I had a single Russian tortoise for almost a year and, like you, wondered if she was getting lonely. Then I rescued a second Russian tortoise who needed a good home. When I put him in with her, she got very territorial and started bullying him. She does a lot of head bobbing and stealing food out of his mouth, and I've seen her bite at his head. I've also seen her walk up to him, pull her head into her shell, and then launch herself at him to knock him out of her way. They've been together for six months now, and they mostly get along with each other with just a little bit of head bobbing from her, but I think she resents having to share her living space (which is a 2 foot by 6 foot area) and would have preferred to remain alone.