Found mouse dead and believe littermate is injured

Question
Good morning.  We purchased 3 female mice from our local pet store eight days ago.  They were all from the same tank, but two were black and one was grey.  The grey one seemed shy and timid with humans, while the two black ones were easily being handled within a few days.  The grey one also squinted a lot, and the black ones didn't at all.  They all seemed to be getting along fine, but the gray one didn't ever play with the other two.  They all slept in the hut together though.  My daughter keeps them in her room, checked on them last night about 11:00 before bed and they were fine.  In fact, the grey mouse seemed to be getting less shy the last two days; finally taking food from her hand and letting her pet and briefly hold her.  This morning when my daughter woke up she checked on them (8:30)  She found one of the black ones belly up with one side of her face chewed to the bone, and also the ears chewed off.  The gray one was sleeping outside the hut and is acting very strange, not moving normally, hardly opening her eyes.  We separated her from the living black mouse, and she kinda hobbled around for a minute and then settled into a corner of the cage.  She reminded me of the way an old hamster moves toward the end of it's life.  Her back is hunched.  Something doesn't seem right.  The living black mouse is acting totally normal, and was sleeping in the hut, but came out when we opened the cage.  I'm thinking the grey and black got into a fight and the black one was killed, but the grey one may have internal injuries from the fight.  My children are devastated, it really is awful.  I am not sure what to do now.  If the grey mouse gets better is it safe to put her in with the other mouse?  Is it possible that the black mouse who appears to be fine was the aggressor?  And if the grey mouse dies, will the one still living be okay alone?  And if not, how do I go about introducing new mice?  I am sorry for so many questions, but I never expected female mice to fight like this, especially with no warning.  They are so cute and it's amazing how attached we got to them in a week.  I have 3 children walking around with red, puffy eyes, and I'm trying not to cry myself.  So sad.  Any advice will be appreciated!  Thank you in advance!

Answer
Dear Teresa,

I am so sorry for your loss. Mice are just as loving and wonderful as any larger pet, though many other people don't understand that. Their loss is just as sad.

We definitely don't know how the black mouse died. It could have been unrelated to a fight. It seems that the grey mouse may be sick from something she brought from the pet store, and the black one may have contracted it but showed no symptoms. Mice are prey animals, and thus can't afford to show symptoms of illness until they are very ill- or they would be targeted by predators. So we also don't know if the remaining black mouse is ill.

It is natural for mice to eat a corpse, for various reasons, including that it might attract predators to the nest. So even with a completely natural death, you may find a horrible sight which makes you believe something terrible has happened.

Put them back together and watch. Right now they are both lonely, which not only depresses the grey mouse's immune system, but also the black mouse's. If the black mouse has contracted the illness, which is quite likely, she also can't afford a compromised immune system. But you do want to watch, because although it is rare, occasionally a mouse will be mean to a sick one. Also, give them two nests. It is sad if the black mouse shuns the grey one, though understandable; and I do not want the grey one to have to sleep outside of the nest. Make sure the cage stays very warm and there are no drafts.

If you do need to keep them separate, keep the cages side by side. They actually communicate in a frequency we can't hear. This way they will be less lonely.

If the grey mouse dies, which seems likely, you willl have to quarantine the poor black mouse for two weeks. She will be sad and lonely, so you really have to give her a lot of attention. You and her children will be her mouse friends. A mouse will often have a change of personality within a few days after losing her last girlfriend, though this girl is already sweet. But this doesn't mean she doesn't need friends.

A dying mouse may ask to be picked up to die in your hands. She also may kick or even spasm violently. Do not worry about this.

You had exactly the right idea to get three mice. With three mice, when one dies, there is not left a mouse who is not only devastated at the loss, but also lonely. Both conditions can lead to illness, depression, and mites. But I suggest you find another pet store. You should let this one know that they sold you a sick mouse. Some places care; others don't. When you choose a mouse, its  fur should be immaculate; no wounds; its eyes should be bright; there should be nothing around its eyes or nose; it should not be lethargic; and the same must be true for every mouse in the cage.

Now you are going to introduce the mice. You have one sad and lonely mouse who wants friends, so she is unlikely to be a problem. That means if there is aggression, it is most likely to be from the new ones. What I suggest is to introduce them as usual- which means clean cage, clean accessories, and REAL vanilla on their necks and rumps. And watch them carefully. When mice are establishing dominance, there can be a whole lot of chasing and squeaking. Usually no one gets hurt. The criteria for separating the mice are these:

1. Blood
2. Depressions, lethargy
3. Someone is kept from wheel, nest, water bottle, or food.
4. The chasing and squeaking is nonstop.

If one of these happens, if the original mouse is the victim,  take them all out and put them in the OTHER cage, which you have left dirty from Mouse 1. That should be the smaller cage. So what I am saying is: let mouse 1 make the smaller cage dirty. Clean the bigger cage and put them all in it, but do not clean the small cage. If there is a problem and the black mouse is being targeted, put them in the dirty cage, with no vanilla. This gives mouse 1 the home court advantage.

There are usually no problems. Let me know if there are, and I can help further.

Best of luck. Best case scenario, the grey one gets better, and you don't have to get more than one new mouse, and you can take your time (if you are heeding my advice to always have 3 mice).

Squeaks,

Natasha