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[serpenttongue]

I dont believe this behaviour to actually be yawning, the way we yawn. They're most likely just stretching their jaws.

As craig said they will do it several times after a feed to put their jaws back in place, but i also agree with Moosey, snakes will stretch their jaws when they think they are getting fed. This will be most noticeable if you let them know its feeding time, but dont offer the food right away. Instead keep them in anticipation for 5 minutes and you'll usually get one or two yawns out of them. I'd imagine they're stretching the jaws in preparation for the strike.