It's not adaptive for anger to be communication though. For instance, a small dog may not be willing to fight to the death against a much larger creature. But the visceral response of anger comes automatically, and the reason it's useful is because it doesn't convey information, but conceals it. You snarl so that people think you might be combative, but you are quite definitely not communicating that you are combative. A world where a growl really was communication would be a world where anyone who didn't growl could be abused.