Dog owners may think their pets can tell a smile from a frown, but scientific evidence has been lacking. Now, researchers have trained dogs from a variety of breeds to look at a pair of photos arranged side by side—one showing the upper half of a woman’s face looking happy and the other showing the upper half of the same woman’s face looking angry—and pick out the happy expression by touching their snouts to it (pictured). When then shown the lower halves of the faces or pieces of other people’s faces, the perceptive pooches could still easily discern happy from angry. Another group of canines similarly learned to identify angry faces. Dogs in a previous study that distinguished expressions on whole faces could have done so using simple visual clues that reappeared in every face: the white of teeth in a smile, for instance, or creases in angry skin. Identifying emotions from photos of different parts of the face requires a more holistic understanding of expression, argue the authors of the new study, published online today in Current Biology. While primates are known to recognize faces, dogs may have been especially adapted for emotional sensitivity to humans during their domestication. The researchers plan to investigate how common this ability is by testing pigs and other animals.