The Alpha Mouse: How to tell if your mice are fighting or playing
Should you separate your mice?
Mice fight for the same reasons humans do. Power. Girls. Territory. Sometimes they're bored and need entertainment. Females will fight for their children's safety. And sometimes, they just need to take a swing at the jerk who stepped on their tail. Whatever their reasons, if you keep more than one mouse in a cage, understand that they may fight. If they're males, expect it. But also like humans, mice will display different levels of aggression which can range from playful wrestling to fighting to the death. Since mice often live healthier lives in social settings, separating them can lead to loneliness and depression, but if you can't identify when fighting becomes serious, keeping them together might threaten their lives.
Female mice tend to be less aggressive. I've only seen them fight to prevent another female from getting close to her babies. This situation resolves itself after time. Both mice played well together in settings other than the mother's cage. Chances are if this happens, once the babies are old enough to fend for themselves, mom won't feel the need to fight for them.
If you house two or more males together, the situation isn't as simple. Boy mice have an instinct to establish a pecking order. Not being as democratic as humans, they will probably fight each other until one mouse--the alpha--subjugates all the others. This is normal, and you have to let it go. Sometimes my mice would squeal in terror in the middle of the night. I separated them, but it didn't do any good. I tried to train the wanna-be alpha that any time he made the others squeak I would put him alone in the travel cage for five to ten minutes. It didn't work. For mice, as well as humans, order needs to be established, and you should just sit on the sidelines and observe until it's over.
Is my mouse gay?
Please note that these animals practice male-on-male mounting as a way of dominating each other. If you see this, your mouse isn't gay. While the subordinate mouse may not like it, it doesn't harm him and breaking up the event probably will make the dominance process last longer.
Once the mice have sorted out which one is king, they probably will continue to fight now and then. Sometimes they're only playing, other times it serves as a challenge or reminder as to who's in charge. But again, this won't hurt them.
So when are they fighting for real?
When an aggressive mouse fights beyond what is necessary, we call it a "bully mouse." Watch the subordinate mice in the cage. They may call uncle by crouching submissively, holding their front paws together in front of them, or even lying on their backs, spreading their legs to the aggressor. If this doesn't stop the fighting, you may have a bully.
If you catch it early, you can break up a serious fight before it starts. Mice will face-off by rearing up on their hind legs. They'll slap their tails against the floor to threaten each other. If you see this, grab them before they bring their teeth into the situation. You won't find an easier time to stop the fight.
Never take the submissive mouse out of the cage. When you see bullying, you don't want the bully to think he won, so you have to pull him out for a while--ten or fifteen minutes will usually cool him off. I recommend having at least one cage--even a small one--for each mouse that you own. This way you never have to scramble for a makeshift holding pen if your mice start squabbling in the middle of the night.
You should be concerned if one of your mice starts bleeding. If someone bites them hard enough that it opens a wound, immediately take the bully out of the cage--use gloves. If they can wound other mice, they can wound you. Keep a pair of leather work gloves by the cage if you have two or more male mice.
Sometimes it can be hard to determine the aggressor. Don't assume. I took my alpha out one night because he constantly picked on one of the smaller mice in his cage. When I held him for a few minutes I noticed his back and bits of his tail were oozing blood. The younger, submissive mouse just got tired of being picked on and he fought back. (That's them in the picture below.) When this happens, don't ever put them in the same cage again. It may be possible that the fight was a single event, but three days later, the submissive tried to finish the job and gave the alpha a severe beating. If that can happen, don't risk the health of your pets. Always keep a cage on hand, and be prepared to separate them permanently at any moment. You may have read about techniques for introducing strange male mice to each other--don't try it. These mice aren't strangers to each other. Look at the picture--I wanted to put the mice close together to stave off loneliness in the former submissive. It just made him mad. Any time the former alpha came close, he would swipe at him and bite through the bars, hoping to resolve their fight.
It was an unfortunate situation, but they've lived quite happily in separate cages, so sometimes you should just let an antisocial mouse be antisocial.