Mice Gender & Fighting

Question
My parents recently bought two pet mice. We thought they were female, however now we're unsure. We've only had them for about three days, and they recently started fighting--seriously fighting. They fought before, but before they'd squeak for a few seconds before they stopped and went about their own business. However, now, they start fighting and I listen to them squeak for a few seconds before I have to go over and intervene to make them stop fighting. I thought maybe they were fighting over their food, because I'd recently refilled their food bowl, only with less than the first time they came home because they only ate a small fraction of it. So I figured maybe I should put less in. Once they started fighting, I put more food in just in case maybe that was what they were fighting about. I'm not sure if this worked, because they haven't fought since but that was only a couple of hours ago. Aside from that, there are other times when they're completely fine with each other. They'll sleep next to each other or start grooming each other and everything is fine. And then other times they fight until I intervene.

My questions are: If they're male, will they go far enough to kill each other for dominance? And if they're female, do they fight for dominance the same way that males do?

Answer
Hi Marissa,

Every now and then males can get along, but when they start fighting, they don't usually stop until one or the other is dead.  This kind of fighting is very serious and usually results in blood very quickly.  Even if they are separated, wounds should be tended to as soon as possible to prevent infection.  Males will usually not return from fighting to "friendly" behaviors, instead taking up their own territory.

Females, on the other hand, usually prefer living with other females, but some do like to live alone better.  Every now and again fighting can turn serious, but this is usually about incompatibility rather than a dominance struggle.  Pecking orders are usually resolved in the first few days by normal scuffling and sometimes squeaking - dangerous fighting includes biting (not nipping, but actually hanging on - you can sometimes see scabs from bites by blowing the fur backwards), any injuries that result in open wounds, boxing (the mouse being chased stands on its back legs and "punches" out at the offending mouse), and tail rattling (vibrating the tail against something hard in the cage - this is not dangerous but is a strong warning sign of "back off" that means things could turn south suddenly).  At the first sign of real fighting that could lead to injury, the mice should be separated.  Try to figure out who is doing the chasing - typically one mouse is picking more on the other and instigating the trouble.  Who is being chased?  Are they injured anywhere?  Check the tail and under the fur for bite marks or scratches.

Sometimes pecking order troubles can be resolved using a little vanilla extract (real vanilla, not the substitute!) - dab a tiny, tiny amount on the rear end and chins of both mice, so they smell the same, and change out the cage bedding.  This turns all scents to neutral and gives them a chance to settle out.  There will always be a dominant mouse in the cage, but given a few days and some careful observation, it should be more or less peaceful.

Here is a great guide on determining their genders:  http://www.thefunmouse.com/info/sexing.cfm

Another thing you can try is spreading the food throughout the bedding (they like digging for it anyways, and may already have started hiding food around the cage), or using multiple food bowls.  You can give them as much as you want - obesity is genetic in mice and they will never overeat.  One thing to be sure of is to give a treat to each of them when you give them goodies - even the best of friends will argue over a yummy yogurt chip!

Let me know if there is anything else I can help with!
-Tam