Steroid hormones could be a proximate explanation for variation in male mating success in many species of birds. High levels of testosterone may enhance mating success but then reduce the ability of individual males to cope with pathogens which may, in turn, decrease long-term survival. In one unusual mating system, males form groups and court visiting females at traditional display sites (or 'leks'). In lek-mating birds, males typically defend territories that contain no resources for females, only a limited subset of males mate successfully, and females care for all offspring. The effects of natural and experimental testosterone levels in a wild population of greater prairie-chickens (Tympanuchus cupido), a lek-mating species of prairie grouse will be measured. The study design will use three approaches to determine the links among testosterone levels and the behavior, mating success and survival of males. First, path analysis, a statistical method, will be used to distinguish between direct benefits of naturally high levels of testosterone on mating success and indirect benefits through testosterone-enhanced traits (comb size, behavior, and territory attributes). Second, experimental manipulations of testosterone will verify causal relationships among testosterone, male attributes, and mating success. Last, the costs of naturally high and experimentally increased testosterone levels will be tested by examination of yearly return rates. This funding will support field work on testosterone implants and behavioral observations, and laboratory analyses of fecal samples to measure testosterone levels.
The intellectual merit of this study accrues from the fact that this is one of the first experimental manipulations of testosterone in a lek-mating bird. This study combines robust statistical methods with experimental manipulations of testosterone to elucidate causal relationships among hormones, hormone-mediated traits, and reproductive performance. The costs of naturally high or experimentally increased testosterone levels will be examined by impacts on male survival. The physiological role of testosterone is of great interest because it could explain well known life history trade-offs between current and future reproduction. Multiple testosterone samples are needed from individual males, and fecal samples are a noninvasive approach that reduces unnecessary stress to the birds but still allows measurement of individual and seasonal variation in testosterone. Intensive focal observations will contribute to a better understanding of the behavioral ecology of lek-mating birds. The broader impacts of this work include demographic data on an economically important game bird of conservation concern. Greater prairie-chicken populations are declining range-wide, and this study will aid management actions by providing baseline survival and fecundity data from healthy populations in Kansas. Research training will be provided for one female graduate student and two undergraduates. This project will foster research collaboration between Kansas State University and the University of Missouri at Columbia. Results will be shared with the public through guided tours and outreach programs at Konza Prairie Biological Station.
