Male and female testosterone--is one sex made in the image of the other? A comment on Goymann and Wingfield

Abstract

Goymann and Wingfield (2014) provide a welcome review of the relationships between circulating testosterone, sexual dimorphism, and social systems in female birds. They conclude that earlier comparative studies were inconclusive because they failed to incorporate phylogeny (Ketterson et al. 2005). I agree. Goymann and Wingfield also criticize experimental studies that rely on implants to elevate circulating levels of testosterone. Here, I disagree. The distinction between pharmacological versus physiological levels of hormones can be informative rather than problematic; if a novel phenotype were to arise, would it spread? Further, evolutionary endocrinology has advanced substantially beyond its earlier focus on circulating levels of hormones to investigations of local steroid production, tissue-specific sensitivity, and hormone-induced patterns of gene expression. The current challenge is not to replace one approach with the others but to integrate them. When Maria Sandell, Val Nolan, and I were writing about female–female competition and female testosterone a decade ago (Ketterson et al. 2005), we considered both cross-species data on male and female testosterone and experimental studies in males. In the dark-eyed junco, we had found that males with experimentally elevated circulating testosterone differed phenotypically from controls in ways that resulted in greater offspring production via extrapair fertilizations, reduced survival, but higher net fitness. Seeking an explanation for why males did not naturally produce these higher levels of testosterone, we proposed that if male and female levels of testosterone were correlated, and higher levels in females were disadvantageous, then correlated responses to selection might constrain male testosterone. Today, the logic for sexual conflict over testosterone remains sound, but the case is far from made. Research on the junco provides conflicting evidence. Females treated with testosterone implants do become more aggressive (Zysling et al. 2006), but