Testosterone Boosts Birds' Attractiveness, But Leads To Shorter Lifespan

Published in the May issue of The American Naturalist, the team’s study found that male birds with extra testosterone were more attractive to females and produced more—but smaller—offspring. Smaller offspring had lower survival rates than larger offspring. The extra testosterone also made the male birds sing more sweetly and fly farther. The testosterone-laden birds proved irresistible to older, more experienced female juncos, but that attractiveness carried some risks. Elevated testosterone levels increased activity—possibly attracting more predators—made the male, dark-eyed juncos more susceptible to disease and shortened their lifespan. “They had lower immune function and paid a cost with lower survival rates,” said Reed.

The increased testosterone also made the dark-eyed male juncos less attentive parents to their offspring as they made fewer nest visits, resulting in less food delivered and less time spent at the nest. The research team monitored more than 400 junco nests in the Appalachian Mountains of Virginia for nine breeding seasons. One group of dark-eyed juncos in the study received tubes implanted under the skin which contained testosterone and the control group of birds received implants that were left empty. Implants were removed from birds recaptured at the end of each breeding season.

Study results are featured by the American Association for the Advancement of Science, in a podcast called Testosterone Tradeoff at http://www.scienceupdate.com/index.cfm. The podcast notes that such feathered Casanovas have a better sex life, but a shorter one than birds not receiving the extra testosterone in the study. Reed led the research as part of her postdoctoral work.

The study is titled “Physiological Effects on Demography: A Long-Term Experimental Study of Testosterone’s Effect on Fitness.” Investigators in addition to Reed include: M.E. Clark, NDSU; P.G. Parker, University of Missouri; S.A. Raouf, University of Washington; M. Arguedas, Ohio State University; D.S. Monk, Washington State University; E. Snajdr, V. Nolan Jr., and E.D. Ketterson, Indiana University.

For more information, see:
“Physiological Effects on Demography: A Long-Term Experimental Study of Testosterone's Effects on Fitness.” The American Naturalist. May 2006, Vol. 167, No. 5, pgs 667-683. http://www.journals.uchicago.edu/AN/journal/issues/v167n5/41049/41049.html