A glucocorticoid hormone released by the adrenal cortex as part of the stress response.

Cortisol mobilises energy, modulates immune function, and affects memory formation. Brief cortisol responses to challenge are normal and adaptive: they mobilise the resources needed to handle the stressor and the hormonal response resolves once the challenge has passed. The body’s normal homeostatic mechanisms then return cortisol levels to baseline.

Sustained cortisol elevation, characteristic of chronic stress, has wide-ranging effects. Persistent cortisol elevation suppresses immune function, impairs hippocampal function (with consequences for memory and learning), alters appetite and metabolism, disrupts sleep, and contributes to a range of physical and behavioural health problems. The negative effects of chronic cortisol elevation are well-documented across the mammalian range, and similar findings have been established in birds.

Cortisol is measurable in blood, saliva, hair, and faeces, which makes it one of the most widely used indicators in animal welfare research across species. Different measurement methods capture different timescales: blood and saliva show acute cortisol responses; hair and faecal cortisol show longer-term average levels. The choice of measurement depends on the welfare question being asked.

Cortisol measurement has limitations. Individual variation in baseline cortisol is substantial, so single measurements are less useful than within-individual changes over time. Cortisol responses can be elicited by positive arousal (excitement) as well as by stress, so cortisol elevation alone does not distinguish between welfare-positive and welfare-negative states. Welfare assessment typically combines cortisol measurement with behavioural observation and other physiological indicators.

In training contexts, attention to cortisol-mediated processes is one of the reasons that memory consolidation of stressful learning events tends to produce particularly durable and inflexible memories. Cortisol modulates how memories are formed under stress, biasing them toward fear and avoidance.