Snake yawning, also called mouth gaping, was once thought to only prepare snakes for engulfing large meals (by stretching their jaws and re-aligning the fangs). However, it is now recognized as a way for snakes to pick up chemical cues from their environment (Graves and Duvall, 1983, Barbour and Clark 2012). By opening their mouths wide, chemicals make contact with the vomeronasal organ (also called Jacobson's Organ), which resides above the roof of the snake's mouth. Tongue-flicking, a well-known snake behavior, also allows the vomeronasal organ to process chemical cues. The tongue catches chemicals in the environment and brushes against the roof of the snake's mouth to transfer these chemicals to the organ. Mouth gaping often occurs after snakes probe their heads forward and tongue-flick.
Yawning also gives snakes charisma. When their "stone cold" countenance is interrupted by a yawn, you can't help but see their softer side. Even more importantly for researchers like me, yawns reveal hidden snakes in their environment. In my research, I use fixed security cameras to record the behaviors of free-ranging snakes. However, oftentimes I lose the snakes in my videos because they are well camouflaged with their surroundings. When they yawn, they open their mouths so wide that a white flash appears on the camera frame, thus revealing their location to me!
So in conclusion, I love snake yawns because they are adorable and informative.
Graves, B.M. and D. Duvall. 1983. Occurrence and function of prairie rattlesnake mouth gaping in a non-feeding context. Ethology 227:471-474.
Barbour, M. and R.W. Clark. 2012. Diel cycles in chemosensory behaviors of free-ranging rattlesnakes lying in wait for prey. Ethology 118:480-488.