Many different kinds of vertebrates yawn, from fish to humans, but they yawn for different reasons. With some, it seems to be an aggressive act, a non-violent way to say, "Look what big teeth I have." For others, especially reptiles, it is thought to help with cooling their body. For many animals, including humans, we are still not sure why they yawn. We do know that it is quite contagious, and often even reading about yawning can cause you to yawn. Let me know if you yawned while you read this.