Testosterone Mediates Mating Success in Greater Prairie-Chickens

Testosterone plays a key role in influencing behaviors that enhance male breeding success, but elevated testosterone levels can also reduce immunocompetence and survival. In socially monogamous species, males with higher levels of circulating testosterone experience advantages during both female choice and male–male competition activities. The role of testosterone influencing male mating success in lek-mating systems is largely unknown. Over three years, we quantified natural and experimental levels of testosterone among Greater Prairie-Chickens (Tympanuchus cupido) at five leks. Our project had three objectives: (1) to quantify natural variation in testosterone in blood plasma and to determine whether testosterone levels are correlated with male traits and mating success; (2) to experimentally elevate testosterone levels to determine causal relationships between testosterone levels, male traits, and mating success; and (3) to analyze return rates to determine if increased levels of circulating testosterone are costly in terms of annual survival. Using a before– after control-impact (BACI) experimental design, changes in aggression, display behavior, territory size and location, mating success, and survival were compared between males with testosterone implants (15 males) and sham implanted controls (13 males) over two years. Natural level of testosterone was a strong predictor of male mating success, but testosterone level in unmanipulated males was not related to any morphological, behavioral, or territorial traits considered. Comb area, tarsus length, and distance to the center of the lek were also significant predictors of male mating success. Similar results were also found in our field experiment: Testosterone-implanted males tended to gain more copulations than sham-implanted males, although the difference was not significant. In addition, the T- and sham-implanted males did not vary with respect to their behavior or territory attributes. Neither naturally nor experimentally elevated T levels were related to annual return rates of males. Our project is one of the first manipulations of testosterone levels in a lek-mating bird. Future work should search for links between testosterone and male attributes in lek-mating systems because females are unlikely to be able to assess testosterone levels directly.