With the arch of an eyebrow or the curl of a lip, humans can convey some 27 emotions, ranging from happily surprised to angrily disgusted. Now, it turns out, horses also use the muscles underlying their facial features—eyes, nostrils, and lips—to alter their expressions, scientists report today in PLOS ONE. Those equine expressions are far richer and more complex than previously believed, and some are surprisingly similar to those of humans, the researchers say. To understand how horses use their faces to communicate, the scientists analyzed videos of the animals behaving naturally (like the horse in the photo above). They identified all the possible movements horses can make with their faces and the underlying musculature. Using these data, they then created a coding system for each individual facial movement—just as other researchers have done for humans and several other species. Horses, it turns out, can make 17 discrete facial movements, whereas cats make 21, dogs make 16, and chimpanzees 13. Further, even though a horse’s face is structured differently from that of a human—an equine’s eyes, for example, are placed on the sides of the skull—it can nevertheless produce some expressions strikingly similar to ours, the scientists discovered. Their study overturns the old notion that species distantly related to humans would have more rudimentary facial expressions. Now, the researchers are working on identifying the emotions or social information horses are communicating through their expressive faces.