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

Abstract

Identifying valid indicators to assess animals emotional states is a critical objective of animal welfare science. In horses, eye wrinkles caused by the contraction of the inner eyebrow raiser have been shown to be affected by pain and other emotional states. Whether individual characteristics of a horse systematically affect eye wrinkle expression has not yet been studied. Therefore, the aim of this study was to assess how age, sex, breed type, body condition and coat colour affect the expression and/or the assessment of eye wrinkles in horses. To this end, we adapted the eye wrinkle assessment scale from Hintze et al. (2016) and assessed eye wrinkle expression on pictures taken from the left and the right eye of 181 horses in a presumably neutral situation, using five outcome measures: a qualitative first impression reflecting how worried the horse looks, the extent to which the eyebrow is raised, the number of wrinkles, their markedness and the angle between a line through the eyeball and the topmost wrinkle. All measures could be assessed highly reliable with respect to intra- and inter-observer agreement. Breed type affected the width of the angle (F2, 114 = 8.20, p 0.05). Consequently, horses characteristics age, sex and coat colour did not systematically affect eye wrinkle expression, whereas breed type explained some variation in angle; how much eye wrinkle expression is affected by emotion or perhaps mood needs further investigation and validation.