Fortunately the young ones are the ones most likely to do it, so they haven't yet managed to break through and give me flat tires. If it was an adult, that would be a different story. When I feel like they're getting a bit too comfortable chewing, I periodically shoo them off - which can be a bit tricky since I don't want to actually scare them away as I'm observing them. If I ever do get a flat tire while sitting at a den, I'm not entirely sure what I'll do ;).