This is a bit of a long one, but I promise there is a really strong reason for me wanting to post this in so much detail, and a big part of this is as a warning to anyone who ever wants to keep male mice together. As many of you know, I've been keeping mice together for a good few years now (8 and a half years) and over that time have kept numerous groups of males - of varying sizes - together with lots of success. The groups that couldn't be kept together were separated early one with little more than a few bite marks at the worse as a sign that things weren't working out - just about everyone I speak to mice about has always went by the adage "No blood, no foul" when it comes to mice.
When Laura the vet started neutering I would get the males who fought neutered, then started getting all my males neutered early on so that they could continue to enjoy one another's company. Recently, Laura the vet left and although the other vets said they could still neuter, I opted against it for my future mice as they warned me that it would be a "learning curve" for them to operate on any mice (apparently none of them ever helped out when my mice were being operated on and had left Laura to it - they said she had lost "dozens" of her own mice when she started out operating on them and said it would be likely they would face similar problems until they worked out how best to deal with them under anaesthetic.) I know they would have been careful regardless of the animal but I didn't want to take the chance.
A few months ago I got a little group of 4 male mice, all youngsters under 8 weeks old. One (Figaro) turned out to be a major bully towards one boy in particular (Ares) and I separated him when the squabbles started escalating, exactly as I always had done in the past. The three remaining boys (Ares, Whiskers and Dobby) I left together and only saw the odd bit of squabbling over bits of food, nothing to be worried about. Then I came home from work and was heading to bed when I found that Ares has been attacked by Whiskers (Dobby was curled up with Ares when I found him and Ares was running off whenever Whiskers moved, no way it wasn't him)- it's one of the worst things I've ever seen. Ares had had his scrotum bitten into in such a way that one of his testicles has been pulled out from under the skin. I separated them immediately and rushed him to the vets, Ares was taken in for an emergency castration and died under anaesthetic. The vets say the damage was significant and it looked like there was chewing to the base of the testicle, his penis had significant wounds and there was no way of knowing how much other damage there was internally, then of course there was them not having any experience in operating on mice to boot. So my beautiful little Ares, who wasn't even a full 3 months old, was gone.
So yeah, I'm sorry that was so long. It happened a few weeks ago and to be honest I was so devastated that I couldn't bear to even come on the forum to warn anyone else wanting to keep males together. But I really know male mice, really know them - I think around 85% of all the mice I have had in my time of keeping them (which numbers into at least a couple of hundred) have been male. I've never, ever seen an odd squabble (and I do mean odd, I only heard them two or three times in the course of the week the three of them were left together) over food escalate to a major wound. And I've never seen a wound like this. I hope I never do again. I love my mice so much, and there is something about male groups that is very special, they become so much closer to their cage mates than the girls I've had ever do, but it's not worth that risk. Apparently knowing how to read male mice wasn't enough, and my poor Ares suffered a painful death because of it. The vets kept saying it was sheer fluke that a bite could result in an injury like that, but it's not worth taking the chance that anything like that could ever happen again. So the group of 9 neutered males I have will be the last all male group I will ever have, because it's either keep whole males together knowing that this (or worse) could happen again, or get them neutered when the vets have said it's highly likely that I will end up losing most of the ones I put in to be operated on at least until they figure out the best way to neuter.