Meeting Abstract
Hormones, such as testosterone, are integral in mediating physiological processes and behaviors that facilitate reproduction. Thus, an understanding of the relationships between hormones and reproductive success may provide an avenue for understanding the evolution of hormonal profiles. Some previous studies have established direct links between reproductive success and testosterone levels by experimentally manipulating testosterone levels and investigating the impacts on reproduction. However, measures of individual variation in testosterone levels are also needed as they are necessary to assess how selection might act on hormone levels. In addition, testosterone manipulations may elevate testosterone levels beyond an individual’s natural maximum capacity. Gonadotropin-releasing hormone (GnRH) challenges provide a method of measuring the natural maximum testosterone levels that an individual can produce. Recent work in dark-eyed juncos (Junco hyemalis) has demonstrated that males with higher testosterone produced in response to GnRH had higher reproductive success. Relatively fewer studies have investigated relationships between reproductive success and testosterone levels in females, but to understand how hormones evolve in a species, it is important to study both sexes. We conducted GnRH challenges on male and female eastern bluebirds (Sialia sialis), and related GnRH-induced testosterone levels to clutch size, number of offspring fledged, and offspring mass. Unlike the previous study on juncos, we did not find a positive relationship between measures of reproductive success and GnRH-induced testosterone levels in male and female bluebirds. Our findings suggest that the relationships between reproductive success and testosterone in both sexes are complex and may vary among species.
