For the first time ever, researchers have established that dogs can tell the difference between angry and happy human faces. Dogs may have developed the skill during their domestication by humans, according to a study published today in the journal Current Biology. The study also suggests that dogs had memories of what the expressions looked like and were able to associate them with the appropriate emotions.

In the study, nine dogs (mainly Border Collies) were shown sections of pictures of sad and happy human faces on touchscreens. One group of dogs was trained to touch the happy face to receive a treat while the other group was trained to touch the angry face. After the training period, the dogs were able to select the correct face with a 70 to 80 percent accuracy, explains lead author Corsin Müller, a cognitive scientist at the University of of Veterinary Medicine in Vienna, adding that the emotions being tested had been selected because they were "quite relevant for the dogs."

The set-up of the experiment. (Muller et al, 2005)

Similar studies have been carried out in the past, but those experiments tested the dogs by showing them full pictures of neutral and happy faces, Müller told The Verge. This means that the dogs might not have been recognizing expressions, but merely single qualities in the pictures. "[With the past studies] we're not sure whether they discriminated the face based on human expression," says Müller. "The very simple explanation is the dogs were just identifying teeth."

To exclude this possibility in the new study, the dogs were not only tested on their ability to recognize whole faces but also parts of the face in isolation. If the dogs recognized expressions simply because of the presence or absence of teeth, then they wouldn't be able identify angry or happy faces just from looking at the eyes, explains Müller. Thus, the dogs were trained with just one part of the face and one expression — either happy or angry mouths or faces. Then they were tested to see if they could recognize that same expression in other parts of the faces or whole faces, a task they managed with 70 to 80 percent accuracy.

"They seemed to realize that the smiley eyes have the same meaning as a smiley mouth and angry eyes have the same meaning as an angry mouth," Müller says.

There were also clues that the dogs were recognizing more than expressions. Dogs that were tasked with identifying angry faces were far slower to learn than dogs identifying happy faces. "This seems to indicate that they do have some associations [between expressions and emotions]," says Müller, "and that an angry face is something they wouldn’t want to approach and touch."