Just as people can be either left- or right-handed, a similar phenomenon exists among horses. This preferred usage of one side of the body is correctly termed laterality.
Most commonly, scientists assess which leg a horse prefers to place forward when grazing. A foal’s long legs can prevent his muzzle reaching the ground, so he will place one foreleg in front and one behind. This stance becomes habit; while he will switch leading legs, he quickly develops a favourite.
The selective breeding of sport horses for longer legs and a smaller head means that many graze with one foreleg much further forward, even as adults. The advanced foot typically develops a longer toe and a collapsed heel, while the foot placed behind becomes more upright in shape. What effect this has on a horse’s soundness, symmetry of movement and balance, we don’t yet know.
During repeated walk-to-halt transitions you may notice that your horse prefers to stop with one foreleg last. This foot is then usually the first to take a forward step, unless the horse has been standing for a while.
Another way to look at laterality is to evaluate a horse’s reaction to a novel object. The right brain hemisphere is associated with the fight or flight response, while the left is linked to more investigative responses. Each hemisphere connects to the opposite side of the body.
A horse who chooses to investigate something with his left eye is, therefore, using the right brain hemisphere, which has been shown to result in a more emotional response than using the right eye. Rodents tend to turn left when stressed, and we already know that most horses who spook or rear tend to turn or land to the left — so is there a link between laterality and emotional state?
Food for thought
One aspect of emotion is cognitive bias, which refers to how optimistic or pessimistic a horse may be.
This can be tested by training the horse to stand behind a barrier with a bucket visible to either his left or right. The bucket should contain food when placed on one side but remain empty when on the other. After a few training sessions, the horse will rush to the bucket on the “food side” but may not even approach if it is on the “empty side”.
When a single bucket is then placed in the centre, an optimistic horse will reach it in a similar time to when it is on the food side — while a more pessimistic horse will take longer.
Scientists recently tried to identify a link between laterality and cognitive bias in horses, by first noting the preferred foreleg placed forwards as they grazed. They then ran the cognitive bias test, using boxes instead of buckets, noting which leg each horse used to take his first step when released and which was placed forward on reaching the box.
The horses stepping first with their right foreleg when the box was placed centrally were more likely to be optimistic — that is, they took the same time to reach the box as when it was in a location where they knew it contained food.
Those using the left foreleg (and so the right brain hemisphere) took longer, suggesting that they thought it less likely that they would find food inside.
Laterality and cognitive bias are exciting new areas of research, although more work is needed before we can be confident about connections between them.
Ref Horse & Hound; 7 March 2019