Pupil size changes reveal dogs' sensitivity to motion cues.

Certain motion cues like self-propulsion and speed changes allow human and nonhuman animals to quickly detect animate beings. In the current eye-tracking study, we examined whether dogs' () pupil size was influenced by such motion cues. In Experiment 1, dogs watched different videos with normal or reversed playback direction showing a human agent releasing an object. The reversed playback gave the impression that the objects were self-propelled. In Experiment 2, dogs watched videos of a rolling ball that either moved at constant or variable speed. We found that the dogs' pupil size only changed significantly over the course of the videos in the conditions with self-propelled (upward) movements (Experiment 1) or variable speed (Experiment 2). Our findings suggest that dogs orient toward self-propelled stimuli that move at variable speed, which might contribute to their detection of animate beings.