Equine eye wrinkles linked to emotional states by researchers

Can the eye wrinkles of horses provide insights into their emotional state?

A study carried out in Switzerland has found that the “eye wrinkle expression” of horses does indeed change between positive and negative conditions. The researchers have laid out a defined set of measures to reliably assess eye wrinkle expression in horses.

The scientists have described their findings in a paper entitled, “Are eyes a mirror of the soul? What eye wrinkles reveal about a horse’s emotional state”, published this week in the open-access peer-reviewed journal, PLOS ONE.

Researchers Sara Hintze, Samantha Smith, Antonia Patt, Iris Bachmann and Hanno Würbel said finding valid indicators of emotional states was one of the biggest challenges in animal welfare science.

They set out to investigate whether variations in the expression of eye wrinkles in horses caused by contraction of the inner eyebrow raiser muscle reflected their emotional state.

Hintze and her colleagues hypothesized that positive emotions would reduce eye wrinkling whereas negative emotions would increase it.

Sixteen horses from the Swiss National Stud Farm in Avenches, Switzerland, were used in the study. Fifteen were stallions and the other was a mare. The horses had Franches-Montagnes, warmblood and trotter bloodlines.

They were individually exposed in a balanced order to two positive situations (grooming and food anticipation) and two negative conditions (food competition and waving a plastic bag).

Each situation lasted 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.

They were then scored in random order by two experimenters who did not know the circumstances under which each picture was taken.

The pictures were each assessed for qualitative impression, eyelid shape, markedness of the wrinkles, presence of eye white, the 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, the researchers reported.

The narrower angle during grooming and wider angle during food competition compared to the control phases indicated relaxation of the inner eye brow raiser during grooming and contraction of this muscle during food competition.

No effects on the other outcome measures were detected.

“Taken together,” they said, “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.”

They continued: “Variation in eye wrinkle expression might provide valuable information on horse welfare but further validation of specific measures across different conditions is needed.”

The researchers said their study was, to their knowledge, the first to systematically investigate the effect of different emotional conditions on eye wrinkle expression in horses.

In doing, so, they established six measures, of which five allowed the reliable assessment of aspects of eye wrinkle expression. The five were: qualitative assessment of eyelid shape, markedness of eye wrinkles, the presence of eye white, as well as two continuous measures: the number of wrinkles and angle between the line through the eyeball and the highest wrinkle.

“We found that the angle between a horizontal line through the eyeball and the highest wrinkle caused by contraction of the underlying inner eyebrow raiser was consistently affected in two out of the four conditions: Grooming resulted in a narrower angle through muscle relaxation while food competition resulted in a wider angle through muscle contraction. The other outcome measures were not affected by the four conditions.”

They said further research was needed to investigate which aspects of eye wrinkle expression reflected emotional valence in general or were more reflective of specific conditions and thus discrete emotional states.

“Moreover, it is important to assess how longer lasting conditions affect these measures in view of using them as indicators of emotional well-being in horses.

Hintze and Würbel are with the University of Bern in Switzerland; Smith is with the University of Edinburgh; Patt is with the University of Maryland; and Bachmann is with Agroscope in Switzerland.

Hintze S, Smith S, Patt A, Bachmann I, Würbel H (2016) Are Eyes a Mirror of the Soul? What Eye Wrinkles Reveal about a Horse’s Emotional State. PLoS ONE 11(10): e0164017. doi:10.1371/journal.pone.0164017

The full study, published under a Creative Commons License, can be read here.