Immunity, Hormones, and Life History Trade-Offs

Michael P. Muehlenbein, Sean P. Prall, and Hidemi Nagao Peck

Abstract Immunity is an integral part of organismal life histories because it is crucial for maximizing evolutionary fitness, and because it is energetically expensive to develop, maintain, and activate. This chapter orients the reader to the roles of immunity in human life history trade-offs, and specifically the utility of sex hormones in mediating variation in immunity. Hormones are central mechanisms that contribute to the onset and timing of key life history events, fine-tune the optimal allocation of time and energy between competing functions, and in general modulate phenotypic development and variation. Here we describe the roles of testosterone, dehydroepiandrosterone, and estradiol in moderating immunocompetence from a life history perspective, illustrating how correlated changes in immunity and gonadal function reflect the manifold interactions between these two systems. The immunomodulatory actions of these hormones are complex and varied, and we attempt to provide explanations for this variation from the literature. Although our evidence comes from clinical medicine, our basic prediction is derived from life history theory: altering the hormonal milieu may result in differential susceptibility to both infectious and chronic diseases. Furthermore, the immunological costs associated with hormone supplementation are worthy of greater consideration by both clinical practitioners and evolutionary ecologists alike.

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