Gerbils have to constantly chew, their two front teeth, like beavers, and rabbits, never stop growing. He may be chewing on his tail to clean, or itch. But make sure you provide your gerbil with lots of stuff to chew. They start out with crumpled paper, then switched to old cereal boxes, next thing you knew, I would throw in my old woden tea boxes, here a lot of our tea are sold in small woden boxes, and it also loved blue, anything blue, and my old white vitamin bottles. Chewed them into tiny little bits. It was fun to watch. Otherwise they will often attack barred cages. They need something, but I find most of the chew toys I bought, woden toys, didn't go neat. Seemed like a mini garbage disposal system LOL