Horses understand human facial expressions, and they can tell the difference between happy and angry faces, according to a new Biology Letters study published this week.

Previous work has revealed that horses are able to produce complex facial expressions and also perceive these in other members of their species. They’re also sensitive to signals from us, too. After all, the ability to read emotions across the species barrier would be especially helpful for social, domesticated species – no matter how different our faces might look from theirs.

To see if they can discern our expressions, a University of Sussex team led by Amy Smith showed photographs to 28 domestic horses (Equus caballus) aged four to 23, recruited from five stables in Sussex and Surrey in the U.K. These color photos were of two unfamiliar men smiling or frowning (pictured to the right). In order for the researchers to get spontaneous reactions, the horses received no training for this experiment. The team measured the horses’ heart rate, which is correlated to stress, and recorded their responses with camcorders.

When the horses saw photos of the men making an angry face, their heart rate increased faster than when they looked at photos of the men smiling. Additionally, many of them also moved their heads to look at the angry photo with their left eye – a behavioral response previously linked to negative stimuli perception.

The right brain hemisphere, which handles information from the left eye, is specialized for processing threatening stimuli. "It is particularly important for animals to recognize threats in their environment," Smith explains in a statement. "In this context, recognizing angry faces may act as a warning system, allowing horses to anticipate negative human behavior such as rough handling." This is called the left-gaze bias, and dogs do it too. Neither species showed a gaze bias towards the happy facial expressions, since these aren't threatening cues.

There are several likely explanations for why horses have the ability to discriminate certain human facial expressions. “Horses may have adapted a pre-existing (ancestral) ability to respond appropriately to the negative emotional expressions of conspecifics and, throughout their coevolution with humans, transferred this ability onto a morphologically different species,” the team writes. Or, individual horses may have simply learned to interpret our expressions over the course of their lifetime.

Image in the text: A.V. Smith et al., 2016 Biol. Lett.