By Maryna Mullerman ’20

Mammalian facial expressions are known to correlate with animals’ internal states. Substantial similarities have been previously identified between chimpanzees and humans, but facial expression similarities between more distant mammalian species is unknown. Caeiro Cátia and researchers from the University of Lincoln in the United Kingdom investigated whether domestic dogs produced certain facial expressions in response to different external emotional stimuli. Furthermore, the study aimed to find similarities in facial muscle structures and movements among humans and domestic dogs.

The researchers analyzed numerous videos from online databases to gather 150 individuals, consisting of 50 humans and 100 family dogs. Neutral or relaxed individuals were used as members of a control group. All videos had high stimulus quality and clear response, minimum editing, and high quality. Amateur videos provided data on the natural and spontaneous behavior of people and dogs. The Facial Action Coding System (FACS) and DogFACS were used to analyze expressions based exclusively on anatomy.  Four categories of emotion with different triggers — fear, frustration, positive anticipation, and happiness — were explored. Canine morphological differences were assessed by head type, jowls length, ear morphology, and breed. Action units (AU) was used to code for emotion types.

Compared to the relaxed condition, humans showed higher rates of facial expressions associated with happiness, fear, and frustration categories, but not with positive anticipation. Dogs displayed higher rates of facial action in fear, positive anticipation, and happiness categories, but not with frustration. Direct comparisons between human and domestic dog facial expressions revealed significant differences in all categories. Morphological differences did not impact facial actions.

The results suggested that domestic dogs displayed distinct facial movements in response to different emotional stimuli, but they did not display equivalent facial actions to humans when reacting in similar emotional contexts. This demonstrated limitations in the homologies between humans and domesticated canines. This was indicative of phylogenetic inertia, or constrained evolutionary pathways. The researchers noted that the species employed different, non-facial expressions of similar emotions that could not be directly compared. Still, this study provided new insight into the evolution of emotional expression and human perception of non-human emotions.

References:

1. C. Cátia. et al, Dogs and humans respond to emotionally competent stimuli by producing different facial actions. Scientific Reports 7, (2017). doi: 10.1038/s41598-017-15091-4.
2. Image retrieved from: https://www.pexels.com/photo/animal-blur-canine-close-up-551628/