According to new study results from French equine behavior scientists, stress affects horses' working memory--the short-term memory used in training and in associating signals and commands with actions--especially if the horse already has a basic fearful nature.

"We observed that stress had a strong deleterious effect on working memory performance," said Mathilde Valenchon, MS, PhD candidate with the behavior, neurobiology, and adaptation team at the French national agricultural science institute in Tours. Valenchon presented her findings at the French Equine Research Day held March 1 in Paris. "From a practical point of view, this means that a horse under stress could have a compromised capacity to respond to the demands of his trainer, especially if working memory is required."

In her investigation Valenchon and colleagues tested 30 Welsh pony mares' ability to remember in which of two buckets they would find a carrot. The researcher would stand with one bucket on each side of her. She would drop a carrot into one of the buckets randomly, and the horse would have to wait between zero and 20 seconds before going to the bucket to retrieve the carrot. The researchers tested the horses first in a calm, familiar environment, and then later in a more stressful environment with unfamiliar sights and sounds (including a barking dog and a waving white sheet, among other stressors). The horses also underwent temperament tests to determine their basic fear factors, to see what role a fearful temperament had in working memory.

The team found that in the calmer environment the horses had an average working memory of 16 seconds--meaning they could still find the carrot after a 16-second delay--but in the stressful environment the horses weren't able to perform at all.

They also found that while the more fearful horses had much worse performance under stressful conditions--frequently unable to find the carrot even after only a four-second delay--these horses actually had the highest performance in the calm environment.

"This last phenomenon actually isn't new with animals," Valenchon said during her presentation. "The same results were also seen in fearful or anxious mice versus calm mice. Our hypothesis is that fearful horses always have a little bit of stress, and in fact, a little bit of stress is actually helpful for the working memory. But when actual stressful factors are added, their stress level goes beyond what they can manage in order to maintain good working memory."

In all cases the horses had a tendency to pay less attention to the carrot and the buckets when stressed, Valenchon added. Thus, the stress affected working memory but also the ability to concentrate on the task at hand, and perhaps even physiological factors because the horses were no longer interested in food.

"It's also worth noting that we saw a huge difference from one horse to another," Valenchon said. "So it's really worthwhile for trainers to treat horses individually, recognizing whether they're fearful or not, and how they react to stress, in order to optimize and adapt the training program to each individual horse."