I don't know if someone has already answered when you were talking about your gerbil chewing on it's skin that you thought might be a tumor, but I just got this off of a website. See if it can help.
Tumors on the Scent Gland Gerbils all have a scent gland on they're underside that looks like a section of rough yellow skin without fur on it. Gerbils use these to mark they're territory. In older, and mostly male gerbils, sometimes tumors will develop on the scent gland. They start off small, but can grow to become bothersome to the gerbils, and sometimes they will chew the tumor off. If it gets very large, a vet can remove these tumors safely, but it is not always necessary, and I recommend watching the gerbil that has a scent gland tumor very closely to see if it irritates him. Hope it helps.... heather
