title: Are Eyes a Mirror of the Soul? What Eye Wrinkles Reveal about a Horse's Emotional State.
abstract: Finding valid indicators of emotional states is one of the biggest challenges in animal welfare science. Here, we investigated in horses whether variation in the expression of eye wrinkles caused by contraction of the inner eyebrow raiser reflects emotional valence. By confronting horses with positive and negative conditions, we aimed to induce positive and negative emotional states, hypothesising that positive emotions would reduce whereas negative emotions would increase eye wrinkle expression. Sixteen horses were individually exposed in a balanced order to two positive (grooming, food anticipation) and two negative conditions (food competition, waving a plastic bag). Each condition lasted for 60 seconds and was preceded by a 60 second control phase. Throughout both phases, pictures of the eyes were taken, and for each horse four pictures per condition and phase were randomly selected. Pictures were scored in random order and by two experimenters blind to condition and phase for six outcome measures: qualitative impression, eyelid shape, markedness of the wrinkles, presence of eye white, number of wrinkles, and the angle between the line through the eyeball and the highest wrinkle. The angle decreased during grooming and increased during food competition compared to control phases, whereas the two phases did not differ during food anticipation and the plastic bag condition. No effects on the other outcome measures were detected. Taken together, we have defined a set of measures to assess eye wrinkle expression reliably, of which one measure was affected by the conditions the horses were exposed to. Variation in eye wrinkle expression might provide valuable information on horse welfare but further validation of specific measures across different conditions is needed.
authors.affiliation: Division of Animal Welfare, Vetsuisse Faculty, University of Bern, Bern, Switzerland.|Agroscope, Swiss National Stud Farm, Avenches, Switzerland.
authors.forename: Sara
authors.initials: S
authors.lastname: Hintze
authors.affiliation: Royal (Dick) School of Veterinary Studies, University of Edinburgh, Edinburgh, United Kingdom.
authors.forename: Samantha
authors.initials: S
authors.lastname: Smith
authors.affiliation: Department of Animal and Avian Sciences, University of Maryland, College Park, Maryland, United States of America.
authors.forename: Antonia
authors.initials: A
authors.lastname: Patt
authors.affiliation: Agroscope, Swiss National Stud Farm, Avenches, Switzerland.
authors.forename: Iris
authors.initials: I
authors.lastname: Bachmann
authors.affiliation: Division of Animal Welfare, Vetsuisse Faculty, University of Bern, Bern, Switzerland.
authors.forename: Hanno
authors.initials: H
authors.lastname: Würbel
mesh_terms: D000827:Animal Welfare; D000818:Animals; D004644:Emotions; D005127:Eye Color / Q000502:physiology; D005143:Eyelids / Q000502:physiology; D005247:Feeding Behavior; D005260:Female; D006736:Horses; D008297:Male; D015595:Skin Aging / Q000502:physiology*