Eyelid activity shows promise as an indicator of stress in horses, the findings of a Canadian suggest.
Katrina Merkies and her colleagues at the University of Guelph have described an experiment in which they monitored blinking and eyelid twitching in horses placed in stressful situations.
The study team, writing in the open-access journal Animals, noted that eye blink rates have been used as an indicator of stress in humans.
Because it is non‐invasive, it could be useful to measure stress in horses, they said.
Horses are known to exhibit both full and half-blinks, as well as eyelid twitches.
For their study, the researchers exposed 33 horses to stressful situations – separation from herd-mates, denial of access to feed at their regular feeding time, and the sudden introduction of a novel object, a ball, thrown in front of them.
The heart rates of the horses were also monitored, as was their behaviour.
Analysis of video recordings of the horses showed that full and half eye blinks decreased under these scenarios when compared to the control situation, in which the horses were monitored in their normal paddock environment.
“Feed restriction was the most stressful for the horse as indicated by increased heart rate, restless behaviour and high head position,” they reported.
“The decrease in eye blink rate during feed restriction was paralleled with an increase in eyelid twitches.”
On average, horses performed full blinks 8–9 times per minute in the absence of any stressors. This rate decreased to 5 blinks per minute in the presence of any external stressors.
Conversely, eyelid twitches increased from about two per minute in the control situation to six per minute during feed restriction. There was no increase in eyelid twitches or heart rate with the other scenarios, indicating the horses did not find these overly stressful. However, they did focus their attention more during these situations.
Discussing their findings, the study team said further research investigating specific eyelid movements in relation to level of arousal could provide insight into the emotional responses of horses.
“For example, in humans, facial electromyography has been successfully used to correlate facial muscle activation to positive or negative emotions.
“Since we cannot ask horses to self‐report how they are feeling, physiological measures that differentiate between pleasant and unpleasant experiences may allow us to infer underlying emotions.”
Further investigation of changes in spontaneous blink rate and eyelid twitches over varying time spans is needed to identify patterns.
In conclusion, they said that horses exposed to stressful environments decreased their spontaneous eye blink rate and increased the frequency of eyelid twitches.
“However, if the environment is simply visually stimulating, eyelid twitches do not appear to increase even if eye blink rate decreases.”
Monitoring of the spontaneous blink rate proved to be a sensitive metric of neural activity, they said.
“Observation of eye blinks and eyelid twitches can provide important information on the stress level of horses,” they concluded.
The full University of Guelph study team comprised Merkies, Chloe Ready, Leanne Farkas and Abigail Hodder.
Eye Blink Rates and Eyelid Twitches as a Non-Invasive Measure of Stress in the Domestic Horse
Katrina Merkies, Chloe Ready, Leanne Farkas and Abigail Hodder.
Animals 2019, 9(8), 562; https://doi.org/10.3390/ani9080562