Social communication in domestic horses: the production and perception of facial expressions

Living in complex societies is thought to promote the development of sophisticated social, cognitive, and communicative skills. Investigating the extent of these skills across taxa is critical to understanding the evolution of the advanced abilities found in some species, including humans. Facial expressions are rich sources of social information for humans and some primates; however whether this is true for other animals is largely unknown. Horses are an ideal study species for these questions: they form valuable social relationships and display some advanced socio-cognitive skills, but are phylogenetically distant from primates and so might be expected to communicate quite differently. Here I present a method for quantifying and coding horse facial movements (EquiFACS), which reveals that horses have an extensive capacity for producing facial expressions. I then utilise EquiFACS to demonstrate that horses produce facial actions that mirror the emotional content of auditory stimuli, providing evidence for a perception-action representation of emotional information. Through my experiments on the perception of facial expressions in horses I show that these expressions display meaningful information to conspecifics, which influences their behaviour in functionally relevant ways. I also shed light on the physiological processes involved in the perception of emotional conspecific facial expressions, showing that viewing negatively valenced conspecific emotional expressions raises resting heart rate. This is indicative of emotional contagion, which may be a mechanism through which information is obtained and social interactions are regulated. Collectively, my research demonstrates the ability to produce and use complex facial expressions as a source of social information is not limited to primates, but is present in at least two phylogenetically distant groups. This suggests these skills may either be an evolutionarily conserved trait or have evolved under common selective pressures. As well as their scientific significance, these findings have implications for horse management and welfare