Theory of mind is a distinct concept that isn't necessary to explain this behavior. Of course, it may follow naturally, but it strikes me as ham-fisted projection of our own cognition onto others. Ironically, a rather greedy theory of mind!
If apes started communicating mongs themselves with sign-language they learned from humans that would measn they would get more practice using it and they could evolve it over aeons. Hey, isn't that what actually happened?
