Gizmo and Scooter (both neutured) like to bite Tigger (spayed when she was 8 months) on the back of her neck. The boys are 1 and Tigger is 2. Is this just them showing their dominance or what? I worry, because she is smaller than them, and we have to break it up when she cries. I just don't want them to hurt her..... She doesn't fight back. Any ideas?