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About five months ago I purchased a female mouse for my daughter. We both felt sorry for the lone mouse and went back to the pet store the following weekend to purchase another female. Not being well versed in mouse genitalia, we allowed the store assistant to pick out a female for us.

Six weeks later we had eight babies. We separated the male "Sniffy" but not soon enough. A few weeks after that the female had another litter of 13.

We managed to locate a breeder and gave away 18 mice leaving us with two girl babies that we caged with their mom and 1 boy baby, Rupert, that we put with his dad in another cage. I had read that male's should not be housed together, however, I had observed the males closely, and knew that Rupert would get along with Sniffy. They slept together constantly, groomed one another, played on the wheel together. Sniffy would fight with his other sons but always left Rupert alone.

The plan worked. They got along extremely well, we even joked that the boys were in love with one another. They did everything together and not once did I witness a fight.

Then I went away for five days. Cleaned the cages beforehand and left the mice in the care of a friend. When I came back they seemed fine but I didn't get around to cleaning out their cages until a few days after my return. When I cleaned out the boys I noticed that Sniffy's eye was half shut. I figured it was infected because he had been living in his own filth for too long. Guiltily, I swabbed the eye clean with saline solution and put him back in his clean cage. I swabbed his eye again the next morning.

That evening, I went to clean his eye again, and to my horror, Sniffy was caked in blood on his rear quarters, both eyes were glued shut and his ears looked chewed and bloody.

I immediately removed him, cleaned his wounds gave him food & water and placed him on his own in a quiet, dark environment so he could heal. We checked on him around the clock, cleaning out his cage twice a day, tending to his wounds, feeding him water with a dropper, trying to warm his cold little body.

All our efforts were in vain. Sniffy died in my hand 36 hours later.

What happened? Could Rupert really have killed him? I examined Rupert and there wasn't a single scratch on him. Could it have been a wheel accident? The guilt is killing me. Rupert is acting stressed. He keeps pacing his cage as if he's looking for Sniffy.

Sorry this is so long, but it helps to write, I got so attached to the little guy. Any advice would be helpful.

Yes, it does sound as if the one mouse killed the other. Don't feel guilty about this. Contrary to popular belief small colonies of male mice CAN be kept together, especially males that are related, such as father and sons, and brothers. I myself have two tanks of males, one with three and the other with five, and all get on well.

HOWEVER, male mice ARE territorial, and sometimes you can just get one that doesn't like company, period. The rule with housing male mice together is space. They must have plenty of space and if possible several houses, wheels, and food bowls, so that each can have its own territory. Keep an eye out for signs of stress or attack, and seperate them if you see those signs.

Don't feel badly about this. You did what you could with the info you had, and will learn from it. It sounds like you just had one of those boys that, once he hit full on puberty, decided he'd much rather be alone.