Most attention to animal stress focuses on techniques that involve obviously aversive stimuli. In human medicine, however, it is known that lack of external stimuli can cause psychiatric disorders. Researchers have demonstrated that laboratory animals sometimes develop severe signs of distress from a lack of essential stimulation. (This topic is discussed in greater detail in Chapter 3.) The recent focus on the "psychologic well-being" of nonhuman primates has raised the issue repeatedly, and many have proposed to improve psychologic well-being with enrichment devices or programs. Although nonhuman primates have received nearly all the attention, it should be asked whether other laboratory animals are in distress as a result of environmental monotony in standard housing or lack of other unidentified components of the environment. Understimulated animals of many species show abnormal behavior patterns (stereotypes and displacement behaviors) that are largely absent in enriched environments (Wemelsfelder, 1990). Animals in unenriched environments are also more passive, and their behaviors might be less diverse than those of animals in more stimulating surroundings.
When stress is present, the first change most likely to be reported is a change in an animal's activity pattern. Behavior can range from inactivity to hyperactivity and from adaptive to maladaptive, depending on the source and severity of the stressor and the species. Changes can be seen in sleep and eating behavior. The animal might be nonresponsive, listless, lethargic, and depressed, or it might be unusually restless, excitable, anxious, apprehensive, hypersensitive, or aggressive. It might constantly move about the enclosure or repeatedly stand and lie down. As one approaches a normal animal's cage, it should respond in a usual and predictable manner that enables assessment of its gait, inquisitiveness, vocalization, and posture. An animal in stress that previously would have investigated a new visitor to the room or a change in the environment might now fail to do so or attempt to escape.
Nocturnal species, which during daylight hours exhibit very little species-typical behavior other than sleeping, should be observed at times that coincide with their active period, e.g., early in the morning, when the animals are still awake and moving about. During initial observations, respiratory and activity patterns should be assessed without investigator-produced stimulation or the escape behaviors that are often associated with capturing and handling.
The definition and measurement of distress are in their infancy. Few attempts have been made to place the assessment of distress on a scientific and systematic