“We can ask human athletes how they’re handling everything that’s going on—like their preparation for Olympic trials or how they’re finding their training regime—but we can’t ask a horse,” said Amy Holtby, PhD, at Plusvital in Dublin, Ireland. “The only thing we can do is make assumptions based on things that we perceive to be true.”

Results from her team’s study provide evidence that even within the same breed and similar bloodlines, horses can have individual personality differences that could significantly influence their health, welfare, and performance as they go through training, she explained.

“Some of those differences are actually in the DNA,” Holtby said. “And knowing that might increase awareness and inform our decision-making about questions like: Are we giving them the best opportunities to thrive in their environment?”

The Stress of Entering the Racehorse Training World

At the start of training, young Thoroughbreds are exposed to potential stressors related to changes in environment, social structure, housing, diet, and exercise. While some of the horses appear to cope well with those changes—showing initial stress reactions but gradually settling in and adapting to the new routine—others continue to experience stress over extended periods, Holtby said.

“Some of us (humans) deal with situations more comfortably than others—and with horses it’s similar,” she said. “They’re individuals, and how they respond to the same event can be very different.”

Prolonged stress can affect horses’ physical and mental health, Holtby said. It could also lead to training delays and, ultimately, affect their performance.

Breeders might already select for horses that cope well based on behavior, she explained. But identifying the specific genes related to such personality differences could improve genetic selection and help determine which horses have the best DNA for coping with the racing environment.

Comparing DNA With Stress Hormones and Behavior Assessments

To find the genes responsible for racehorse personality, Holtby and her colleagues first sent a questionnaire to three senior handlers, asking them to describe the coping behavior in 96 young Thoroughbreds they trained and managed at the same flat-racing yard in Ireland. All the horses—48 colts and 48 fillies—were born in 2017 and were yearlings at the time of the study.

The researchers also took salivary samples from 34 of these horses—22 colts and 12 fillies—to test their concentrations of cortisol—the “stress hormone”— shortly before and about 30 minutes after the first time they had a rider on their backs.

The team then ran genomic analyses on all 96 horses, which allowed them to identify genes known to be responsible for fear responses and behavior traits in other species.

Poor Link Between Stress Hormone and Behavior

Somewhat unexpectedly, the study horses’ cortisol results didn’t always match up to the questionnaire responses—meaning some horses that showed high stress levels in their saliva samples exhibited good coping behavior, and vice versa, Holtby said.

That could be due to an “anticipatory response” at a molecular level that comes before any true physical reaction to stress, she explained. As such, it’s important to decipher which is more relevant in horse training—the cortisol reaction or the behavioral reaction—through additional research, she said.

“Actually, it probably isn’t that surprising because, really, the work captured all the facets of the horses’ responses to what was going on, and those measures just show the complexity of behavioral traits and the fact that there are things going on, on the surface, that may be visible, and there are also things going on underneath,” she said.

A Gene for Better Coping to Training Stress?

Regarding their genomic analyses, the researchers found relatively strong associations between certain genes and the results from the cortisol and behavior assessments, said Holtby.

In general, the DNA regions having a high level of significance with regard to either coping behavior or cortisol concentrations were near genes that have already been associated with personality and behavior, she said.

Previous study results have linked those genes—specifically, GABARAP, NDM, OAZ1, RPS15A, SPARCL1, VAMP, CEBPA, COA3, DUSP1, HNRNPH1, and RACK1—with other such biological social behavior functions as the autism spectrum, suicide, stress-induced anxiety and depression, Alzheimer’s disease, neurodevelopmental disorders, neuroinflammatory disease, fear-induced behaviors, panic disorders, and alcohol and cocaine addiction, said Holtby. The team found the strongest association with the NDN gene, which is known to be connected to temperament in cows.

While the research hasn’t homed in on a specific “coping gene” or “personality gene” that’s ideal fit for Thoroughbred flat racers, it strongly supports the theory that such genes exist, she said. And those genes certainly play a critical role in Thoroughbreds’ ability to adapt to their training environment and, therefore, represent genetic markers that could improve racehorse welfare.

“This is by no means a silver bullet, but it will contribute to a greater understanding and being more informed about how we make selection decisions—whether it’s management or breeding or maybe even changes in routine or training,” Holtby said. “So while more research is required, the idea would be that these results could be developed into a screening tool to help give horses the correct opportunities, for them.”

Further studies building on this research could investigate similarly beneficial traits for jump-racing horses and sport horses of various disciplines, she added.

The findings could also lead to a better image for racing as trainers make selections based on genes associated with better coping, said Holtby. “Because not only will this improve animal welfare but also the social license to operate, which is hugely important,” she said.

The study, “Integrative genomics analysis highlights functionally relevant genes for equine behaviour,” appeared in Animal Genetics in March 2023.