Male Mouse Suddenly Attacking Sibling

Question
QUESTION: Hi, Jedediah:
Usually, I ask Natasha questions because I find her advice reasonable and helpful, but I thought I would ask you first because you have some experience with exotics.
I have several cages of deer mice. If I sexed them correctly, they are two 20L tanks of males and two 20L tanks of females respectively.
My problem is with one of the male tanks. There are 5 males in this 20L tank with a shelter, hollow log, and three wheels. One of the males is the father, and the rest of them are his offspring.
They have all lived together in peace their entire lives for at least two years now I would say with the occasional brawl over favorite foods like nuts.
Recently, one of them developed an eye infection and needed me to apply medicine to his eye twice a day. This was no easy task. He isn't used to being held and didn't like me putting medicine in his eye once I was able to grab him after much effort. I separated him from the others into another cage to make it much easier to grab him, even though, I was worried about reintroducing him back to the others when he got well. I just couldn't treat him otherwise.
Anyway, after about 5 days of giving him treatment in a separate cage, I noticed that one of the other mice in the male cage was suddenly attacking another of the males.
I hoped it was just a squabble that would soon blow over. I decided to stop the mouse's treatment and return him back with the other males. He was accepted back by all as if he never left.
However, that one male still continued to attack the other male and chase him around the cage on a regular basis. I could see it was really stressing out the one who was being attacked.
I watched them over the next several days, hoping things would calm down with eveyrone together again, but it didn't. Last night, I saw another attack. I also noticed that the one being attacked had blood on his tail. I knew I had to separate the aggressive mouse from the pack. I now have him in a separate cage. Hopefully, I did get the right one. He looks so similar to another mouse.
Do you think he could ever be put back with the pack again? I'm guessing not. Is this behavior because I took one of the mice out of the cage for several days? Did I upset the order of things?
Is it spring fever and the smell of the females next door is what's driving the male mouse crazy? Did I not sex them right?
Is 5 males in one cage just too much now? I hate to add another cage to clean, but maybe I need to keep the aggressive mouse in another cage with one of the males he seems to still get along with. I assume the aggressive mouse would be miserable living alone. What do you think?

ANSWER: Dear Derek,
thank you for your question.
I have very little experience with deer mice and have never kept them for a longer time, but I do suspect that housing the female group close to the males may be part of the problem. Removing the first male probably made them fight over dominance again and removing the aggressor may not be a solution since the others will need to establish a new pecking order again. But I can't really offer a solution right now. I will ask a friend who has more experience with deer mice than I have, she has kept them for years, and I will let you know if she has any suggestions.
I hope I was of some help to you and I'll get back to you as soon as possible.
Jennifer

---------- FOLLOW-UP ----------

QUESTION: Thanks for your reply! I a considering reintroducing the offending male after a cooling off period and hoping that his isolation will cool things off. If that doesn't work, then, I will have to divide the males and hope they can get along in two separate cages. Still, I would really appreciate hearing what your friend thinks about the situation. The sooner the better, I would think. I'm flying blind with this problem, and don't want to make things even worse.
Any advice you can pass along would be greatly appreciated!

Derek

ANSWER: I haven't reached my friend yet, but I think that a slow re-introduction of the whole group is probably a good idea. It's what I would do with pretty much all mouse species in that situation. Try offering them only a part of the cage and little or no furniture at first, so that they have to get used to the cage all over again, no-one will defend any territory that way. I hope that thing work out well! I'll let you know when I hear from my friend.
Jennifer

---------- FOLLOW-UP ----------

QUESTION: Thanks for being on the case. I would be still curious to hear what your friend thinks, although, I believe the problem has worked itself out. I decided to try and reintroduce the offending mouse after keeping him in a separate cage for three days. I did it early in the afternoon before the other mice got active. I didn't clean the cage since the offending mouse had been in it. That way, the others would hopefully still have some of his scent, but, he would have lost some dominance in the cage. I also put them in a separate room apart from my other cages. He was nervous when I first put him in with the others. He ran around sniffing things. He went into the nest were the others were resting for a brief moment, then, he came out again. One of the other mice came out soon after to check him out, then, went back in the nest. He marked a corner of the cage. After moving around the cage some more, he finally went inside the nest and stayed there. It has been two days since I reintroduced him back into the group. So far, they all seem to be getting along. Hopefully, the peace is a lasting peace!

Answer
Hi,
how are things with the mice? I hope they are still peaseful and get along!
I've reached my friend and she also believes that the removal of the sick mouse upset their dominace. It would be her recommendation to keep the male group in a different room that the females to keep them from fighting. I'm glad to hear that the reintrodution went so well, let me know if there are any further problems.
best wishes
Jennifer