A model for assessing the impact of behavioral stress on domestic animals

Animal scientists need a reliable measure of behavioral stress in domestic animals if they are going to be able to assess the stress of various management practices and to answer public concern about the well-being of animals used in agriculture. Popular measures of stress, alterations in behavior or changes in hormone secretion, are not adequate because of a failure to establish any direct correlation between changes in these characteristics with adverse effects on animal well-being. Further complicating the use of these indicators of stress is the variation in their pattern of response to different kinds of stressors. Even the same stressor can elicit divergent responses in different animals because of inter-animal variation in the stress response. To address these problems, a model of animal stress is discussed and tested. From this model it is proposed that the best indicator of an animal suffering from stress is the development of a pre-pathological state; i.e., a stress-related change in biological function that threatens the animal's well-being. Examples of such pre-pathological states would be a suppression of the immune system, the loss of reproductive events critical for normal reproduction, or the development of behaviors that would lead to such undesirable acts as tail-biting or excessive fighting. Although determining the existence of such pre-pathological states is not convenient, their existence is currently the only defensible indicator of an animal suffering from behavioral stress.