The Evolution of Dogs “smile”: Breed-specific Variations in the Relaxed Open Mouth Signal

Facial expressions play a crucial role in social communication, and animals have evolved a variety of signals to communicate with conspecifics. This study investigates the Relaxed Open Mouth (ROM) expression in domestic dogs ( Canis familiaris ), a conserved facial signal observed during playful interactions, by examining its variation across 30 breeds and comparing it to wolves ( Canis lupus ). Using the Dog Facial Action Coding System and phylogenetic comparative methods, ROM execution was analysed to determine whether domestication and selective breeding have influenced its expression. The results reveal significant inter-breed variability in ROM execution, with some breeds exhibiting muscle recruitment closely resembling wolves, while others display distinct divergence. Notably, these differences do not always align with genetic relatedness, suggesting that artificial selection has shaped facial expressivity in ways that transcend phylogenetic proximity. While domestication is often linked to enhanced communicative abilities, these findings indicate that some breeds have simplified ROM execution, potentially due to morphological constraints or selective pressures. This novel approach could also be applied to compare facial expressions across species with existing FACS systems, including other carnivores and primates.