Testosterone and survival: a cost of aggressiveness?

Free-living male brown-headed cowbirds were captured during the breeding season, implanted with testosterone-filled Silastic tubing, and released. Radioimmunoassay verified that the implants maintained circulating plasma testosterone values at maximal breeding season levels well beyond the normal time of decline. Survival to the following year of these implanted males was compared with survival of unimplanted birds captured in other years, and also with survival of males given empty implants. Androgen-implanted male cowbirds exhibited significantly reduced survival to the following year compared with either of the control groups, and also exhibited severe injuries not seen in other years. It is suggested that the increased risks associated with prolonged high testosterone levels act as a selective force to maintain reduced androgen levels except during the period of aggressive intrasexual interactions that characterizes the reproductive season. Further, it is proposed that the nature and importance of the different risks vary with a species' mating system.