Dogs are able to recognize human facial expressions, a new study determined.

Canines can tell the difference between happy and sad human faces. This is the first time that a species, other than human beings, has been shown to recognize emotions through facial expressions in another species. Previous studies examining emotional recognition of human emotions by dogs were inconclusive.

Pictures of people, exhibiting happy and sad faces, were used in the study. Investigators began by training the dogs to react to photos. Pictures of people with neutral expressions were shown to dogs, along with the back of the same subject's head. When the pup touched the picture showing a face, they were provided with a small treat.

The photos were then shown just from the nose down, while expressing emotion. A total of 15 pairs of pictures were used during training. Four trials were then presented to the animals, as researchers showed the dogs the upper half of the photos, half-pictures of the same emotions showing on other people, as well as full photographs, and the left half of faces in the original photographs.

The canine subjects reacted to happy and sad photographs of people to a greater extent than would be expected if they did not recognize emotion. The animals were also able to transfer their learning to unique cues in the environment, including unfamiliar faces.

"We think the dogs in our study could have solved the task only by applying their knowledge of emotional expressions in humans to the unfamiliar pictures we presented to them," Corsin Müller of the University of Veterinary Medicine Vienna said.

Canine subjects reacted the most to expressions of happiness, and to anger the least. That may be because the dogs try to avoid angry humans.

The dogs in the study were able to recognize emotions not just by looking at mouths, which are the most obvious sign, but also by looking at eyes.  This suggests the animals actually recognize emotions, rather than just reacting to physical characteristics.

"We can rule out that the dogs simply discriminated [between] the pictures based on a simple salient cue, such as the visibility of teeth," Müller said.

A total of 24 canines were originally included in the experiment, but 13 needed to be removed from the study for various reasons.

Researchers are uncertain why dogs have developed the ability to recognize facial expressions in human beings. One theory is that living in close quarters with people has provided the animals with the ability to determine when our emotions ride one way or another.
 
Examination of emotional recognition in dogs was profiled in the journal Current Biology.