Temperamental Thoroughbreds show a higher hormonal stress response to track workouts than their calmer counterparts, researchers have found.
Exercise is known to increase plasma levels of the stress hormone cortisol in horses.
However, the findings of the Hungarian study raise interesting questions around whether apparent discrepancies seen between some study results using exercise as a stressor are due to a lack of standards in experimental designs or individual-related factors for each horse.
Zsófia Bohák and her colleagues, writing in Acta Veterinaria Hungarica, noted that the effect of temperament on changes in cortisol response to exercise has not been reported in horses so far.
They hypothesized that horses with a more excitable temperament would show a higher cortisol response to the exercise than calmer horses.
“Exercise means not only physiological stress for horses,” they said. “Horses are also exposed to environmental, social, and psychological factors during their daily training.
“Based on our results, responsiveness to these effects may affect the strength of the cortisol response.”
The study team described an experiment in which 20 healthy Thoroughbreds, comprising half stallions and half mares, were assessed under a 25-item questionnaire for characterizing equine temperament on a 7-point scale.
Eight were assessed as temperamental and twelve were assessed as calm.
Each was warmed up for 10 minutes and galloped alone around a 2300m sand track without restraint or encouragement by their regular rider. Blood samples were taken at four intervals to test for plasma cortisol levels.
The first sample was taken at rest, before the experiment, the second after the warm-up, the third after completion of the gallop, and the fourth after a 30-minute recovery period.
Baseline levels of the stress hormone cortisol were no different between the two groups before their individual exercise session, indicating that human presence before the workout did not cause higher stress for temperamental horses than for calm ones.
The authors acknowledged that the exercise intensity was not completely standardized, given that the rider avoided encouraging the horses. However, it turned out that the horses performed similar workouts, with no statistically significant differences in speeds or maximum heart rates between the groups.
A distinct increase in serum cortisol concentrations occurred in both temperamental and calm horses in response to the exercise.
However, differences between the groups were noticeable both after the warm-up and immediately after the track workout, with the more temperamental horses showing a higher cortisol response. Thirty minutes later, cortisol levels were still higher for the temperamental horses.
“The effect of temperament on changes in cortisol in response to exercise has not been reported in horses so far,” the authors said.
“Serum cortisol seems to be a relevant marker to quantify individual differences in racehorses,” they said.
The research team comprised Zsófia Bohák, Ottó Szenci, Andrea Harnos, Orsolya Kutasi and Levente Kovács, variously affiliated with the MTA-SZIE Large Animal Clinical Research Group, the University of Veterinary Medicine, Budapest, and Szent István University, all in Hungary.
Effect of temperament on cortisol response to a single exercise bout in Thoroughbred racehorses – short communication.
Bohák Z, Szenci O, Harnos A, Kutasi O, Kovács L
Acta Vet Hung. 2017 Dec;65(4):541-545. doi: 10.1556/004.2017.052.
The study, published under a Creative Commons License, can be read here.