Horses' Eye Blink Rate Could Reveal Stress Levels
While cortisol measurements have long been the gold standard in measuring stress in horses, getting those measurements can be stressful in and of itself. As researchers look into less invasive ways to study stress levels, a new method has recently caught their eye: blinking rates.

“Horses in our study showed a reduced amount of half and full eye blinks along with an increase in eyelid flutters when exposed to certain stressful situations,” said Katrina Merkies, PhD, associate professor and equine program coordinator at the University of Guelph, in Ontario, Canada. She presented the work of her student, Amelia Garnett, BSc candidate, during the 2017 International Society for Equitation Science Symposium, held Nov. 22-26 in Wagga Wagga, Australia.

Merkies and Garnett investigated 23 horses’ blinking characteristics when exposed to three kinds of stressful situations: separation, feed restriction, and sudden scary object. In separation, they took the horse away from other horses where he could no longer see them. For feed restriction, they withheld the horse’s meal at the regular feeding time. And for the sudden scary object test, the researchers threw a ball in front of the horse while he was alone in an arena.

The scientists recorded the horses’ eye movements on video during these tests, as well as while the horses were at rest in the paddock with other horses, as a basis of comparison. To check their stress test’s accuracy with blink rate, they also recorded cardiac data with a heart monitor and evaluated the horses’ behavioral responses.

They found that full blinks and half blinks (any movements of the upper eyelid that did not lead to fully closing the eye) decreased significantly during the stressful situations, Merkies said. But eye flutters (a kind of rapid muscle movement in the “eyebrow” area) increased—especially during feed restriction.

“The eye flutters are reminiscent of what we see in the horse grimace scale (HGS) with regard to what they call a triangular eye,” she said. “Horses had more worry wrinkles or piqued eye shape when they were in pain in the HGS study, and that’s similar to what we saw in our study with the eye flutters—likening the stress to worry or pain.”

Eye blink rate has already been used as a measure of studying stress levels in humans and in cattle, but scientists have rarely evaluated it in horses. Merkies and Garnett first presented the concept in a poster at the 2014 International Society for Equitation Science conference.

“These are preliminary results, but they indicate that eye blink rate could probably be used in the future as a measure to assess welfare in horses,” Merkies said.