"Physiological Effects on Demography: A Long-Term Experimental Study of Testosterone's Effects on Fitness," published in the May issue of The American Naturalist.
Does an extra dose of testosterone pay long-term dividends? A higher level of the hormone increases sex drive and attractiveness of males, leading to more offspring and increased evolutionary fitness; it also weakens the immune system, amplifies stress, and encourages recklessness, increasing the risk of departing the gene pool altogether. Looking at songbirds, Wendy Reed, a physiological ecologist at North Dakota State University, set out to determine whether, evolutionarily speaking, a little extra machismo is really worth it. And since testosterone has pretty much the same effect across all species, her results probably extend to humans as well.
A group of Reed's colleagues went to the Appalachian Mountains to jolt the testosterone levels among breeding males in a population of small, two-toned songbirds known as dark-eyed juncos. Each year the researchers caught about 100 males in seed-baited traps and nets. They made a small incision under the wing and inserted two small tubes. In about 50 males, the tubes contained crystalline testosterone; in the others, the tubes were left empty. Birds were tagged with colored rings on their legs and released.
By continually trapping and releasing the birds over an eight-year period, Reed was able to measure the relative survival rates of the two groups. Males with normal testosterone levels had a 44 percent chance of surviving to the next year, compared with only 38 percent for the males who received a boost. On the other hand, 38 young males on testosterone successfully courted older females, compared with only 28 untreated young males.
Dark-eyed juncos are socially monogamous—they nest and raise a family with one partner. But DNA paternity testing reveals they have a lot of sex on the side. The "extra-pair fertilizations" of normal males in the monitored group resulted in a total of 68 offspring. Testosterone-treated males fathered 83 young outside the nest.
So does living fast compensate for dying young? Overall, high-testosterone birds had a notable 35 percent increase in reproductive fitness, passing on more genes to the next generation. In other words, says Reed, "it's OK to die young if you make up for it by having more sex."
Ellen Ketterson of Indiana University leads the group that goes into the field and makes dark-eyed juncos more macho. She and her students spend "every day in the field almost literally dawn to dusk" from the end of April to the middle of August; one way or another, she has been working with the birds for 35 years. Wendy Reed and her husband, Mark Clark, stay at home and crunch the numbers—in fact, they've never met the birds they study. Reed and Clark collaborate on a lot of projects, including their two children. He contributes his fair share and is reasonably attractive, Reed says. "I don't have any desire for extramarital fertilizations."