We talk very glibly these days of stress, especially the ‘stress of modern-day living’, and in the scientific literature one often finds mention of things such as ‘temperature stress’, ‘water stress’ and ‘the stress of reproduction’. It has proven extraordinarily difficult, however, to agree on a common definition of the term, let alone develop methods for measuring the incidence and intensity of stress in animals.

There is general agreement among biologists that ‘stress’ is an important ecological factor, often contributing to the extinction of rare and endangered species (Bradshaw, 1996, 1999). There are thus a number of important questions that need to be addressed concerning stress, including:

1. What is stress?

2. How do we define it and measure its effects?

3. How does one measure its incidence and severity?

4. What effects does stress have on an animal's ability to maintain homeostasis?

5. Does prolonged stress reduce fitness?

6. Are threatened and endangered species more susceptible to the effects of stress than are other species?

7. Can we use instances of stress physiology to gauge the level of susceptibility of animal species to environmental change and their likelihood of extinction?

Definitions

It was Walter Cannon again who, as early as 1914, first developed the idea that organisms react to unfavourable situations in terms of highly integrated metabolic activities. Cannon's concept of stress was derived by simple analogy from Newtonian physics where an imposed force (stress) produces a deformation (strain) in an object, with the strain being proportional to the stress.