helpful to omit the first step and begin with the second, a small box. This step is finished successfully if the mice all cuddle in one place, usually the plug of the bath tub.
If the mice fight immediately the first time they meet in the bath tub, you shouldn't use this method but try another. If the mice hate each other from first sight also other methods might also fail. Therefore keep in mind, that you may need a new partner if other methods fail, too.
If the mice do get along together they should stay in the bath tub overnight if possible.

Next step is a small box where they stay altogether for at least 12 hours. Serious trouble or even fights shouldn't occur. If they do, put the whole group one step back to the bath tub. If you leave the group in the box overnight you should put the box right next to your bed to notice trouble immediately.
Here you can find some pictures of the development of this step.

If they behave well in the box you can move them to a small cage of about 40 x 30 cm up to 60 x 30 cm depending of the size of the group and the species you introduce. If the mice continue to behave freindly towards each other, they can move to a bigger cage (80 cm or more) 24 hours later. Had there been any serious trouble they should move back to the step before. Were there any petty quarrels the group should stay at this step some time longer.

If the group has spent the night together without trouble in the big cage, they can move to their final home which hasn't got any furniture yet. The furniture is added by and by and piece by piece. Depending on the size of the cage and the harmony in your group you can add several steps to the steps described.

It's one advantage of this method that your mice aren't so stressed because they can avoid each other when meeting first if they want to. They have to share small spaces not until the first contact has ended peacefully. The mice have enough time to get to know each other and get familiar with the new buddies. Thus the result is more solid.
A disadvantage for you as the keeper is the relatively long period of time you may need. And not everybody owns each size of cages he or she may need. But maybe you can borrow some from friends or an animal shelter.
Is there a step where quarrely ir fights always occur even though you moved the group one step back for 3 or even 4 times this method has failed.
This method fits to all species in principle.

Using this method you bring the mice together in a very small cage or even a box.
This method is based on the characteristic of some species to begin turf wars on bigger areas. Additionally the odour of the new mice mixes up with the one of the old because they are so close in touch.
A disadvantage is the high level of stress caused by this method. Furthermore there is a significant risk of injury, if the mice fought in this small space.
That's why you should use this method only if other methods failed or in combination with another method (e.g. step by step).
The method has failed, if there are further serious aggressions occurring after 15 or 20 hours.
The small boxes method doesn't fit for all spiny mice, grass mice and also for male pygmy mice, harvestmice and some other species because of their aggression within small spaces.

This method works with a cage separated into two halves by a mesh. To avoid aggressions and injuries you should use a double mesh consisting of two meshes in the distance of 1 cm for aggressive species.
This method bases on the fact that the mice can see and smell each other but can't fight. You put the individuals (seldom groups) each on one side of the mesh and then change them every 12 to 24 hours. So they get a bit faster used to the strange odour and also get it onto themselves. Some days later you remove the fence for test and watch the mice. If they still react in an aggressive way you have to put in the mesh again.
This method failed if your mice still fight after two weeks of changing sides every day.
This method is a good completion for the step-by-step-method for hard cases. Simply use it a week or two and then you can begin with step by step.
Additionally, this method works for barely aggressive species and individuals.

Using this method your mice meet on neutral ground none of them knows. Neutral ground can be an empty, cleaned cage with fresh bedding, the bath tub or something like that. Furthermore there shouldn't be any furniture at the beginning.
That's the best chance that they can meet without irritation. Especially during the first 2 or 3 hours you should watch them attentively because aggressions might occur.
After 24 hours being together without fight, there are two possibilities to go on. Either you leave them in the cage where they met first and give them furniture by and by or you move them to their nearly empty final cage and then add the furniture.
This method works well for genders and species which are said to be peaceful.
It is not so stressful und you don't need much time for it.
But you won't get peaceful groups in every case using this method. If it doesn't work alone you can combine it with other methods like the „split cage“-method.

There are several ways to use this method but all aim at a similar scent for all animals of the new group.
Some sources suggest to make the rodent swim in a perfumed solution (usually made of tea or essential oils). These ways you should definitely avoid because the mice can easily get a cold and also panic.
It´s not sure whether essential oils are safe, unsafe or even toxic. So be careful to use them even because acerb oils may irritate skin and lungs of your pets.
Another way to neutralize the inherent smell of each individual is to use a spray for bitches in heat. It neutralizes the smell for 2 or 3 days. Up to now I've never heard of any incompatibility of such sprays but you should be aware that there might be some.
The most natural way to practice this method is to take bedding from the toilet of the individuals and rub the other in with it.
Please notice: If you decide for this method you have to prefer areas with scent glands (e.g. at the anus, under the belly or the chins).
Even though the mice might not fight at the very beginning watch them attentively. Aggressions may break through later on.
The low level of stress for the rodents is an advantage of this method.
The method has failed if the mice become aggressive again after loosing the strange odour and get back their own or they fight though all have the same smell.
This method works together very well with other methods as the step by step for example.

The panic box is a special form of the small boxes method. A very small box of a size of about 20 x 10 cm e.g. for Mongolian gerbils is used. In some occasions it is filled additionally with bedding or hay so that the mice haven't got enough space to fight.
The mice have to stay in the box for about 12 to 48 hours.
This method is based on the characteristic of some species to begin turf wars on bigger areas but also on the fear - as the name says - the mice experience in the tiny box. Additionally the smell of the new mice mixes up with the one of the old because they are so close in touch.
A disadvantage is the high level of stress caused by this method. There is also a significant risk of injury if the mice fight in the small space. Since the box is filled with bedding, providing water is difficult. Use vegetable for this purpose.
This method is suitable if mice show aggressions on bigger space such as you use in the step by step method and the neutral ground method. You can also use it as first step in the step by step method.
The method has failed, if there are further serious aggressions occurring after 15 or 20 hours.
The small boxes method doesn't fit for all spiny mice, grass mice and also for male pygmy mice, harvestmice and some other species, because of their aggression within narrow spaces. For mice biting their fellows out fo fear this method doesn't work either.

As the name says you take some of the bedding out oft the cage every day and change it with some of the bedding of the other cage. Bedding from the toilet of your pets will work best for this purpose. Effect and success are similar to the fence-in-between-method.
You can use this method also to prepare a difficult introduction.
It makes sense to put the cages side by side as long as the animals don't show aggressive behaviour.

This method works similar to the bedding-change-methode, but in this case the mice move from one cage to the other not the bedding.
Success and suitability are the same as the bedding-change-method.
It makes sense to put the cages side by side as long as the animals don't show aggressive behaviour.
You can use this method to prepare difficult introductions.

Translation
Angelus Noctis
Proofreading
Jedediah