Supplementary data for "Like deer caught in the headlights: Human evaluation of the intensity of emotions in brachycephalic and normocephalic dogs – A pilot study"

While the acoustic communication of dogs across a wide variety of contexts has been studied intensively, visual communication is, to date, less understood. Current research has focused on human assessment of canine signals or factors affecting the ability of humans to understand them. In this continuation of our previous pioneering study (Eretová et al., 2024), we have examined the human-perceived intensity of basic emotions (Happiness, Anger, Fear, Sadness, Curiosity) in two dog breeds (brachycephalic - Boston Terrier, normocephalic – Jack Russell Terrier) via short, soundless videos showing canine behaviours in four simulated situations. Participants of the study submitted their assessment via online survey, using a 7-point Likert scale and also provided information about their demographics, attitude towards dogs, and dog ownership history. Statistical analysis was conducted in R 4.2.2. using one full cumulative link mixed model per each inspected emotion. Distributions of positive and negative emotions differed across situations in both breeds, suggesting good recognition of the presented materials of contexts even without any auditory stimuli present. Results showed that owners of normocephalic dogs viewed Boston Terriers as having lower intensities of positive emotions (Happiness, Curiosity; P < 0.05) than brachycephalic dog owners, while also assessing Jack Russell Terriers with higher scores of positive and neutral emotions (P < 0.05). Older participants also tended to assign lesser intensity to positive emotions than their younger counterparts (P < 0.001). Despite predicting that the Boston Terriers would be rated with an overall higher intensity of positive emotions, this was not the case. This pilot study points out that context and the valence of the emotion are critical in its perceived intensity, and that the experience and attitude of assessors are important factors. More research which employs the scalability of emotions needs to be conducted on a wider variety of canine phenotypes to properly understand human-perceived dog emotions and the factors affecting them.