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.
Highlights
Animal welfare comprises both physical health and emotional well-being
We investigated in horses whether variation in the expression of eye wrinkles caused by contraction of the inner eyebrow raiser reflects emotional valence
Our study was not designed to investigate laterality effects, for further validation of ‘angle’ and possibly other measures of eye wrinkle expression as indicators of emotions in horses, it should be examined whether laterality can affect eye wrinkle expression depending on the valence of the conditions the horses are exposed to. To our knowledge this was the first study systematically investigating the effect of distinctively valenced conditions on eye wrinkle expression in horses
Summary
Animal welfare comprises both physical health and emotional well-being. Whereas physical health can be assessed directly, the subjective experience of emotions must be inferred [1].PLOS ONE | DOI:10.1371/journal.pone.0164017 October 12, 2016Eye Wrinkle Expression in HorsesIdeally, indicators of emotional state are non-invasive and can be assessed and reliably in various contexts.Charles Darwin already observed that ‘man and animals express the same state of mind by the same movements’ ([2], p.352), highlighting facial expressions as potential indicators of animal emotions. Animal welfare comprises both physical health and emotional well-being. Whereas physical health can be assessed directly, the subjective experience of emotions must be inferred [1]. Indicators of emotional state are non-invasive and can be assessed and reliably in various contexts. Charles Darwin already observed that ‘man and animals express the same state of mind by the same movements’ ([2], p.352), highlighting facial expressions as potential indicators of animal emotions. [3,4]) and non-human primates [5]), facial expressions have been largely neglected in other species until more recently While studied extensively in humans (e.g. [3,4]) and non-human primates (e.g. [5]), facial expressions have been largely neglected in other species until more recently (e.g. [6,7])
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