We find it easiest to keep litter mates together (especially males) from birth. That way by the time they are adults they have been together for a long time and as a result this generally seems to reduce the incidence of males fighting. I'm not sure if that is at all helpful for your situation but I hope it's a little bit helpful :).

In terms of females I use mice with a C57BL/6 background and they seem perfectly content to be moved around/between cages with new and old friends.

I have pretty much the same answer as Jacinta. In my experience male mice only tolerate each other if they are littermates, and even then if they grow old they might start picking on one particular cage-mate from time to time. So if you want to have many males together they would have to be born together in the same cage, but not necessarily from the same mother - so if you breed your own mice I would try to time the pregnancy of 2 females in a breeding trio and keep all the males from those two litters together.

For female mice I have only very rarely seen any aggressive behaviour when they are re-caged together, so I would just combine females of the same age (I have seen older females bite very young just weaned females).

I agree with Jacinta and Freja.   Also, even if you use male littermates, I suggest avoiding stirring up the bedding (except at cage change time) and adding some nesting material sufficient to provide a 3-D nest.  Dominant males will urinate in a pattern that establishes their dominance in the cage.   Nestlet squares don't work as well as newer materials (such as a commercially-manufactured crinkle paper) to enable mice to create a larger nest.  

Investigators want to house males together to save money, but combining mature, unrelated B6  males in a cage ultimately risks combat.

Hi Katherine

I've been an animal technician for many years and I would not recommend combining cages of adult males of any strains unless you absolutely have to. C57BL/6 I wouldn't do at all as they are prone to fight, sometimes even when they have knows each other since birth. If you try you are quite likely to see some very distressing injuries and you will probably have to euthanize animals. As people have already said, females are much less likely to fight, but they are also territorial, and they might also attack smaller and younger females.

Good luck!

I completly agree with Jacinta.

Indeed keep littermates in the same cage from the weaning is the best way to avoid fighting and may help also with your behavioural test.

 In case you already have adults the best is to move all the adults males in a new cage and never add males in a cage where theare are already other adults males housed.

Moreover is better not to house in the same cage males that have been mated recently.

Using this two advice you should reduce probabilities of males fighting.

Indeed females have no problems at all.

I don't recommend putting male mice especially if they have B6 background. Some people suggest using B6 mice from Jax rather than CRL, they are less aggressive. Suggestion of use of littermate isn't a bad idea but isn’t sure to 100%. In some case, you’ll be require to stop experiment because of fight.

Thanks for the responses. 

We always keep litter mates together. If we need to combine litters, we normally do so at weaning. Part of the issue is that the strains we're using are not good breeders and in order to use the Intellicage efficiently we'd like to test 8-12 females or 6-8 males at a time. We've been told by the Intellicage manufacturer that others have combined adult cages, but they didn't know which researchers. I will continue with my literature search and try to contact the groups that did combine adult cages. 

So you have to strike a balance with all the components and challenges you might face with your experiment. Probably you could just order the males of the same litter from vendors instead of relying on your own breeder stock. Data obtained from males is equally important and might have some significant differences with one from females. There is pretty much little or nothing you can do to prevent males from fighting as everyone has suggested, you could probably put some dividers within the cage but that would reduce your floor space if you had 6- 8 animals at a time.

Agree with Brian. Also remember that changing the social structure in a way that is likely to cause aggression will be incredibly stressful to them. Severe stress causes physical changes that could change your experimental results unless you're very careful. If you do do this you should probably report it when you publish your results.

I agree with everyone above that it is generally fine to rehouse females but not males. However, I manage multiple colonies of transgenic mice that are on several different C57B6 backgrounds.  We were running into huge per diem costs housing 1-2 animals in a cage after culling. As a result, I had to try to rehouse some animals to save costs. What I found is this: Females of any age can be placed together with rarely any problems.  When it comes to males, you have to screen much more closely for age. We found that males can be re-caged with other males very easily at weaning.  We also found that re-caging had significant success with males up to six weeks of age.  After that it becomes more difficult. At 7-8 weeks, you may still be able to house some together but it will be almost impossible once they are able to reproduce. Male mice over 8 weeks of age cannot be easily re-caged together without significant fighting and bullying and should be avoided. If you decide to re-cage males due to your caging paradigm and testing needs, you should always place all of the males into a fresh cage with fresh bedding and try to keep their ages/sizes as close together as possible. . They should be closely observed over the next few days for any evidence of fighting/bullying. Generally if they make it through the first week with no significant fighting, they should be ok together. If you see fighting in the cage you can try to identify the problem male and remove him or simply go back to the previous housing situation.  You also must give ample time for acclimation to the new caging situation/social order to become established and stress levels to equilibrate before beginning behavioral or molecular studies.  This is by no means a foolproof approach but in a pinch it may work for your needs. Good Luck!

You can try to give the animals some toys, like wooden stick or nest material. The toys may reduce fighting a little bit. You can also provide a shelter for weak animals to hide, which is made by a red plastic and is a commercial available shelter. Mice can’t see through red color, but we can observe the animals inside the shelter.

 Translational Neuroscience (2014) 5/3, 185-196.

This paper might be helpful for your experiments.

Best,

Tateki kikuchi

The Mouse Cubby https://mousecubby.com/ has been shown to reduce aggression 50 to 90% in mice. It has been used to recombine male sentinel animals that had been separated for fighting and eliminated the fighting in those cages.

Dear Mr Tallent:

I agree with your comments on the mouse cubby and suggest Ketherine to try it . My best, Tateki

Hey there,

I was facing the same dilemma a while back and I'd just like to add that I agree with Renee Donahue here, with age being crucial on whether the animals would tolerate mixing.

I have mixed several cages of male b6 mice either at weaning (~3wks) or up to 7 weeks old and this was completely fine. It's important to monitor the animal closely the first couple of weeks.

Again, as Renee pointed out, combining males older that 8 weeks didn't work, they fought.

Good luck!