Parasitic Stress and Self-Medication

In the physical sciences, "stress" is defined as the force per unit area, or pressure, acting upon a solid body, resulting in the deformation (strain) of the solid. At low stresses the strain is said to be elastic, directly proportional to the stress and reversible; the solid returns to its original shape after the stress is removed. As the stress increases the elastic limit is reached, after which the strain is said to be plastic, increasing exponentially with increasing stress and non-reversible. Plastic deformation continues until the rupture strength is reached, at which point the material breaks. The term stress was adopted by biologists to refer to factors that interfere with the maintenance of homeostasis, the effects of which range from the minor, temporary and easily reversible, to the complete breakdown of homeostatic mechanisms (Cannon, 1935). As applied to vertebrates, the term "stress" is generally used to denote stimuli that elicit a specific set of physiological responses, particularly the release of corticosteroids (Vander, 1981; Kopin, 1995; Mims et al., 1995). However, these responses are not characteristic of all taxa, so this definition is not inclusive. Stresses can also be defined more broadly as aversive stimuli (McGrath, 1970; Selye, 1976), regarded as selective forces, and studied along with the adaptations that have evolved to reduce their negative effects (see Thornhill and Furlow, this volume). Under this view, stresses can take a myriad of forms, as indicated by the wide range of topics included in this volume. Along with competition and predation, parasitism is one of the main sources of biotic stress facing all organisms. For the purposes of this discussion, parasites will be functionally defined as organisms that live in or on a heterospecific