Using eye tracking hardware to monitor the reactions of 43 individual canines, the scientists presented the dogs with images of human faces. The faces, which were either smiling or frowning, produced different reactions in the dogs, with pupil size suggesting the emotions the dogs were feeling when shown the faces. As you might expect, frowning or angry faces resulted in a sharp reaction from the dogs, with increased pupil size and attentiveness, while smiling faces put them more at ease, but that’s not the entire story.
The researchers supplemented their initial findings with further experiments using oxytocin to gauge how the “cuddle hormone” might change the dogs’ reactions. They found that, as it would in humans, oxytocin made the same frightening faces seem less intimidating, and made them more interested in the smiling faces than they had been previously.