Hmmm... no, I don't think they could have done that by digging... I usually had my gerbils in glass aquariums or bin cages, and the only case of raw nose that I had was a compulsive chewer that would chew anything in sight. Of course, this girl I'm talking about would actually fall asleep while chewing the running wheel and was an extreme case. Have they lost hair just on their nose or are there any other body parts that are also missing hair? What type of bedding do you have them in? Is there any other weird thing like porphyrin around their eyes and nose? (porphyrin is a reddish secretion).
No, no porphyrin, and the hair is already growing back, before I could see their dark colored skin, now it's turned the color of their fur and is starting to get some stubble back. They are on whats called Cellsorb bedding, its a recycled paper product and highly absorbant, its also low dust so I don't know if they are allergic to it or not...
Hmmm, yeah, the only thing that occurs to me is allergies. Gerbils are very prone to those. Most places say that corncob is the only hypoallergenic bedding, but I know someone that had a gerbil allergic to corncob... and she also had a gerbil allergic to aspen, so it is possible. If they are getting their hair back, I'd say leave it as that and continue to observe them. If it continues to happen, try a different bedding to see if they get better on that.
mine has fur missing from just above her nose as did her mother it is caused by excessive chewing but it is deffiantly nothing to worry about just the more energetic chewers get a lil fur rubbed off of their nose