(newscientist.com)—Why the long face? Horses can remember the facial expressions they see on human faces and respond differently if you smiled or frowned when they last saw you. Leanne Proops, now at the University of Portsmouth, UK, and her colleagues at the University of Sussex showed in 2016 that horses respond differently to photographs of happy or angry human faces. Now they have studied whether horses can form lasting memories of people that depend on their facial expressions. First, they showed horses a photo of one of two human models, displaying either a happy or angry face. Several hours later, the model visited the horse in person, this time with a neutral expression. As a control, some horses saw a different model in the second part to the one they saw in the photograph. Crucially, the models didn’t know which photo the horse had seen earlier. In the early 20th century, a horse called Clever Hans amazed audiences by appearing to answer simple mathematical problems by tapping his hoof. It turned out he was responding to involuntary cues from his trainer. Proops’ study aimed to eliminate such cues. The team found that the horses remembered the models’ previous facial expressions.