Male-to-female testosterone ratios, dimorphism, and life history—what does it really tell us?

Testosterone is a key hormone for the development of secondary sexual characters and dimorphisms in behavior and morphology of male vertebrates. Because females often express detectable levels of testosterone, testosterone has been suggested to also play a role in the modulation of secondary sexual traits in females. Previous comparative analyses in birds and fish demonstrated a relationship between male-to-female testosterone ratios and the degree of sexual dimorphism. Furthermore, female maximum testosterone was related to mating system and coloniality. Here, we reevaluate these previous ideas using phylogenetic analyses and effect size measures for the relationship between birds’ male-to-female maximum testosterone levels. Further, we investigate the seasonal androgen response of female birds (the difference from baseline to maximum testosterone), which in males is strongly related to mating system. We could not confirm a relationship between male-to-female testosterone, maximum female testosterone, or the seasonal androgen response of females with any life-history parameter. We conclude that the expectation that testosterone regulates traits in females in a similar manner as in males should be reconsidered. This expectation may be partially due to hormone manipulation studies using pharmacological doses of testosterone that had similar effects in females than in males but may be of limited importance for the physiological range of testosterone concentrations occurring within ecological and evolutionary contexts. Thus, the assumption that circulating testosterone should covary with ecologically relevant secondary sexual traits in females may be misleading: selection pressures on females differ from those on males and females may regulate behavior differently.