The Modern Equine Vet
November 2023
Vol 13 Issue 11 2023
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Why Are Some Mares More Reactive Than Others?

By Tom Rosenthal

Older Thoroughbred broodmares were more likely to have fearful responses to the stress of encountering unfamiliar objects than younger broodmares who recently retired from their careers, according to research presented at the BEVA Congress, held in Liverpool, England.

And while the foal is at foot, it appears to mimic the mothers’ startled reactions to the novel objects, said Alexandra Moss, DVM, an internal medicine specialist currently working on her PhD in clinical genomics at the University College Dublin. Whether this similarity in response is maintained into adulthood is soon to be investigated.

Dr. Moss hypothesized older broodmares, who have had more opportunity for environmental exposures would have a lower startle response than younger mares, but found the opposite to be true, perhaps, because the younger mares were more used to unfamiliar situations due to their recent careers, while an older mare had spent more time away from these stressful situations.

“What was interesting for us was that we actually found with increasing age, we were getting a higher startle response time and a lower behavior score (indicative of increased stress-associated behaviors), which was actually counter to what we would expect when you think older horses are more likely to have experienced more in their lives,” Dr. Moss said.

She posited if this was because the longer the broodmares have been out of racing, the less exposure they have to unknown objects so, were more startled when encountering novel objects.

The study is part of an overall effort to examine the concept of behavioral plasticity—the ability of animals to adapt to environmental stimuli with a reduced stress response, Dr. Moss explained. “Some animals cope better with stress than others. We don’t necessarily know why. But we’re interested in that, and our group in particular is interested in if there is an epigenomic contribution to that difference in stress response.”

Dr. Moss said that much of the research conducted in this field has been done in the study of people.

“What we know is that early life stress leads to significant deleterious neurobiological effects, specifically on the hippocampus,” she said. “And the hippocampus is crucial in life for regulating emotional responses, but also in forming learning pathways. And so that can lead to poor stress adaptation later in life.”

Dr. Moss said that when it comes to horses, “obviously that’s a welfare concern, but they’re also performance animals, so that has significant economic consequences as well.”

She added, “Our aims were to test the hypotheses that fearfulness could be established in Thoroughbred broodmares using both subjective and objective responses to a novel object. And we were particularly interested in age, parity and the time since they were last raced and whether these could be used as predictors for the level of fearfulness in these horses.”

The study involved 25 Thoroughbred broodmares ranging from 5 to 16 years old that were outfitted with an ECG heart rate monitor system and let loose in a 5-by-5-meter box. Seventeen were multiparous, having had 2 or more foals, previously, and 8 were maiden or had only 1 foal previously.

Their responses to the introduction to a novel object—a self-opening bright red umbrella—were videotaped for 5 minutes. The ECG recorded the horses’ responses during the 5 minutes and a 30-minute recovery period. And the tapes were reviewed by blinded observers. All the mares were introduced to the umbrella before it opened.

The behavior score and the startle response time, which averaged about 13 seconds, were proxies for stress response, and the positive linear relationship found with heart rate variability data obtained during the exposure validated their use, according to Dr. Moss. The older mares tended to be more reactive and took longer to settle down than the younger ones.

The mean stress response was 12.8 plus or minus 11.9 sec, mean peak heart rate was 110 plus or minus 34.3 beats per minute. Both the stress response and behavior were moderately correlated to age (r=0.49, P=0.01) and (r= 0.53, P=0.007).

“Ultimately what we can conclude from that is that subjective and objective behavioral responses in our cohort at least can be used to classify the Thoroughbred’s response for fearfulness response to a novel object,” she said.

Dr. Moss noted that the foals emulated their mothers’ behaviors in responding to an unknown object but that by the time the yearlings had reached 18 months, anecdotally, they appeared to be moving away from that behavior. MeV

The study was funded by the Science Foundation, Ireland, the Frontiers for the Future Program, and a University College Dublin Veterinary School Summer Research Grant.