Huddled in our jungle retreats, we are all getting ready for sleep. But dangers always lurk in the jungle, and so we'd do well to deceive potential predators into thinking that we are still hyper-aware of our surroundings (despite the fact that the exact opposite it true!). So what do we do? We open our mouths wide, we bare our canines, we stretch, thus making ourselves appear bigger, and we let out a low groan, which also suggests we are bigger and more dangerous than we actually are. We see our mates engaging in this deception, and we follow suit. Soon enough, due to the survival-enhancing value of this deceptive signal, the behavior becomes entrenched. That’s why we yawn.