An ethogram of facial behaviour in domestic horses: evolutionary perspectives on form and function

Understanding cause and function of animal facial behaviour can provide key insights into the animal’s cognitive and emotional state. The Equine Facial Action Coding System (EquiFACS) has characterised a wide range of equine (Equus caballus) facial movements (Action Units (AU) and Descriptors (AD)). However, there is still a lack of systematic documentation of whether and how these AUs and ADs are combined to produce discrete configurations of facial behaviour in horses. The aim of this study was to provide a systematically derived ethogram of equine facial behaviour in the domestic horse across positive, negative and neutral social interactions. Facial behaviour was recorded during horse-horse interactions occurring during affiliative (non-play), play, agonistic and attentional contexts, based on the coding of contextual behaviour. Using EquiFACS, a bank of 805 AU/AD combinations, across 22 distinct behaviours, was created. Network analysis techniques (NetFACS) were used to establish the facial movements significantly associated with each context. Domestic horses use a wide range of distinct facial behaviours, which are defined and described in our ethogram. Interestingly, there were marked similarities between the play faces of horses and the open mouth play faces of primates and carnivores, adding weight to the hypothesis that these facial behaviours are deep rooted in mammalian biology. We also defined a new EquiFACS Action Unit, AUH21, facial tightener (platysma), which makes the underlying facial structures appear more prominent. This AU is found in humans and gibbons, but no other species studied, and its addition to EquiFACS will enhance cross-species comparisons and potentially facilitate the attribution of emotional state and assessment of pain in horses. Our equine facial behaviour ethogram will be invaluable in future work exploring equine emotion, welfare, social behaviour, and perception, as well as having direct applications for those working with and around horses.