Humans have been living and working with horses for more than 5,000 years, since the first domesticated equines had their teeth worn down by primitive bridles in northern Kazakhstan. Hands could not have built modern civilization without the help of hooves—to haul ploughs, pull carriages, march soldiers into battle, and carry messages of love and war across hundreds of otherwise-insurmountable miles.

An unlikely pairing of wily predator and one-ton prey, humans and horses have managed to successfully communicate across the species barrier because we share a language: emotion. Experienced riders and trainers can learn to read the subtle moods of individual horses according to wisdom passed down from one horseman to the next, but also from years of trial-and-error. I suffered many bruised toes and nipped fingers before I could detect a curious swivel of the ears, irritated flick of the tail, or concerned crinkle above a long-lashed eye.

Horses can read human emotions, too, often in uncannily accurate ways; alerting us to our sadness or nervousness, sometimes before we’ve even consciously registered it. As Herman Melville wrote in Redburn, "No philosophers so thoroughly comprehend us as dogs and horses."

Today’s domesticated horse, Equus caballus, is a tamer version of the snorting, shaggy beast who once roamed the plains in the Ice Age; selectively bred over generations for agility and a human-friendly temperament. But horses’ ability to form complex social relationships stems from an older evolutionary legacy, of living in close-knit bands of five to 10. Within these bands, foals, fillies, stallions, and mares—who were the observant matriarchs, despite the males’ eye-catching flashy necks and noisy displays, as Wendy Williams notes in The Horse—all formed close emotional partnerships. Now, humans are part of the herd: Domestic horses respond to the tiniest change in tone of voice, quality of touch, or stiffness of their rider’s body.

The equine ability to read human emotion through sound and touch is exquisite. But horses can also read the expression on a person’s face—as a Biology Letters paper earlier this month confirmed for the first time. This sophisticated capacity has only ever previously been demonstrated in dogs—and even further disproves the myth (that I believed until now, despite riding for 15 years) that horses have bad eyesight. While horses can’t see color, and have a blind spot directly in front of them as a result of eyes positioned on the side of their head, their vision is actually more acute than domestic cats’ or dogs’.

A University of Sussex research team, led by Amy Smith alongside the veteran animal-behavior scientist Karen McComb, showed a group of 28 horses large photographs of man’s face making either a positive (smiling) or negative (angry, brows furrowed) emotional expression. The results showed that horses were able to automatically distinguish between the two expressions, and what they meant.

The horses tended to look at the angry faces out of their left eye—a response well-documented in horses and in dogs, indicating that an animal is engaging the right hemisphere of its brain where novel and fear-provoking stimuli are processed. The horses’ heart rates also rose more quickly when they were presented with the angry face. Being able to tell a smiling handler from an angry one is a useful skill for a domestic horse—being approached by a frown rarely results in happy consequences.

The authors speculated that horses may simply have been applying an ancestral ability to read the facial expressions of their own species “onto a morphologically different species,” in this case, humans. These same University of Sussex scientists found last year that horses have 17 distinct facial expressions—more than dogs’ 16, or chimpanzees’ 14—many of which are similar to humans’ 27 facial movements, like creased brows or eyes widened in fear. (So far, this method of coding facial expressions hasn’t been applied to wild animals, and those that aren’t part of humans’ evolutionary lineage.)

However, since the horses tested in this particular study came from riding schools, where they interact with humans all day every day, their face-reading ability may also have been learned within their lifetime. Previous research has demonstrated that familiarity was a significant factor in dog’s ability to recognize human expressions—they performed better when faced with their owners than with a stranger.

While these latest findings probably come as no surprise to those who work with horses on a daily basis, they are one more piece in the increasingly complex picture of the emotional language humans share with other species. Horses’ ability to read faces perhaps also refines the explanation for horses’ therapeutic and even spiritual effects on humans, documented in our art and stories since the first etchings on cave walls.

They may speak in humming sighs or twitches of whiskered lips instead of words, but it’s increasingly clear that horses are highly emotionally attuned—and not all that different from the animals they carry on their backs.