Identification of facial expressions in response to rewarding stimuli in dogs

The morphological features of facial musculature enable mammals to produce facial expressions as part of their responses to various stimuli. Dogs are among the most suitable animals for analyzing facial expressions due to their expressiveness, ease of recording, and social and veterinary importance. In the present study, we aimed to identify dogs’ facial expressions in response to rewarding stimuli. Owners of the pet dog subjects recorded the subjects’ facial expressions under three conditions at home. In the food and non-food conditions, the subjects were given a sit-and-stay command and then presented with either their preferred food or non-food stimulus, respectively. In the control condition, the dogs were given the same command, but no stimulus followed. Their facial expressions were coded using DogFACS. We found that both the male and female subjects showed increased AU25 (lips part), AU26 (jaw drop), AD19 (tongue show), and EAD102 (ears adductor) in both the food and non-food conditions, suggesting that dogs show these action units in response to rewarding stimuli. We also found sex differences in facial action unit changes. Specifically, the male subjects additionally showed increased AU109+110 (nose wrinkle & upper lip raiser), AU116 (lower lip depressor), AD37 (lip wipe), and AD137 (nose lick) in both conditions. In addition, while the male subjects consistently showed the changes throughout the test period, changes in the female subjects were primarily observed during the first half of the test. Taken together, the present study provides insights into how rewarding stimuli influence dogs’ facial expressions. • Dog facial expressions to preferred food and non-food stimuli were analyzed. • Four action units increased in both sexes in response to both types of stimuli. • These changes were considered expressions to rewarding stimuli in dogs. • Males showed consistent changes, while females changed primarily in the first half. • Only males increased four additional units to both types of stimuli.