An animal may be considered to be in a state of stress if abnormal behavior or extreme adjustments in its behavior or physiology are necessary in order to cope with adverse aspects of its environment. Methods used to determine if an animal is stressed can be either behavioral or physiological. Behavioral methods may be highly erroneous due to their subjective nature since alterations in behavior do not necessarily prove that an animal is stressed. There is no single measure of stress that can be used in all situations at this time. Every measure must be critically evaluated to ensure that it is valid for each application. An integrated approach using both behavioral observations and physiological measures is necessary.
Friend, T.H. (1980). Stress: What is it and how can it be quantified? International Journal for the Study of Animal Problems, 1(6), 366-374.