(CNN) - Over time, we've learned how to read the body language of horses -- from understanding whether the movement of its ears, head, legs and tail mean its relaxed, anxious, angry or alert.
But it turns out horses are capable of pulling faces just like humans, too -- which may shed more light on what they're feeling.
In fact, horses can make 17 facial movements -- which is at least three more than our relatives, the chimpanzees, and just 10 less than humans.
In order to try and identify whether horses can pull more than just a long face, researchers at the University of Sussex in the United Kingdom created the Equine Facial Action Coding System (EquiFACS) to determine any discrete expressions made by horses.
By dissecting a horse's head and identifying its facial musculature, in addition to watching 15 hours of horse behavior in 86 horses ranging in breed and age, they were able to log any possible faces that the animals can make.
The coding, which has been used on cats, dogs, chimpanzees and humans also found specific "evolutionary parallels" in the way different species use the face to communicate.
"It's a nice system because it's an objective anatomical base code system that anybody can use," said Anne Burrows, a professor at Duquesne University in Pennsylvania who was involved in the study.
With her Ph.D in biological anthropology, Burrows was able to help dissect the horse head that was provided by a veterinarian school.
"We spent, I think, over a week working on that horse dissection, extensively documenting with photographs, written notes and measurements."
The domestication of horses around 5,000 years ago could have dramatically influenced the social, cognitive and morphological characteristics of the animal, as explained in the report which was published on PLOS One in 2015.
"Surprisingly their (horses) use of facial expressions has been largely overlooked," the study said.
"It was previously thought that humans possessed the most complex repertoire of facial expressions and that, in phylogenetic terms, the further away an animal was from humans, the more rudimentary their use of facial expressions would be. However, through the development of EquiFACS it is apparent that horses have an extensive range of facial movements."
Burrows says that there is growing evidence to suggest that animals do indeed communicate intentionally, not just with each other -- but with humans as well.
"It makes sense because humans domesticated these animals, but it's fascinating. The literature on dogs is so extensive but it seems to be spreading out to other domestic animals as well."
The research provides an ideal framework, the study says, for investigations into whether the facial movements are associated with positive or negative emotional states.
Most common expressions
Of the 17 facial expressions identified, such as "chin raiser," "sharp lip puller," "lip pucker" and "jaw drop," the team narrowed down some of the most common expressions and what they may represent.
Lip corner puller
This particular movement is what makes up a human smile, and in horses, the report found it was displayed as part of a submissive gesture.
Researchers said they found younger horses tend to pull this expression to older horses.
Burrows said that the upper lip was one of the most interesting things she found during her dissection.
"The upper lip is almost like a finger, the musculature around the upper lip reminds me to some extent of what we see in chimpanzees."
Upper eyelid raiser and eye white increase
Another movement identified by researchers were two separate expressions which are often seen together at the same time -- the "upper eyelid raiser" and an "eye white increase."
In horses, the outer white layer of the eyeball is generally not visible.
Researchers found that like humans, widening of the eyes in horses is often associated with fear.
Raised inner brows
While horses do not have eyebrows or a prominent brow ridge like humans and other primates, the study identified that they do have an expression which raises the inner corner of the eye.
From what the team were able to identify, it's thought that this particular expression is usually pulled when the horse is in a negative emotional situation.
Similarly, humans make the same expression when they're feeling sad, or fearful.
How the findings are helping vets, trainers
The report, it says, is able to help inform those working in the horse community or in veterinary practices.
"EquiFACS provides those working in the horse community with a standarized language through which information can be shared, facilitating the investigation of questions relevant to horse management and welfare," it said
Shortly after the report, Burrows became a horse-owner herself as her daughter took up riding -- and, thanks to her involvement in the study, she says she's able to identify the same facial expressions and behaviors that she learned through the research.
"Now I see very up close what we were dissecting years ago and it's amazing," she says.
"I'm surprised with how much movement of the ears is related to what they think is emotional intent -- the aggressive way of the ears that says 'if you don't stop that I'm going to bite you' and certain movements of the ears seem to be very tightly tied to him being pleased or curious."
Horses can read your expressions
Since the report was published in 2015, more research has taken place into understanding not only horse facial expressions, but how they interact and identify the expressions of other species -- including humans.
It was found earlier this year that horses can read and remember a person's emotional expressions.
Experts at the University of Sussex and the University of Portsmouth presented horses with photographs of people with a happy, or angry face and then introduced the horse several hours later to that same person -- who this time exhibited a neutral expression.
Researchers found that despite the fact the person was now exhibiting a neutral expression in front of the horse, its gaze revealed that it still perceived the person in a negative light if they were angry in the photo.
It was based off previous research that found that horses usually view negative things with their left eye due to it being sent messages from the right side of the brain which specializes in processing potentially threatening stimuli.
"What we've found is that horses can not only read human facial expressions but they can also remember a person's previous emotional state when they meed them later that day -- and, crucially, that they adapt their behavior accordingly," Professor Karen McComb said of the findings.
"Essentially horses have a memory for emotion."
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