Tenseness Relaxed by Vocalizing, Illustrated by Horses (Equus caballus) Whinnying

Animal calls are commonly considered indicators of a sudden change in their arousal state. However, reports evidencing associated internal physiological changes are rare. By homology with human "emotional" vocal productions (e.g., laughter and crying), we predicted that animal vocal productions may result in relaxing tenseness or excitement. In this study, 15 stallions equipped with a heart rate monitor were presented a mare at some distance so as to prevent contact, thereby increasing the males' arousal. Stallions' heart rates increased just before whinnying but returned to baseline rates while vocalizing. We found that sudden changes of a caller's arousal state could be associated with vocalizing, and this opens new lines for noninvasive research concerning the self-regulation of emotional via vocal production in humans and animals.