Horses, like humans and chimps, have a highly-complex system of emotional facial expressions, a new study finds.

Previous research has showed that horses have a coding system that corresponds to different facial expressions they use to communicate. In this study, researchers identified 17 distinct "action units" -- identifiable facial movements -- in horses. Similar research has revealed 27 action units in humans, 16 in domesticated dogs and 13 in chimpanzees.

"Horses are predominantly visual animals, with eyesight that's better than domestic cats and dogs, yet their use of facial expressions has been largely overlooked. What surprised us was the rich repertoire of complex facial movements in horses, and how many of them are similar to humans," study co-lead author Jennifer Wathan said in a press release.