Stress response systems

The integrated physiological systems that activate when an animal encounters a challenging stimulus.

The two main components of the stress response are the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis. The sympathetic nervous system produces the fast “fight or flight” response within seconds, raising heart rate, redirecting blood flow to skeletal muscles, dilating pupils, and releasing adrenaline. The HPA axis produces the slower cortisol response over minutes to hours, mobilising energy and modulating immune function for the sustained part of the response.

Both systems run across mammals and birds in broadly similar ways, which is why stress physiology research generalises well across species. The same basic architecture is present in horses, dogs, cats, rodents, primates, and birds, with species differences mostly in the details of timing, magnitude, and recovery rather than in the fundamental mechanism.

The two systems are integrated. The fast sympathetic response handles the immediate moments of a challenge, while the HPA axis handles the longer aftermath. Recovery from a stress event involves both systems returning to baseline, and the time course of recovery is an important welfare consideration. Animals who can recover quickly between stressors tend to have better welfare than those whose stress response systems remain activated for prolonged periods.

The parasympathetic nervous system, the counterpart of the sympathetic system, is responsible for the “rest and digest” state and for recovery after stress. Vagal tone, a measure of parasympathetic activity, is increasingly used as a welfare indicator across species because it reflects the animal’s capacity to engage social and recovery systems.

Chronic activation of the stress response systems, characteristic of chronic stress, has wide-ranging negative consequences for welfare and health. Acute, short-term activation in response to specific challenges is normal and adaptive.