Horses share surprisingly similar facial expressions to humans and chimps, according to researchers.
Like humans, horses use muscles underlying their nostrils, lips and eyes to alter their facial expressions in a variety of situations, mammal communication researchers found.
The findings, published in PLOS ONE, suggest evolutionary links in different species in how the face is used for communication.
The Equine Facial Action Coding System (EquiFACS), which was devised by a University of Sussex team along with researchers at the University of Portsmouth and Duquesne University in Pittsburgh, identified 17 discrete facial movements in horses, compared with 27 in humans, 13 in chimps and 16 in dogs.
The study's co-lead author, doctoral researcher Jennifer Wathan, said: "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.
"Despite the differences in face structure between horses and humans, we were able to identify some similar expressions in relation to movements of the lips and eyes. What we'll now be looking at is how these expressions relate to emotional states."
Researchers analysed video footage of a wide range of naturally occurring horse behaviours to identify different horse facial movements. They also carried out an anatomical investigation of the facial muscles that underpin these movements.
Co-lead author Prof Karen McComb said: "It was previously thought that, in terms of other species, the further away an animal was from humans, the more rudimentary their use of facial expressions would be.
"Through the development of EquiFACS, however, it's apparent that horses, with their complex and fluid social systems, also have an extensive range of facial movements and share many of these with humans and other animals. This contributes to a growing body of evidence suggesting that social factors have had a significant influence on the evolution of facial expression."
She added that the findings should ultimately provide important information for veterinary and animal welfare practices.