A Long-Term Study Using Hormones to Study Life History Trade-Offs and Sexual Conflict
In natural systems, time and resources are finite and must be allocated between competing life history demands. Allocation is frequently mediated by the endocrine system, and study of the relationships among hormones, behavior, and fitness has helped to reveal how conflicts inherent in life histories are resolved. From 1987-2007, we employed experimental manipulations of testosterone to study trade-offs in a natural population of juncos. Phenotypes of male juncos with experimentally elevated testosterone differed from controls. Alterations in male physiology and behavior resulted in reduced fecundity with social mates and decreased survival, but greater overall reproductive success due to more extra-pair offspring. Female physiological and behavioral responses to increased testosterone were somewhat similar, but the fitness consequences were highly detrimental. These results suggest that increased testosterone is favored in males but not females, suggesting sexual conflict, and perhaps accounting for maintenance of testosterone at levels below the male optimum. However, the evolutionary dynamics underlying this system are almost certainly more complicated. The degree to which testosterone-mediated traits are beneficial to males may be frequency dependent or may vary among years. These additional considerations may shape both the proximate regulation and the evolution of hormone-mediated life history traits.
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