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PloS one2016; 11(10); e0164017; doi: 10.1371/journal.pone.0164017

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.
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Publication Date: 2016-10-12 PubMed ID: 27732647PubMed Central: PMC5061373DOI: 10.1371/journal.pone.0164017Google Scholar: Lookup
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Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

This research investigates if the expression of eye wrinkles in horses can be indicative of their emotional state. The researchers hypothesized that positive emotions would decrease eye wrinkle expression and negative emotions would increase it. The study did show changes in eye wrinkle expression under different conditions, reinforcing the idea that it may be a valuable indicator of horse welfare.

Research Methodology

This study involved a set of 16 horses that were individually subjected to different emotional conditions:

  • Two positive conditions: Grooming and food anticipation
  • Two negative conditions: Food competition and wavering a plastic bag

Each condition lasted 60 seconds and was preceded by a 60-second control phase. Photos of the horse’s eyes were taken during both phases.

Eye Wrinkle Evaluation

Four pictures per condition and phase were randomly selected for each horse. These pictures were then scored by two experimenters who were blind to the condition and phase. Six outcome measures were used to assess the eye wrinkle expression:

  • Qualitative impression
  • Eyelid shape
  • Markedness of the wrinkles
  • Presence of eye white
  • Number of wrinkles
  • The angle between the line through the eyeball and the highest wrinkle

Research Findings

The research found that the angle of the eye wrinkles reduced during grooming and increased during food competition, as compared to the control phases. However, there was no difference during food anticipation or exposure to the plastic bag.

Interestingly, none of the other outcome measures showed any significant changes across the different emotional states.

Conclusions and Further Research

Overall, the study suggests that eye wrinkle expression in horses may serve as a measure of animal welfare. Specifically, the research demonstrated that variation in the angle of eye wrinkles can reflect different emotional conditions.

However, the authors do caution that further validation of this specific measure is required across different conditions. Further research also needs to be conducted to understand whether the other outcome measures can provide similar insights into a horse’s emotional state.

Cite This Article

APA
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. https://doi.org/10.1371/journal.pone.0164017

Publication

PloS one
ISSN: 1932-6203
NlmUniqueID: 101285081
Country: United States
Language: English
Volume: 11
Issue: 10
Pages: e0164017
PII: e0164017

Researcher Affiliations

Hintze, Sara
  • Division of Animal Welfare, Vetsuisse Faculty, University of Bern, Bern, Switzerland.
  • Agroscope, Swiss National Stud Farm, Avenches, Switzerland.
Smith, Samantha
  • Royal (Dick) School of Veterinary Studies, University of Edinburgh, Edinburgh, United Kingdom.
Patt, Antonia
  • Department of Animal and Avian Sciences, University of Maryland, College Park, Maryland, United States of America.
Bachmann, Iris
  • Agroscope, Swiss National Stud Farm, Avenches, Switzerland.
Würbel, Hanno
  • Division of Animal Welfare, Vetsuisse Faculty, University of Bern, Bern, Switzerland.

MeSH Terms

  • Animal Welfare
  • Animals
  • Emotions
  • Eye Color / physiology
  • Eyelids / physiology
  • Feeding Behavior
  • Female
  • Horses
  • Male
  • Skin Aging / physiology

Conflict of Interest Statement

The authors have declared that no competing interests exist.

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