The stress of elaborate male traits: integrating glucocorticoids with androgen-based models of sexual selection

Stress hormones are emerging as major factors regulating the expression of elaborate male traits. Surprisingly, however, the effects of glucocorticoids on such traits have not been formally integrated with androgen-based models of sexual selection. Here we point out that consideration of glucocorticoid-mediated effects on the phenotype provides new insight into long-standing hypotheses and controversies associated with such models. In particular, androgen-based ‘handicap’ models of sexual selection characteristically hinge on graded effects of androgens on male traits, but few studies have found support for such a relationship, suggesting that androgens may not be a primary target of selection. We propose, however, that in many instances, androgens may not appear to have a graded effect on the phenotype because elevated glucocorticoids mask the effects of androgens. Glucocorticoids may be inextricably linked to elaborate traits because the energetic demands associated with such traits promote glucocorticoid production. We thus propose that glucocorticoid-mediated effects on male traits warrant re-evaluation of androgen-based models of sexual selection. In particular, we argue that androgen-based handicap models cannot be dismissed based on the lack of evidence for a graded relationship between androgen level and the extent or magnitude of the trait. Our review of the literature indicates that most studies have examined either the effects of androgens or the effects of glucocorticoids, but not both. A more integrated approach involving the effects of both steroids is necessary to fully understand the role of androgens and the endocrine targets of selection associated with the expression of elaborate male traits.