mental procedures, social interactions, feeding regimens, lighting, odors, noises, and so forth. It is not possible to list all possible stressors, because the variety of experiences and responses of animals is vast. This chapter will therefore concentrate on environmental and ecologic factors that have been demonstrated in humans and animals to be important contributors to stress. They are not all-inclusive causes of stress, but are intended to represent major categories that can be useful options for consideration. The categories are derived from what is known about causes of stress in animals in general, including wild animals and humans, as well as laboratory animals. Even ecologic stressors that have no obvious impact on laboratory animals (e.g., predator-prey relations and foraging) have implications for research animals that should be considered.

Stress is a normal feature of life and serves important adaptive functions. The physiologic processes involved in the flight of an antelope being chased by a cheetah and in the cheetah that pursues it are examples of the normal adaptive functions of stress. Those processes allow both animals to maximize their physiologic resources in a situation of vital concern to each of them. Stress is also common in captive environments. It can be produced by pain and by extreme variations in ambient temperature, illness, demanding tasks, and almost any situation that an animal perceives as threatening or that puts it in a state of uncertainty and conflict (Hennessy and Levine, 1979; Weinberg and Levine, 1980). It is important to recognize the presence and varied sources of stress for ethical reasons and because the physiologic changes associated with stress are likely to affect experimental data. Research designs and experimental procedures should be planned to minimize stress.

Although stress has normal adaptive functions, stress in captive environments can lead to pathologic changes, such as gastric ulcers, and to outcomes that are maladaptive. When that occurs, it can be said that the animal is not only stressed, but distressed. As stated in Chapter 1, distress is a state in which an animal is unable to adapt completely to stressors; it differs from stress only in the manifestation of maladaptive behaviors or other pathologic processes.

Pain is an example of a stressor. The primary biologic function of pain is to signal potential or actual tissue damage. An animal in pain characteristically shows postures or behaviors that alleviate or terminate the pain (see Chapter 4). When an animal in pain is prevented from assuming those postures or performing those behaviors, or if they are not effective, it might show maladaptive responses. It is then in distress.

Most environmental stressors lack the specificity of pain, both with respect to the sensory systems that mediate them and with respect to the physiologic and behavioral reactions they elicit. The potency of many stressful conditions or events can change quickly. Moreover, one animal adapts readily to a particular environmental stressor, and another does not. The difference can be the result of habituation, a learned association with other environmental events, or the acquisition by one animal, but not the other, of the ability to cope with the stressful circumstance.

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