Behavioral aspects of stress.

Many researchers view stress theory as having evolved from a specific response for each specific stimulus, through a standardized general adaptation syndrome for a wide variety of stressors, to a newly hypothesized model that suggests a standardized physiological response for psychological stressors and specific homeostatic responses for physical stressors. Stressors can originate from within an individual (endogenous) or from the environment (exogenous). A sense of control over one's environment (exogenous stimuli) and predictability of stimuli are extremely important in determining the potency of psychological stressors. The performance of highly repetitive, stereotyped behaviors has been shown to help animals cope with environments containing little or unvaried stimulation. Chronic psychological stressors are the most problematic area of stress research and animal care. Animals may respond behaviorally to chronic stressors by either achieving adaptation or by showing such conditions as learned helplessness, intensification of drives, stereotyped behavior, and absence of normal behaviors. A chronic endogenous buildup of motivation to perform specific behaviors that an animal has been prevented from performing can be quantified and result in animals displaying signs of chronic stress, i.e., altered metabolism (hormones), suppressed immune system, and learned helplessness. It is of paramount importance that the behavioral and physiological variables utilized in stress research be relevant to the particular species and questions under study.