The ability to use complex facial expressions to convey emotion isn't unique to humans -- chimpanzees, dogs and cats also possess this ability. Scientists are finding that horses, too, show their feelings on their faces.

In a new study, psychologists from the U.K.'s University of Sussex found that horses use a range of muscular movements to form dynamic facial expressions that are sometimes nearly identical to those of humans.

The researchers compiled a directory of horse facial movements, published online this week in the journal PLOS One, which offers a glimpse into the complex socioemotional lives of horses.

"Horses are undoubtedly emotional animals," Jennifer Wathan, a doctoral researcher at the University of Sussex and one of the study's authors, told The Huffington Post. "But what they feel and how that is expressed is a question that we have yet to pin down."

In a variety of social situations, horses move the muscles in their faces -- largely the muscles around the nostrils, lips and eyes -- to create multiple different expressions that communicate information to other horses.

Credit: University of Sussex Researchers found surprising similarities between the muscular structures of horse faces and human faces.

Wathan and her colleagues devised a comprehensive coding system to better understand common equine facial actions, which they named EquiFACS (Equine Facial Action Coding System). They identified 17 different facial actions -- including "ears forward," "eye white increase," "tongue show" and "lip presser" -- that horses combine to create their expressions.

"Facial expressions are made up of a number of different complicated movements that overlap in a dynamic way," Wathan said. "The Equine Facial Action Coding System gives us a way in which we can make sense of these dynamic and complex forms of communication."

The new research could be put to good use in veterinary settings or in horse training programs. An animator is even using the findings to help create more realistic horse faces, Wathan added.

"Even people who know horses really well sometimes can't articulate exactly what it is about an expression that they're seeing," Wathan said. "This gives them a way of articulating it."

Without further ado, here are a few of the most common equine facial movements and the emotions that they might express:

Credit: University of Sussex

Inner brow raiser: Horses tend to raise the inner brow of the eye in negative emotional situations as an expression of sadness or fear.

Dogs and humans are known to use a similar action in expressions of surprise, sadness and fear, Wathan noted.

Credit: University of Sussex

Upper lip raiser: This can be an expression of fear in horses and is often combined with a widening of the eyes, increasing the visibility of the whites of the eyes. Cows also have been shown to display this facial action in stressful situations.

Credit: University of Sussex

Lip corner puller: This subtle movement is generally a gesture of submission in horses, otherwise known as "snapping." In humans, pulling the corners of the lips is a key component of the smile. The expression is also seen in primates, cats and dogs.

Watch the video below for a demonstration from the researchers: