Researchers who videoed the faces of horses during transport and social isolation identified changes in expression that suggest stress.
Johan Lundblad and his colleagues set out to determine the influence of transportation and social isolation on the facial expressions of healthy horses.
The researchers, writing in the journal PLOS ONE, said horses have the ability to generate a remarkable repertoire of facial expressions, some of which have been linked to pain.
They chose transportation and social isolation for their study, as they are considered stressful to some degree but are also ordinary management procedures.
Twenty-eight horses were taken on a short road trip in a horse trailer. Ten of the horses were also subjected to short-term social isolation.
During all procedures, a body-mounted, remote-controlled heart rate monitor provided continuous heart-rate measurements. The horses’ heads were video-recorded for later analysis.
A substantial dataset was generated from the video, looking for the facial action units described in the Equine Facial Action Coding System (EquiFACS).
EquiFACS, first reported in a scientific paper in 2015, provides a method of identifying and recording facial expressions in horses.
EquiFACS comprises 17 “action units” – or discrete facial movements – in horses. This compares with 27 in humans, 13 in chimpanzees and 16 in dogs.
The authors said heart rate increased during both transport and social isolation, confirming that they caused disruption to the sympato-vagal balance.
Several expression changes observed during both interventions were indicative of stress: An increase in the amount of visible eye white; flared nostrils; raising of the upper eyelid; raising the inner brow; and showing the tongue.
There was also an increase in ear flicking and blink frequency.
“Most facial features identified correspond well with previous findings on behaviors of stressed horses,” the researchers said. Some, such as dilated nostrils, eye white increase and raising of the inner brow eyebrow, are used as indicators of pain in some face-based pain assessment tools, they noted.
“In order to increase performance parameters in pain assessment tools, the relations between facial expressions of stress and pain should be studied further.”
The study team also used the EquiFACS codes to train a machine learning classifier to discriminate between the high-arousal interventions and calm horses, which achieved at most 79% accuracy.
“It proved possible,” they said, “to induce and objectively record the presence of facial expressions in healthy horses under field conditions, using simple equipment and ordinary management practices.”
The study team comprised Lundblad, Marie Rhodin and Pia Haubro Andersen, all with the Swedish University of Agricultural Sciences in Uppsala, Sweden; and Maheen Rashid, with the University of California, Davis.
Lundblad J, Rashid M, Rhodin M, Haubro Andersen P (2021) Effect of transportation and social isolation on facial expressions of healthy horses. PLoS ONE 16(6): e0241532. https://doi.org/10.1371/journal.pone.0241532