Sometimes, mice go a little neurotic - and it's most likely to happen when they are bored. They will run laps, do backflips, and self-mutilate.
Kiki, one of my mice, had a companion, Bea, but she was constantly getting sick (she went blind in one eye, she had an absess, she had sniffles). Despite Bea's doting attention, she overgroomed herself and scratched a wound into the back of her neck. We (Bea and I) nursed her back to health, and the next issue was she was cleaning her face so much, she rubbed the whiskers and the fur off her nose.
After six months of the two of them living together, Bea remaining fat and happy, and Kiki spending most of her time grooming and sleeping, I decided that it was worth the risk of doing an integration with my other mouse girls.
Now, seven of them live together, and have for about 10 days. Kiki is now incredibly active - spry is the word - climbing bars, running in the wheel, and seeking food and my attention. When I reach in to pet her, because she's blind in one eye, the first thing she does is bite me before she lets me pick her up. She is almost always the first to the food bowl and the last one to leave. She is gaining weight, and I am hoping that soon her whiskers will grow back.
It turns out that though fat Bea is a good cage mate, Kiki needed to have society. And soon she will have two more, and be part of a big group of 9.