Physiological responses to stress by wild mammals.

The biological interactions that enable wild mammals to maintain homeostasis when confronted with stressors depend on precise regulation by the CNS of neural and endocrine systems. Immediate physiological responses to acute stressors are characterised by the activation of the sympathetic nervous system with release of epinephrine and norepinephrine from the adrenal medulla. Persistent stressors are countered by ACTH release and consequent adrenocortical activation. Whether a stress response is produced by psychological perceptions, environmental factors or latent infections, stimulation of the PA axis and increased levels of total corticosteroids will result. Selye's GAS partitions these responses into an initial 'alarm' reaction and a prolonged 'resistance' stage. The intensification of the stimulus, or the superimposition of further stressors, leads to the 'exhaustion' stage. This was initially thought to be characterised by adrenal exhaustion and a depletion of glucocorticoids. It is now known that depressed immune functions occur due to increased adrenal activity and high levels of biologically-active glucocorticoids. Social stressors that initiate endocrine responses have been proposed as being capable of regulating animal numbers in captivity. Such a regulatory role for stress in natural populations has not been adequately tested. However, studies that have correlated individual behaviour and social structure with physiological data offer some insights into possible mechanisms of population regulation.