Horses communicate primarily using body language and they display a number of facial expressions. 

Their eyes, in particular, can be very expressive and are thought to reveal how a horse is feeling. Many equine behaviourists and horse owners believe the wrinkles above a horse’s eye may be associated with discomfort, fear, stress or anxiety – they are often called ‘worry lines’. However, until recently, there has never been any scientific study researching the links between these facial expressions and the horse’s emotional state.

A team of researchers from Switzerland, the UK and the USA set out to examine whether eye wrinkles are caused by negative emotional states in the horse, or whether it was simply a case of humans interpreting these wrinkles as they would for themselves.

The wrinkles studied appear above the upper eyelid are a result of the contraction of the underlying inner eyebrow raiser muscles. These eye wrinkles are commonly seen, but may differ in shape, number and severity in different individual horses.

The team placed 16 horses at the Swiss National Stud Farm into two ‘positive’ situations:
* Anticipation of a food reward;
* Being stroked.

And into two ‘negative’ situations:
* Competing for food;
* Having a plastic bag waved nearby. 

These four test conditions were chosen as they were situations the horses were reasonably likely to encounter in their day-to-day management.

Photos of the horses’ eyes were taken while they were tested and the images analysed based on overall impression, shape of the eyelid, eye whites, number of wrinkles, markedness and angle. 

In the situations presumed to be positive, the researchers found that while stroking did reduce the expression of eye wrinkles, wrinkles were sometimes seen when food was present. This may have been as a result of the horses being frustrated around food. The negative situations did increase the likelihood of the eye wrinkles occurring. 

The researchers found the angle of the wrinkles changed, depending on the situation. The angle was wider in negative situations, compared to positive, due to a stronger contraction of the inner eyebrow raiser. More eye white was seen in negative situations. 

The number and depth of wrinkles and how much white of the eye was shown revealed that some characteristics of eye wrinkling were affected by different emotional states. 

Researchers concluded that a horse’s emotional state could be linked with their eye wrinkle expression and therefore this could be a potential indicator of horse welfare. However, the team indicated that further research was needed.

Research team: Sara Hintze (University of Bern and Agroscope, Swiss National Stud Farm, Switzerland); Samantha Smith (University of Edinburgh, United Kingdom); Antonia Patt (University of Maryland, United States); Iris Bachmann (Agroscope, Switzerland); Hanno Würbel (University of Bern, Switzerland).