Horses experiencing discomfort of the teeth, tongue, mouth or throat may gape their mouth open or twist their jaw as they eat. In this movement, the lower jaw is moved to one side or the other, out of alignment with the upper jaw. In some cases, the tongue protrudes.

This behavior often accompanies slow or difficult eating, or dropping feed. Horses may also do this when they eat something that they do not like, or after being given a dose of oral medication. Some horses with neurologic or severe body-wide disease will also engage in this behavior. A horse that has a foreign body lodged in the mouth will also do this.