previous experience, whereas anxiety usually refers to a generalized, unfocused response to the unknown.
• Of all stressors to which laboratory animals are likely to be exposed, those caused by environmental influences—Environmental Stressors—are probably the most pervasive. They include cage or habitat design, feeding routines, handling techniques, noise, odors, investigative procedures and techniques, interactions with humans, interspecies interactions, and conspecific social interactions, including dominance-subordination relationships. Those and other undetermined stressors can interfere with animals' well-being, in ways that are sometimes manifested as an inability to express species-typical behaviors. Many other environmental stressors are elaborated in Chapter 3. The identification and control of these stressors from the animals' or species' perspective constitute good husbandry and are a primary responsibility of all who care for or use animals in a laboratory setting.
Table 1-1 lists representative examples of three categories of stressors—psychologic, physiologic, and environmental. Any stressor can initiate stress in an animal and, depending on previous states and experiences, have the potential to affect homeostasis profoundly. The categories of stressors are not mutually independent, and stressors can overlap and interact within and between categories.* Interaction of several stressors can act to increase or decrease the net effect of stressors and the resulting stress. Chapters 2 and 3 describe stressors and their potential action as sources of stress and distress for laboratory animals.
An animal's state can vary across a continuum from comfort to distress (Figure 1-1). When an animal is in a state of comfort or discomfort, homeostatic (adaptive) processes tend either to maintain the state of comfort or to return the animal toward a state of comfort. Stressors do not pose a threat to the animal as long as it can maintain an adaptive equilibrium. When that is no longer possible, the animal enters a state of distress, in which its behavior and physiology become maladaptive. No single behavioral or even physiologic measure is an unequivocal indication of distress. Distress should be inferred when there is converging evidence, rather than from a single sign. It is here that judgment and knowledge of an animal and its species-typical behaviors play an important role in determining the state of the animal.