Stress in horses can be tricky to decipher. Each individual displays anxiety and stress in different ways. In people, eye blink rate is a good indicator of stress. In horses, blink rate is a measure of arousal in response to external stressors. Other behavioral measures reveal stress in horses: High head carriage, focused orientation of the ears, increased vocalizations and more active mouth movements.
An equine study at the University of Guelph monitored eye blinks and eyelid twitches as a non-invasive measure of stress in horses that could be useful for welfare purposes. [Merkies K, Ready C, Farkas L, Hodder A. Eye Blink Rates and Eyelid Twitches as a Non-Invasive Measure of Stress in the Domestic Horse. Animals Aug 2019, 9, 562; doi:10.3390/ani9080562] The researchers selected several different stressors to test the validity of using blink rate as an appropriate stress measure. These included feed restriction, separation from other horses and a startle test.
Horse Blink Rate Study Parameters
The study used 33 lesson horses in good health. A handler held all horses on a loose line, standing six feet perpendicular to the horse’s right eye. The study then exposed each horse to four random treatments. It measured blink rate and eyelid twitches for three minutes:
- Controls remained in normal turnout, surrounded by or within sight of paddock mates.
- Feed restriction was accomplished by tying the horse in the stall during regular feeding time while neighboring horses were fed.
- Separation was achieved by placing the horse in an isolated testing arena with no visual contact with other horses although they could hear each other.
- The startle test occurred in a similar environment as the separation experiment – horses could not see others but could hear them. Then the horse stood still on a loose lead line while a large ball was thrown six feet in front of the horse.
In the absence of stress, horses blink 8-9 times per minute. Eyelid twitches occur two times per minute.
Results were revealing:
- In the presence of all three stressors, blink rate drops to 5 blinks per minute.
- Eyelid twitches increased to six per minute with stress and were most evident with feed restriction.
- Separation and the startle test were not accompanied by increased eyelid twitches, in contrast to the results during feed restriction.
In summary, both full and half blinks decreased in the “stressful” conditions. Feed restriction elicited the most stress in the tested horses. They demonstrated other stress signs like high head carriage, restlessness, and slightly increased heart rate. If a horse displays decreased blink rate but no increase in eyelid twitches, the authors suggest that the situation might be visually stimulating but that the horse might not perceive it as stressful.