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.

Sex and age don’t matter but breed type does - Factors influencing eye wrinkle expression in horses

“…However, 407 other explanations need to be considered as well. First, individual horses may have reacted 408 differently to the halter, the human handling and/or the photographing, even though we did not 409 see indications in the horses' behaviour supporting this assumption (in contrast to the differing 410 reactions of the horses to the photographers and the equipment described in Hintze et al, 2016). 411…”

“…Comparison between the outcome measures used in the study byHintze et al (2016) and in the present study.…”

“…In the present study, we used an adapted version of the eye wrinkle assessment scale developed 167 by Hintze et al (2016) resulting in five outcome measures: 'qualitative assessment', 'eyebrow 168 raised', 'number', 'markedness' and 'angle' (for definitions and scoring details see Fig. 1).…”

“…Hintze and colleagues (for a direct comparison of the two scales see Table 1). For the outcome 171 measure 'qualitative assessment' (A), we adapted the definition by Hintze et al (2016): The 172 original definition focused on three particular aspects of the eye wrinkle expression, namely 173 "the number […], markedness, and the angle" (Hintze et al, 2016, p. 6), whereas in the present 174 study we aimed to better capture the overall first subjective impression with respect to how 175 'worried' the horse actually looks ('not worried' to 'extremely worried') without focusing on 176 other aspects at this stage. Moreover, instead of using an ordinal scale with three distinct 10 more sensitive measures (Tuyttens et al, 2009).…”

“…16Therefore, the aim of this study was to assess how age, sex, breed type, body condition and 17 coat colour affect the expression and/or the assessment of eye wrinkles in horses. To this end, 18we adapted the eye wrinkle assessment scale from Hintze et al (2016) and assessed eye wrinkle 19 expression on pictures taken from the left and the right eye of 181 horses in a presumably 20 neutral situation, using five outcome measures: a qualitative first impression reflecting how 21 worried the horse looks, the extent to which the eyebrow is raised, the number of wrinkles, 22 their markedness and the angle between a line through the eyeball and the topmost wrinkle. All 23 measures could be assessed highly reliable with respect to intra-and inter-observer agreement.…”