Abstract
The Criollo is an Argentine horse breed with a calm temperament. Although its temperament is considered to be related to its neurophysiological characteristics, the details of this are unknown. Therefore, we analyzed the heart rate variability in Criollos as a preliminary study to deepen the neurophysiological understanding of their autonomic function. Electrocardiograms were recorded from Criollos and Thoroughbreds, and the power spectrum of heart rate variability was analyzed. Compared with Thoroughbreds, Criollos showed (i) a significantly higher high-frequency component, which is an index of parasympathetic nerve activity, and (ii) tendency toward a lower ratio of low- to high-frequency power, which is an index of the autonomic balance. These results revealed that parasympathetic nerves might be more active in Criollos compared with Thoroughbreds.