biostudies-literatureUnknown13(1)Tomberg CFondation Prince LaurentFacial micro-expressions are facial expressions expressed briefly (less than 500 ms) and involuntarily. Described only in humans, we investigated whether micro-expressions could also be expressed by non-human animal species. Using the Equine Facial action coding system (EquiFACS), an objective tool based on facial muscles actions, we demonstrated that a non-human species, Equus caballus, is expressing facial micro-expressions in a social context. The AU17, AD38 and AD1 were selectively modulated as micro-expression-but not as standard facial expression (all durations included)-in presence of a human experimenter. As standard facial expressions, they have been associated with pain or stress but our results didn't support this association for micro-expressions which may convey other informatScientific reports8625https://www.ebi.ac.uk/biostudies/studies/S-EPMC10224940biostudies-literatureHorses (Equus caballus) facial micro-expressions: insight into discreet social information.PMC10224940Tomberg Cde Selliers de Moranville LAPetagna MfalseHorses (Equus caballus) facial micro-expressions: insight into discreet social information.Facial micro-expressions are facial expressions expressed briefly (less than 500 ms) and involuntarily. Described only in humans, we investigated whether micro-expressions could also be expressed by non-human animal species. Using the Equine Facial action coding system (EquiFACS), an objective tool based on facial muscles actions, we demonstrated that a non-human species, Equus caballus, is expressing facial micro-expressions in a social context. The AU17, AD38 and AD1 were selectively modulated as micro-expression-but not as standard facial expression (all durations included)-in presence of a human experimenter. As standard facial expressions, they have been associated with pain or stress but our results didn't support this association for micro-expressions which may convey other informat2023-01-01T00:00:00Z2023 May2025-05-29T16:22:33.397Z2025-05-29T16:22:33.397ZS-EPMC102249403724493710.1038/s41598-023-35807-z