Endocrine-immune function trade-offs, genetic quality and honest sexual signalling

Sexual selection is a central topic in evolutionary biology, and has fascinated ecologist since the pioneering work of Darwin. Many species possess brightly coloured or exaggerated ornaments that evolved via sexual selection and are used to assess the quality of rivals or potential mates. To understand how these ornaments evolved and are maintained, we need to identify not only the type of qualities that are advertised, but also the mechanisms that prevent cheating. If sexual signalling is to be reliable, signal production or maintenance should be costly. In many species, testosterone enhances the expression of ornaments, but is costly, because it reduces an individual's ability to cope with parasites. According to the 'Immunocompetence Handicap Hypothesis', this dual action of testosterone would prevent cheating. However, the exact mechanisms by which testosterone influences parasite resistance are still little known, and we know even less about how an individual's genetic quality influences the trade-offs between ornamentation and parasite resistance. With the proposed work, my aim is to understand how handicap mechanisms operate to ensure honest signalling via testosterone dependent ornaments, and how the genetic background of individuals moderates the effects of key hormones on parasite resistance, thereby influencing levels of sexual ornamentation. I will focus on the dual effects of two hormones (testosterone and corticosterone) on ornamentation and immune function. I will also focus on Major Histocompatibility Complex (MHC) genes, which are increasingly recognized as a paradigm in studies of sexual selection. I will investigate how MHC genotype influences the trade-offs between hormones, parasite resistance and sexual ornamentation.