ok, i have 9 female mice and 12 males (the females and males are sepreate) i had 13 males but 1 died from his wounds. I started off with a male an female an she gave birth,while she was nursing them she got pregnet again, we didnt have a cage at the time to seprate the males an females which is y we have 21 mice. Anyway they all got along and would work together to take care of the babies, even the tiny babies with hardly any fur would help take care of the pinkys. They were like a big happy family. When i seperated them 1 had already gotten pregnet, while cleaning the cage i realized i had 4got 2 females in there (they were young so luckily they werent pregnet) i put them in the cage and 1 went crazy and attacked all the babies, blood was everywhere. They all lived but 1 and all the other females tryed to fight this 1 mouse off (i named this mouse physco) so i decided to put physco back with the males. Everytime i clean the cage there is no female,could it be phsyco was a male? and at the time he had contracted his balls so i thought he was a girl?
anyway the problem is the males all have bloody/cut up balls/tessticles. they are always fighting. I mean lots of them get along,1 was even cleaning the wonds the others had. Some of them sleep together,sometimes they all sleep together. What i don't get is y r they fighting? are they all fighting? or is one mouse bullying all of them? and can that 1 mouse be "physco"? when i seperated the 3 injured 1s so they wod have time 2 heal,those 3 got along fine,so is there 1 mouse behind all the fighting? what should i do? (the females get along fine,no fights)

You need to separate ALL the ones that do NOT get along, or else someone else will get killed! Males can sometime fight to the death. You NEED to take them to the vet asap to make sure that their wounds get cleaned up and so it doesn't turn into an infection or they could die!! Please take them to one, and separate them right now. Some vets will also take payments, and some even have carecredit cards that you can use (Go to carecredit.com). I really hope they'll be okay! So sorry about the one that died.. :(

Male mice cannot be kept together at all. This is why they are usually sold separately and females in groups. Doesn't matter if the males were born and raised together, they will still fight and brutally injure one another over a territory dispute. I would seperate ALL the males because it's only going to get worse and you'll have a bunch of dead little dudes on your hands with one fierce alpha male left.

Owner of a male mouse