Horses prefer to look at negative and threatening sights with their left eye, and positive social stimuli with their right eye. In the study, when they saw a model they had seen frowning earlier, they spent more time looking with their left eye. They also exhibited more stress-related behaviours, like scratching and floor sniffing. In contrast, when they saw a model they had seen smiling earlier, they spent more time looking with their right eye.

Many other animals have shown an ability to remember human faces, including sheep and fish. Wild crows will hold a grudge for years against people who have treated them badly, and even teach other crows to mob their enemies.

However, the horses seem to form an opinion about people based only on their expression in a photograph. “That’s something we haven’t really seen in animals before,” says Proops.

“The horse family has the most expressive faces after the primates, so logically they pay attention to faces and expressions,” says Frans de Waal at Emory University in Atlanta. “Horses surrounded by people have ample opportunity to learn what our expressions mean.”

Journal reference: Current Biology, DOI: 10.1016/j.cub.2018.03.035