Mice suddenly not getting along

Question
QUESTION: "Hi, Natasha:
I hope it's okay to ask you a question that I also asked another expert. I'm desperate for an answer here. I haven't gotten an answer from him as yet. I asked him first because he has some experience with exotics, although, I have found your advice to be very reasonable and sound in the past.
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? I did somewhat recently rearrange the cage a little bit from what it's always been to make more room and add some variety, but don't know if that would create a problem.
Is 5 males in one cage just too much? It didn't seem to be before. One other tank has 5 in it and they get along well. They are females, though. Males can have more problems I've heard. 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,

It sounds like male deer mice get along better than fancy or pet mice. Male pet mice usually cannot get along and have to live alone or they will fight to the death.

A group of mice has a certain equilibrium. Changing the group can alter the behavior of mice towards each other-- mice who could not get along when they were both in one situation can in another, and vice versa.  It's interesting that the returning male was accepted fine; but not so strange that the order was changed.

Females go into heat all year long, so that shouldn't have changed. Rearranging the cage is a good thing. Mice can get bored.

You are right that he will be sad to live alone. Instead of putting him alone, just divide the mice into two groups, keeping the attacker and the victim apart. You may have to do some juggling to figure out the best arrangement.


"If I have sexed them correctly?"  You had better sex them correctly or you could have hundreds of mice in just a few months! Use this as a quide:

http://www.thefunmouse.com/info/sexing.cfm

Best of luck!

squeaks,

Natasha
<:3  )--~


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


Mice Eating Dinner   QUESTION: Thanks for the quick reply. I will likely have to separate them into two groups. I don't want to, but I may have no choice. I was thinking perhaps I might try reintroducing the offending mouse to the entire group after a few days in solitaire and watch carefully to see if his attitude has improved any. Not sure if that's a good idea or not. I would prefer to keep them together if at all possible.

I know you like to seeing photos of the mice you give advice on, so I've included a photo of my male mice eating together.

I also posted a mouse video you might enjoy. It's called MOUSE PARTY.
It's all of mice I rescued. They were recently released. I wish I could have kept them, but I am overwhelmed with my mice as it is.

Here is the link to the video: http://www.youtube.com/watch?v=BGY30_r_y1c

Take care,

D

Answer
Hi again!

Thanks so much for the update. When introducing mice always either do so in neutral territory or, in the case of an aggressive mouse, in the nice smelly territory of the less aggressive mouse.

And first dab their rumps and chins with REAL vanilla essence. Not the fake stuff. This way they all smell alike. You can even continue to aply the vanilla several times over the first two days.

Good luck!

<:3  )--~  Natasha

PS ADORABLE video! I think I'll post it for my mouse/rat friends on Facebook!

>update<

PS I have decided it would be cool to be Facebook friends with some of my questioners. If you would like to friend me, I am the only Natasha Millikan on Facebook. And you will get to see my mice... and more so, my rats, including mouse-sized rats! Be sure to attach a message or I might not accept. But if you have questions do continue to use this site, so that other people can benefit from the answers.

Preview:

http://www.facebook.com/video/video.php?v=10150091824234803