deleterious effects on the animal’s welfare. Distress can follow both acute and chronic stress, provided that the body’s biological functions are sufficiently altered and its coping mechanisms overwhelmed (Moberg 2000).
The transition of stress to distress depends on several factors. Of clear importance are stressor duration and intensity, either of which is likely to produce behavioral or physical signs of distress. For example, short-term restraint does not cause marked problems in adaptation, whereas prolonged restraint can result in behavioral or physiological distress sometimes expressed by vocalization or gastric ulcers (Ushijima et al. 1985). In addition, predictability and controllability (i.e., the ability of the animal to control its environment) are important determinants in the transition of stress to distress. Numerous studies indicate that, in animals that can predict the onset of a stressful stimulus or control its duration, the behavioral and physiological impacts of stressor exposure are attenuated. Notable among these studies are findings that rats exposed to inescapable shock develop clear signs of distress, whereas yoked rats that can terminate shock exposure do not, despite subjection to the same intensity and duration of shock experience (Maier and Watkins 2005).
Furthermore, the stress response may induce insufficient or inappropriate changes in the behavioral and physiologic control systems (noted above) or inadequate or undesirable responses to their output signals. For example, chronic social subordination has been shown to elicit behavioral withdrawal, prolonged alterations in the hypothalamic-pituitary-adrenal (HPA) axis output, and subsequent immunosuppression (Blanchard et al. 2001), all of which preclude effective coping and adaptation. Further studies have shown that in chronic distress states, such as depression, the glucocorticoid feedback systems fail (Carroll et al. 1976). Thus, if stress responses themselves fail to appropriately cope or produce successful adaptation they may be not merely ineffective but actively deleterious. For example, while corticosteroid responses are essential for the adaptation process, marked or prolonged hypersecretion can produce pronounced metabolic and immune dysfunction (Munck et al. 1984).
Should an animal have the option to behaviorally express a choice in response to a stressful condition and thus exercise some control over its environment, then its adaptive behaviors should be distinguished from maladaptive ones displayed in distress (NRC 2003a, page 22; Mench 1998). However, a cause and effect relationship between various abnormal behaviors and distress or the operationalization and validation of the degree of abnormality associated with distressed states has not yet been established. Distress may not always manifest itself with recognizable “maladaptive behaviors, such as abnormal feeding or aggression” (NRC 2003a, page 16) but instead with subclinical pathological changes, such as hypertension and immunosuppression, which are not behaviorally identifiable. As Moberg