Those who practice Natural Horsemanship, have most likely been told or have observed that a horse licking his lips is processing something. When a horse over comes a fear, he will lick his lips. When he figures out what you’re asking of him, he licks his lips. It’s a very consistent pattern that comes up whenever pressure is taken off of the horse.
Of course there are times when our horses lick their lips after they did something “bad”, and we know that’s not the last time they’ll do it…
“(…) I’ve seen horses kick somebody or buck somebody off and then lick their lips. They may have learned something and become relaxed, but it may not be a positive thing for us. (…) So in the training process we may put pressure on them and they become tight and when they give us positive response we take away the pressure and they relax and lick their lips. This experience can be positive for both us and the horse. We just need to be aware of the situation leading up to them licking their lips and make sure we are getting a favorable response for them to learn from and not something that may be a good experience for them but a bad experience for us. (27)”
– Martin Black, Evidence-Based Horsemanship
But why do they do that? I always thought it was an odd thing to do when you learned something… lick your lips? Really? What if humans did that? Here’s a more scientific angle on it:
“You may be familiar with seeing particular oral movements in the horse (such as lip-licking) that indicate their physiological state. For example, a response to stress will release adrenaline and lead to a relatively dry mouth. The horse starts licking with the return of saliva secretion when the balance of the sympathetic and parasympathetic chemical reactions return to their normal set point. (26).”
– Dr. Steve Peters, Evidence-Based Horsemanship