Listen to part of a lecture on this topic in a zoology class.

(man)
Let’s start with an example you may be familiar with. What’s one thing dogs typically do when they feel threatened? They want to protect themselves. That’s right, they show their teeth. And when we see this, we know this behavior means basically stay away. Now, here’s how this behavior became a recognizable warning. A long long time ago, a dog sensing danger would get ready to bite whatever animal was threatening it. It started by baring its teeth, but the propose of doing it was something very simple: the dog instinctively was making sure that it didn’t bite its own lips when it bit the other animal, OK? But what happened, other dogs, other animals, over time, they learned that this teeth baring always came before a dog was going to bite, and so they started to read it as a signal to be careful, to keep away, or risk getting bitten. This scenario, repeated time after time also led to the dog’s awareness that simply showing its teeth, a lot of the time that was protection enough. What had been a simple preparation to bite had developed into a kind of defense mechanism in and of itself. The dog learned that it did not need to actually attack; but could get the right result by communicating the possibility of attack through showing its teeth. It started to use this behavior as a warning signal.