Mysterious patches

[hannibal]

Hello everyone
I noticed that one of my gerbils is getting at bald patch. Her fur has always been a little messy, so I can't say how long it's been there for sure. I think she has had a dark patch there for a while, but in these last few days it's gotten bigger. And now I can clearly see her skin underneath. There is no fighting going on, she has pups and otherwise she just lives with her mate. She doesn't seem lethargic or anything, the only "warning" sign I've gotten is this patch. 
Has anyone experienced this before, and do you know what's causing it?

[betty]

It looks like the fur has worn away either through barbering, where a tank mate over grooms them, or through rubbing it against something like a piece of wood.