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Introducing and Keeping Male Mice Together

Question and Answer from Mouse@HornsIntroducing Adult Male Mice by Madeline Lewis


If they are introduced at a young age (less than five weeks old), and preferably when they are from the same litter, male mice can be kept together. You will read in some other places that adult male mice must live alone or else they will fight. I can only say that I have kept male mice in both large groups and pairs and that it works. It seems to me that it enriches the animals' lives immensely. All it takes is a little effort.

You can see from this photo how the two bucks are comfortable with each other: Akwete (chocolate) even helped to look after Lithium when he was seriously ill, as you can see in this photo.

Established groups of male mice should not be separated for more than a few hours, or they may start fighting. In particular, you generally cannot take one of a group out to mate with a female and then return him to the group. When I clean my males’ cages, I usually keep them together in a spare tank.

Two bucks meet for the first time: one sniffs the other, who makes a submissive gesture

One of the most common factors which makes male mice more likely to fight is being kept in a small cage. If you keep a group of male mice you really do need to get them a large home, and make sure that there are a number of different nest boxes (boxes, packets etc) so that each mouse can have his own territory if he desires. Despite there being different areas, the mice may still choose to sleep together in one place (all mine have done so).

Mice may jostle for position in the cage hierarchy and can bite each other. More often there will be a lot of squeaking and some squabbling but no real fighting, especially with female mice.

Two adult bucks meet for the first time, the dark buck is biting the other buck on the rump. The bucks had to be separated. (The PEW buck lost most of his hair due to age, not fighting or illness).

Check your mice regularly when they are first put together to make sure that they have not injured each other. I would recommend putting the mice together when you have the time to watch them for at least an hour to an hour and a half. Any fighting will usually begin in this time.

If a mouse is injured she will groom herself more and this will clean the wound. As long as there are no serious injuries where there is blood drawn the mice should be fine. Make sure that there is no excessive bullying though. If this happens you may want to try segregating the bully for a few days as a last resort before reintroducing him to a cage that smells like the other mouse's territory. I must stress that this is a last resort because separating the mice unnecessarily may cause them stress.

The author has kept a number of different groups of male mice together, and would never contemplate keeping one alone unless every other option had been tried.

It is common practice amongst many breeders of fancy mice to keep groups of adult males together, even in small wooden breeding boxes. With a small amount of effort, and a larger cage, the average pet owner should be able to keep male mice together with no problem.

Sometimes even established groups of males fall out, though - you have to watch them carefully. Some strains of mice are more aggressive than others, so your success in keeping males together will depend in large part on the family background of your mice.

This buck is either scared or being submissive towards another buck

There will often be the occasional fight between males who normally live together peacefully. If this fighting seems to get serious, i.e. there are bleeding wounds or a mouse seems depressed and is spending a lot of time sitting still, then the mouse which is bullying him should be taken out of the cage. At this stage you have several choices.

  • Keep the mouse alone and try to give him an interesting life.
  • Introduce the mouse to others who he cannot bully so easily.
  • Put him with an old infertile doe for company.
  • Have him neutered.
  • If the mouse is one you want to breed from, then you could use the following method. Put a doe in with the buck, and leave them together when she has her litter. The doe may well become pregnant immediately after giving birth, so understand that you risk having two litters close together. When the babies reach four weeks, or the doe is about to have her second litter, split the family up leaving the female babies with the mother and the male babies with the father. Because the older buck recognises the babies as members of his own family, he will not attack them and they should be able to live together for the rest of their lives.

Question and Answer - from Mouse@Horns

Question: I understand that you can house two male mice together. I was told it wasn't a good idea because they are territorial. We like our male being by himself as he seems very happy, sweet, and is held a lot. Would you recommend that he have a friend with him? I do realise that this would give him a companion but I don't want him turning aggressive.

Answer: There is NO danger of your mouse turning aggressive towards you if he has a companion. The only danger is that he might fight with the other male, in which case you might - note might, not will - have to separate them.

It is certainly not fair to keep the male alone if you can work out another way of doing things. The poor little thing is living in a highly unnatural state at the moment. For example, one day in a mouse's life is like two weeks in ours. So even if you visit him four or five times a day, he's still basically in solitary confinement for the equivalent of days at a time. Just being left overnight is like spending a week alone.

Black eyed cream (Marco) is sniffed by 2 other bucks from different litters. All are aged 5 weeks and lived happily together until going to new homes


Question: If you think it would be best to give him a friend - another male, of course - could it be one of the male babies from our litter once they reach the age of five weeks - or will he be aggressive towards the baby as it is so small?

Answer: This is usually the most successful way of introducing a friend to a male mouse, as the older male doesn't see the baby as a threat. What I would do is put all the baby boys in with dad (or rather, introduce them somewhere neutral - I would take mum and girls out of their cage, put dad in with the boys so he is on THEIR territory, and put mum and girls in dad's old cage). Do this when they are four weeks old - a bit younger than normal so they're more acceptable to him and less of a threat. Normally this will work out fine. Then when the other boys go to new homes, you can leave one or two with dad.

Occasionally dad will be very aggressive and won't take to the babies - if that's going to happen you'll know within a few hours (serious chasing round, not just the odd squeak) and will just have to separate them. But usually it works out very well.

The other option is to have dad neutered, then after a couple of weeks (to be safe) reunite him with mother and children. This is the safest option all round - castration is a minor operation for rodents, if you can find a vet who uses Isoflurane anaesthetic. It's certainly worth phoning round until you find a vet who will do this.


Question: I have two male fancy mice and I found them continuously fighting. I separated them a while ago and today I tried to put them back together but they kept fighting. One mouse had wounds on his tail and near his neck. Do you have any idea how I could get them to live together successfully?

Answer: I'm afraid that it's very difficult to reintroduce male mice once you have separated them for more than a day or so. The best thing to do is not to separate them, but to rearrange the cage and take other steps to try to stop the fighting.

When male mice fight it's nearly always because they don't have enough space. They need a cage or tank AT LEAST 2' x 12" for a pair of males to live happily, generally; but it doesn't have to be an expensive cage. Lots of people (including myself) have had great success making their own tanks.

I'm afraid that I am not optimistic about your chances for re-integrating the mice, but do read Madeline Lewis's comments on introducing male mice (below).

Group of 5 week old bucks

Introducing Adult Male Mice, by Madeline Lewis

I have done this several times, with varying degrees of success and my 'procedure' is sort of made up as I go along. In all cases, the mice have been fully adult - six months or older.

I take the 'bully' and keep him by himself for a few days (sort of a cooling off period). I give him a fresh nest box (usually a finch nest) to mark with his scent. Then when I clean the cages I put all my males together in a single holding cage (this is my usual practice anyway) to hang out together while I'm cleaning. I leave them all together for around an hour to see how things go. Once I've cleaned and set up the cages (more on this in a minute), I separate the boys into their various living groups.

I have one very large aquarium - 100 gallons (about four or five feet/1.5m long) - and this is the one into which I introduce my troublemaker. As part of my cage set up, I use the troublemaker's own nest box and also his wheel - still stinky from his urine. This way, he has some of his own belongings and apparently doesn't feel the need to assert himself as strongly. He will usually run vigorously on his own wheel and fuss around in his own nest box for a bit. Then, I watch very carefully to make sure that troublemaker isn't too aggressive and that the resident group doesn’t make his life too miserable.

Some things that I'm certain really can make the difference between success and failure:

  • The cage must be very large - male mice are insanely territorial and so there must be ample (if not surplus) space for everyone in the cage. This is perhaps the most important point: it is best to introduce the new male into an existing group of four or more. Since even close-knit male groups are constantly falling out with one another, it's likely that a male on the outs with his group will befriend the newcomer.
  • You need to supply lots of resources - two or three wheels, several nest boxes, and lots of tubes and toys - so that the existing group and the newcomers don't feel that they have to compete strenuously for scarce resources.
  • I don't give them interesting treats as food for a few days - just plain rice. This seems to reduce fighting over tasty bits of food.

All this being said, this plan doesn't always work and then you'll have to remove the old troublemaker (now 'victim') from the situation. Still, I've never had male mice fight to the death and only one of my males has sustained a severe injury as a result of fighting. When I first started keeping mice, I did have two males (brothers) die of what I now recognise to be stress from bullying by an older, domineering buck. Now when I see things are not going well and bullying is going on, I immediately remove the offender in hopes of cooling his jets.

Even though it takes a lot of energy and alertness, I do think making this effort is better than condemning a male to live by himself. I've had injured males who clearly have enjoyed spending a few days by themselves while they rested and healed, but any healthy mouse is bored and depressed on his/her own. They are such intensely social creatures that it seems to me a tragedy to force the males to live out their lives in solitary confinement. Madeline Lewis

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