Blink of an eye: stress level indicator?

Equine behavioural studies often seek to assess the horse’s stress response to various stimuli. Physiological methods such as heart rate, heart rate variability, salivary cortisol concentration all have their place, but for some purposes a more hands-off approach might be preferable.

A study at the University of Guelph, Ontario, Canada looked at the correlation of eyelid movements with heart rate and behaviour. The full report is published in Animals (Basel).

Dr Katrina Merkies and colleagues studied the response of 33 horses to three stressful situations: being separated from their herd mates; not being fed at their normal feeding time; and being startled by a novel object. The responses were compared with those of the horses in their normal paddock environment.

To assess the horse’s response, the researchers monitored heart rate, and counted the number of eye blinks and eyelid twitches. (Each three-minute trial was recorded on video.)

They found that feed restriction was the most stressful situation. Horses showed increased heart rate, restless behaviour and high head position. The horses also blinked less often and had an increased frequency of eyelid twitches, compared with the normal (control) environment.

Neither the novel object test, nor the separation from herd mates test, produced an increase in eyelid twitches or heart rate, which suggests that the horses did not find these too stressful.

The authors conclude that “observation of eye blinks and eyelid twitches can provide important information on the stress level of horses, with a decrease in eye blinks and an increase in eyelid twitches in stressful environments.”

For more details, see:

Eye Blink Rates and Eyelid Twitches as a Non-Invasive Measure of Stress in the Domestic Horse.

Merkies K, Ready C, Farkas L, Hodder A.

Animals (Basel). 2019 Aug 15;9(8). pii: E562.

doi:10.3390/ani9080562