Yawning is usually perceived as a sign of sleepiness or boredom. We yawn when we're tired or not inspired and assume that animals might do it for the same reason.
But there's not a consensus among researchers over the reasons people or animals open their mouths and let loose. Depending on the species, it might be exhaustion or empathy, body temperature regulation or aggressiveness.
Here's a look at some yawning research and trivia, as well as irresistible images of animals (sleepy or not?) yawning.