expenditure of energy. Whether stress and the responses or behaviors it induces should be considered adaptive should be based on professional evaluation and judgment.

Historically, the definition of stress has emphasized physiologic characteristics, especially those related to neuroendocrine systems (Cannon, 1929; Selye, 1974; Levine, 1985). However, it is now known that physiologic measures of stress might not be highly intercorrelated. Differences among physiologic measures in their relation to eliciting stimuli, time course, and adaptive implications have led most scientists to conclude that stress is not a discrete, well-defined physiologic state (Moberg, 1987). Stress remains a useful descriptor nonetheless: it provides a convenient means of identifying, describing, and summarizing important phenomena. For example, it is generally agreed that some environmental conditions or events can act as stressors, cause pronounced or persistent stress in an organism, and lead to alterations in neuroendocrine activities. The neuroendocrine changes can in some instances be severe enough to place the organism in a state of vulnerability to dysfunction or disease, although its behavior might not differ markedly from that typical of its species and might not yet have become maladaptive. Although there is no single measure or manifestation of such a state, some of the pathways and mechanisms through which it is brought about are reasonably well understood. Departures from a normal level of neuroendocrine activity can be considered as one kind of evidence of stress.

Distress is an aversive state in which an animal is unable to adapt completely to stressors and the resulting stress and shows maladaptive behaviors. It can be evident in the presence of various experimental or environmental phenomena, such