The spontaneous blink rate is a valid and fast alternative measure of stress in horses, researchers have found.
However, they say the initial “startle” response must be considered when using this parameter as a measure of stress.
Researchers Richard Mott, Susan Hawthorne and Sebastian McBride, reporting in the journal Scientific Reports, said measuring animal stress is fundamental in assessing the emotional state and welfare of animals.
Conventional methods — measuring the stress hormone cortisol and monitoring heart rate and heart-rate variability — require specialist equipment.
They noted that the spontaneous blink rate has previously been used to measure stress responses in humans. As such, it may provide a non-invasive way of measuring stress in other species.
Spontaneous blinks have several functions, one being corneal lubrication. They also appear important for resetting eye movements. Blinks also reflect cognitive processes during attention.
Mott and his colleagues said the link between spontaneous blinks and stress appears to be mediated by the release of the neurotransmitter dopamine.
For their study, the trio set about investigating the potential use of the blink rate to evaluate stress in domestic horses.
They noted that although most evidence demonstrates that the blink rate increases during stress, other work in horses has reported a decrease when confronted by a potentially stressful event.
“Human studies have shown a reduction in spontaneous blink rate during periods of attention and cognitive effort.
“Potentially threatening stimuli may require cognitive effort to verify it as a threat thus reducing the spontaneous blink rate at the early stage of stressor presentation.”
This, they said, suggested that the blink-rate response to stress could vary depending on when it is measured in relation to the stressor exposure and the type of stressor.
Their work centered on 33 fit riding school horses stabled at Warwickshire College.
Of the 33 horses, 16 were categorised by animal staff as reactive when exposed to the sight and sound of hair clippers and 17 were categorised as non-reactive, based on their previous response to the clippers.
Each horse acted as its own control, with baseline heart readings obtained and salivary samples taken for cortisol measurements in the 15 minutes before the animals were given a 10-minute sham clipping. The left eye of each horse was similarly monitored beforehand to determine the blink rate, with filming also occurring.
These baseline results were compared with those obtained during the sham clipping procedure.
For all horses, there was a reduction in blinking during the first minute of clipping, which the researchers characterised as a startle response.
For the horses reactive to clipping, the initial reduction in blinking was followed by an increase above baseline.
In the non-reactive horses, the blink rate quickly returned to baseline.
For all horses, the blink rate correlated with heart-rate variability and salivary cortisol.
“We have demonstrated that the spontaneous blink rate is a valid, fast alternative measure of stress in horses,” they said, “but the initial ‘startle’ response must be considered when using this parameter as a measure of animal stress.
“The duration of the startle response may vary significantly between low and high-stress reactivity animals,” they said.
They described the blink rate as a simple, cost-free, instantaneous measure of acute equine stress that can support welfare assessment.
The researchers pointed to the significant correlation of blink rate with conventional measures of stress associated with the activation of the hypothalamic-pituitary-adrenal axis and the sympathetic-adreno-medullar axis.
They said a better understanding of the applied use of the blink-rate response, in horses and other species will be improved by future research assessing the effects of positive stimuli on changes in the blink rate, and also further validation work against conventional measures of stress.
Mott is with the Royal (Dick) School of Veterinary Studies at the University of Edinburgh; Hawthorne is with the School of Pharmacy and Pharmaceutical Sciences at Ulster University; and McBride is with Aberystwyth University in Wales.
Mott, R.O., Hawthorne, S.J. & McBride, S.D. Blink rate as a measure of stress and attention in the domestic horse (Equus caballus). Sci Rep 10, 21409 (2020). https://doi.org/10.1038/s41598-020-78386-z