Testosterone and reproductive success in males and females
For logistical reasons, many studies on the relationships between testosterone and fitness have used testosterone implants to experimentally elevate testosterone levels. Such studies that use this "phenotypic engineering" technique have been integral in establishing direct links between testosterone and fitness-related traits. Some previous work has found that experimentally elevated testosterone increases reproductive success in males via increased mating, but reduces reproductive success in females. Measurements of natural levels of testosterone are needed to assess how selection might act on testosterone. A recent study by McGlothlin et al. (2010, American Naturalist 175:687-701) demonstrated that, in male dark-eyed juncos (Junco hyemalis) stabilizing selection acts on GnRH-induced testosterone levels through extra-pair mating success, and positive direction selection acts on GnRH-induced testosterone levels through within-pair mating success. Relatively fewer studies have examined the relationships between reproductive success and natural testosterone levels in females, but to understand how hormones might evolve within a species, it is necessary to measure hormones in both sexes. In both male and female eastern bluebirds, clutch size and fledging success are not related to GnRH-induced testosterone levels. These findings are contrary to those in some studies using testosterone implants. The relationships between reproductive success in testosterone are complex and may vary considerably among species depending on life history traits.