Dogs show left facial lateralization upon reunion with their owners.

Domestic dogs demonstrate behavioral laterality in response to emotional stimuli; those responses include tail wagging and head turning. The dog is the species with the closest relationship to humans; dogs can express strong social emotions (e.g., attachment and separation anxiety) to specific persons, such as their owners. In this study, we examined whether dogs demonstrate more facial laterality when reunited with their owners than when they encounter an unfamiliar person in an unfamiliar situation. We also examined whether the observed laterality was specific to positive social stimuli (i.e., the owner) or a general response to nonsocial positive stimuli (i.e., toys). The dogs' facial expressions were recorded by a high-speed video camera during the presentation of emotional stimuli and the acceleration rates of parts of their faces were analyzed. The results showed that the left eyebrow moved more when the owner was present than at baseline. No bias in terms of eyebrow movement was observed when the dogs saw attractive toys. These results suggest that dogs show facial laterality in response to emotional stimuli. This laterality was specific to social stimuli, probably reflecting the dog's attachment to the owner.