Meeting Abstract
Testosterone, through its pleiotropic effects, plays a crucial role in regulating and coordinating morphology, physiology and behavior. While much has been learned through the use of manipulative experiments, determining the relationship of natural variation in testosterone profiles with other phenotypic traits related to fitness is necessary going forward. The hypothalamic-pituitary-gonadal (HPG) axis, with gonadotropin-releasing hormone (GnRH) initiating the endocrine cascade, regulates testosterone secretion. Circulating testosterone, however, varies over the course of the day and in response to other internal or external stimuli, potentially making it difficult to relate testosterone sampled at any one-time point with fitness-related traits. Over the past decade, researchers have begun to utilize the administration of exogenous GnRH to elicit a testosterone response in order to generate a standardized testosterone ‘phenotype’ to relate to other traits. While this has provided useful insight, it has remained unclear if and how this exogenously stimulated activation of the HPG axis is related with endogenously regulated testosterone capable of influencing testosterone related traits. Here, we ask how endogenous diel variation in testosterone profiles relates to GnRH-induced testosterone secretion, as well as how the relationship between these two measures relate with fitness-related traits in a songbird, the house sparrow (Passer domesticus). Specifically we associated testosterone measures with badge size and sperm concentration and function in captive house sparrows. Data presented will elucidate the usefulness of GnRH-induced testosterone as a ‘phenotypic trait’ in studies aimed at understanding individual variation and selection on testosterone phenotypes.
