One of my rats, Nina, has a habit of gnawing on the bars of the cage. Really trying to chew them up, and making a loud ordeal out of it. Is there some way to discourage this behavior? They have blocks and sticks available to chew, so I'm not sure why she's been doing this. Glee doesn't chew on the bars--just Nina. Is there maybe something safe to put on the bars to discourage chewing?