Introducing mice
There be times when you will want unrelated, same sex mice, possibly of different ages, to live together in one cage. Example reasons could be if you have a lone mouse following the death of it’s companion, or maybe just if you have a new mouse or mice and would prefer for them to live with your existing group rather than in separate cages. It is generally accepted that introducing female mice is easily than introducing males, although that’s certainly not to say that female intros will always go smoothly or that male intros will always be difficult.
The advice on how to successfully introduce mice can vary greatly and I would like to make it clear that the following method is just what I’ve found works for me. I have yet to attempt to introduce entire (uncastrated) male mice but have been successful with females (and females to neutered male/neutered male to entire male) many times over and would follow the same procedure for both. It always makes sense to choose a time when you aren’t in a hurry and where you can be around for several hours to keep an eye on things once the mice are in together.
The first thing I do is to completely scrub, clean and disinfect all parts of the cage and toys. I think that it’s important that none of it smells of the established mouse/mice, because if it does they’ll quite naturally want to defend it. For the same reason if you can change the cage around a bit and/or buy some new bits to go in it that will help too. Also I make sure there are no ‘dead ends’, ie beds and toys have more than one entrance/exit so the mice can’t corner each other. If you are introducing babies to adult mice it can also be useful to offer smaller hiding places that the little one(s) can fit in that the bigger one(s) can’t, so the babies can hide if need be.
Next I introduce the mice to each other on neutral ground, so they can all sniff each other over. Following some advice I was once given I also put a dab of vanilla essence on each of them as this is meant to confuse their scents and make them all smell similar. I have heard of some people using perfume for the same purpose but wouldn’t do this myself as perfume isn’t designed to be taken internally and the mice will almost certainly lick it off whilst grooming.
Once they’ve all had a good chance to check each other out on the neutral ground (how long will depend on the mice) I put the new mice into the clean cage. They then get 10 minutes or so to run around and explore before I put the others back. Basically at this point it’s a case of watching to see what happens! You can probably expect a bit of squeaking, chasing and squabbling as they sort out who’s boss and as the saying goes ‘no blood no foul’. However it was often at this point if at all that the leader of my established mice would decide to attack the new one(s). One time in particular the alpha female got quite irate, tail rattled (they make their tail ridged and flick it rapidly and repeatedly against the floor) and gave a really nasty bite to the new one, so I removed her and put her in the travel cage for a bit to cool down. Its important, if you have to do this, to take out the one doing the bullying and not the new one as otherwise the bully may well think they’ve won!
On occasions where I’ve had to remove a mouse I’ve left them separate for around half an hour to calm down and then put them back in with the group. Sometimes this is enough to calm the situation down but once in particular a mouse instantly did the same thing again, biting and drawing blood, so out she came again! I gave her longer on her own this time and the third time I put her in she chased but didn’t bite so I left them to it.
I have to say that I think it’s perfectly natural to be a little apprehensive during intros but I have found that after a couple of days any scuffles tend to have settled down and everyone will on getting on fine. An occasional chase and squeak after this is nothing to worry about and it’s best to leave them to it unless a mouse seems to be constantly picked on to the point of becoming stressed or if there is any wounds - see aggression.
Further Reading:
What to Expect When Introducing Female Mice by Cait McKeown
Some Tips on How to Introduce Your New Mice to Existing Mice from Little Emmo Mousery
