Horses coexist as herd animals with complex social relationships that depend on visual and auditory cues to tell friend from foe in their environment. their complicated system of visual cues are a type of body language that keeps them in constant communication with the ever-changing moods of the members of the herd.
Subtle cues, such as those of different ear positions, are a fundamental form of communication that contributes to maintaining a class system within the herd without having to escalate to a fight for dominance.
When a new horse is added to the herd, the dominant horse will establish his status in the field by asserting his right to eat or drink first. There might be some kicking, biting, charging, neck snaking and circling of the new horse followed by nose-to-nose contact, striking and squeals. The following day, a raised hoof and bared teeth might be enough to serve as a directed warning to keep the newcomer in his place. The new horse will quickly figure out where his place is in the herd with a signal as subtle as pinned ears.
Most horses are very in tune with the body language of the alpha and their herd mates. The subtlety of specific ear positions sends a very clear message to other horses that can easily be interpreted by their humans, as well.
Learning the subtleties of equine body language provides you with an important key to successful interaction with horses. You have only to become a keen observer to these visual cues to become aware of the message that they send.
A single flick of an ear can transmit several different messages in unison when taken with other body language cues. A horse cannot communicate without using his ears to convey part of the message.
Horses communicate directly through posture, gesture and expressions. Learning to interpret ear positions, and physical postures in general, is not a mystical skill, but an education in observation.