My Hamster Continues to Bite Plastic Tubes. Help.

[Trin]

Okay so a little back story. This is my hamster Muschi’s (moo-she) cage. About two months ago I upgraded him to this four level cage. I did this because whenever I did free roam in my room he loved it and loved just running back and forth. So the idea was with more space he’d have for to explore and more room to run around. I’m the last month I’ve had the re occurring problem of him biting the tubes and I can’t figure out why. He doesn’t bite the wires and since it’s a big cage He’s only ever chewed one zip tie and hasn’t since then. Not even in his previous cage did he chew on any of the plastic and it was significantly smaller. I don’t know what to do or how to prevent this and it’s seriously stressing me out. Please help. (Also the top bin usually has significantly more bedding but I ran short on yesterday’s cage clean)