"The surprising result was that testosterone-treated males had a higher overall fitness than control males," write the authors in a study in the May issue of American Naturalist.
This led to the question of why don't juncos naturally have higher levels of testosterone? Testosterone-treated males produced more offspring, but they were smaller, and smaller offspring had lower postfledging survival. Older, more experienced females preferred to mate with older males and realized higher reproductive success when they did so. While young males treated with testosterone increased their ability to attract older females, it resulted in poor reproductive performance.
"Although testosterone increased male fitness, as measured by lifespan and number of offspring, the extended effects on offspring and female mates were generally negative and may ultimately constrain the evolution of higher testosterone levels in males," conclude the authors.
Founded in 1867, The American Naturalist is one of the world's most renowned, peer-reviewed publications in ecology, evolution, and population and integrative biology research. AN emphasizes sophisticated methodologies and innovative theoretical syntheses--all in an effort to advance the knowledge of organic evolution and other broad biological principles.
W.L. Reed, M.C. Clark, P.G. Parker, S.A. Raouf, N. Arguedas, D.S. Monk, E. Snajdr, V. Nolan, Jr., and E.D. Ketterson. "Physiological effects on demography: A long-term experimental study of testosterone's effects on fitness," The American Naturalist 167:5.