Trade-offs in evolutionary immunology: just what is the cost of immunity?

It has become increasingly clear that life-history patterns among the verte brates have been shaped by the plethora and variety of immunological risks associated with parasitic faunas in their environments. Immunological compe tence could very well be the most important determinant of life-time reprod uctive success and fitness for many species. It is generally assumed by evo lutionary ecologists that providing immunological defences to minimise such risks to the host is costly in terms of necessitating trade-offs with othe r nutrient-demanding processes such as growth, reproduction, and thermoregu lation. Studies devoted to providing assessments of such costs and how they may force evolutionary trade-offs among life-history characters are few, e specially for wild vertebrate species, and their results are widely scatter ed throughout the literature. In this paper we attempt to review this liter ature to obtain a better understanding of energetic and nutritional costs F or maintaining a normal immune system and examine how costly it might be fo r a hose who is forced to up-regulate its immunological defence mechanisms. The significance of these various costs to ecology and life history trade- offs among the vertebrates is explored. It is concluded that sufficient evi dence exists to support the primary assumption that immunological defences are costly to the vertebrate host.