Vagal tone

The level of activity in the vagus nerve, which carries parasympathetic nervous system signals from the brain to the heart and gut.

The vagus nerve is the major nerve of the parasympathetic nervous system, responsible for the “rest and digest” state that counterbalances the sympathetic “fight or flight” response. Vagal activity slows heart rate, supports digestion, modulates inflammation, and contributes to the recovery state after stress.

Vagal tone is typically measured indirectly through heart rate variability (HRV), specifically the high-frequency component of HRV that reflects parasympathetic activity. The measurement has been validated in humans extensively and has been increasingly applied to animal welfare research across species, including horses, dogs, cats, cattle, pigs, and several wild species.

High vagal tone is associated with calmer baseline states, faster recovery from stress, better social engagement, and generally better welfare outcomes. Low vagal tone is associated with the opposite: difficulty recovering from stress, reduced capacity for social engagement, and a range of welfare and health concerns.

Vagal tone is the physiological correlate of an animal being able to rest, recover, and engage socially after challenge. It is in some sense the marker of an animal whose nervous system is functioning in a way that supports welfare, rather than being chronically activated in stress response.

In practical training contexts, attention to vagal tone is less direct than attention to behavioural markers, but the concept matters because it provides a physiological explanation for why some animals recover quickly from challenging sessions while others do not. Training that supports parasympathetic engagement (calm endings, predictable patterns, social connection) tends to produce animals with better long-term coping capacity than training that keeps the sympathetic system chronically activated.