Different equestrian sports horses’ reactivity to startle

Management of athletic horses for a particular sport modality can affect their reactivity to daily stress situations. The present study aimed to compare the stress reactivity in horses of different equestrian sports (polo, endurance, and dressage) through the evaluation of endocrine (cortisol levels), autonomic (heart rate variability) and behavioural responses to a model of startle. Male and female equines of 3 equestrian sports (polo, Thoroughbred, n = 6; dressage, Brazilian Sport Horse Riding, n = 9; endurance, Arabian Purebred, n = 9) were subjected to an experimental model of startle in which an umbrella was opened near the horse. Heart rate variability was analyzed using segments of cardiac intervals of 64 seconds (ultra-short analysis) and the HRV spectra were integrated into low (LF; 0.01-0.07 Hz; index of sympathetic modulation) and high (HF; 0.07-0.50 Hz; index of parasympathetic modulation) frequency bands. Cortisol was measured by radioimmunoassay and the behavioral response was measured by the latency and the duration of the reaction. The Endurance horses showed a less intense startle-induced autonomic response compared to the Polo and Dressage horses (lower LF/HF ratio following the startle test) and higher startle-induced increase in cortisol levels than the Polo horses. The Polo horses had higher baseline levels of cortisol than the other 2 groups and exhibited no change in their cortisol levels in response to startle. The behavioral response following the startle test was similar among the 3 groups. These results reinforce the importance of a combined analysis of behavioral, endocrine, and autonomic data for a global and integrated view of stress reactivity.