Effect of aging on behavioural and physiological responses to a stressful stimulus in horses (Equus caballus)

In humans the expression of emotions varies with aging, but for domestic animals we have little information on this subject. Our aim was to verify the influence of aging (life experience) on the expression of emotions in horses (Equus caballus) in reaction to a stressful stimulus. A total of 98 horses were subjected to the sudden inflation of a balloon in a familiar environment for a period of 5 min, on the assumption that this would induce a negative state. When the balloon was inflated, heart rate variability as an indicator of stress response was monitored. Behaviour related to emotional expression (latency of onset, frequency and length of looking and exploration, frequency of avoidance) was also monitored. The younger horses showed a significantly higher frequency of avoidance as well as greater exploratory activity, while the older subjects were less behaviourally responsive and showed a shift toward control by the sympathetic nervous system, as indicated by lower heart rate variability. The results suggest that aging influences the behaviour and physiology of emotional expression to stressful stimuli in horses. Knowing the effect of aging on stress response could be a critical factor in understanding equine welfare and the development of behavioural patterns.