Pupil size changes reveal that dogs are sensitive to the social conventions behind human gestures

While many animals display behaviours resembling social conventions when interacting with conspecifics, it is unknown whether animals who live in close contact with humans develop sensitivity to our social conventions. With an eye-tracking study (N = 39), we asked whether dogs' (Canis familiaris) eye movements and pupil sizes would show a reaction when observing a violation in the customary response to human conventional communicative gestures ("come here" and "go away"). In the video stimuli, a human actress either approached or walked away from a second human actress. Both these actions (approaching or walking away) could be expected or surprising (within-subject), depending on whether they happened in response to the "come here" or the "go away" gesture of the second actress. While dogs' looking times and number of gaze transitions between the two actresses did not differ significantly between congruent and incongruent responses, their pupil size changes did: dogs' pupils dilated more when watching the incongruent responses (approaching after the "go away" gesture and walking away after the "come here" gesture) compared to the congruent ones. We discuss the possibility that pet dogs, during their ontogeny, implicitly form expectations about the usual response to some conventional human gestures. These expectations are not self-centred, as they were evidenced when dogs observed a third-party interaction between two humans, and they are context-specific, as the same responses elicited surprise following one gesture but not the other. We also consider alternative explanations and limitations of our findings.