Evolution of the androgen-induced male phenotype.

The masculine reproductive phenotype varies significantly across vertebrates. As a result, biologists have long recognized that many of the mechanisms that support these phenotypes-particularly the androgenic system-is evolutionarily labile, and thus susceptible to the effects of selection for different traits. However, exactly how androgenic signaling systems vary in a way which results in dramatically different functional outputs, remain largely unclear. We explore this topic here by outlining four key-but non-mutually exclusive-hypotheses that propose how the mechanisms of androgenic signaling might change over time to potentiate the emergence of phenotypical variation in masculine behavior and physiology. We anchor this framework in a review of our own studies of a tropical bird called the golden-collared manakin (Manacus vitellinus), which has evolved an exaggerated acrobatic courtship display that is heavily androgen-dependent. The result is an example of how the cellular basis of androgenic action can be modified to support a unique reproductive repertoire. We end this review by highlighting a broad pathway forward to further pursue the intricate ways by which the mechanisms of hormone action evolve to support processes of adaptation and animal design.