[question location="via e-mail"] I would like to know what makes a horse do his mouth like he is yawning. Does this help the horse get lots of air in his lungs like with people? Is it a sign of anything in particular?[/question]
[answer]Unfortunately, there is no simple answer that explains why a horse "yawns." There are some veterinary medical professionals that firmly believe that the behavior we attribute to yawning in the horse is actually a manifestation of pain or discomfort. Some believe that so strongly that they propose that a horse only "yawns" when he is uncomfortable. Certainly, many horses will "yawn" when experiencing colic pain, but no one knows precisely why the horse responds this way or what the "yawn" might do to make him feel better.
Horses also might "yawn" if they have oral pain, and some horses "yawn" just after removing the bridle, presumably to stretch jaw muscles. At one time, many hypothesized that the horse might be trying to equalize pressure inside the inner ear much like humans often do when driving in the mountains or riding in airplanes. This has not been proven.
We have all seen our horses "yawning" while seemingly happy and quiet in their stalls or pens. And sometimes horses will "yawn" close to meal times as if they are anxiously anticipating their hay or grain. With so many seemingly contradicting triggers, it is very hard to imagine a single cause for "yawning."