Just as Aristotle’s observations centuries ago led him to recommend moderation in all things, a team of scientists who studied a species of American songbird concluded that Mother Nature favors the middle ground when it comes to testosterone. In research conducted at UVA’s Mountain Lake Biological Station, biologists found that extreme levels of testosterone in male dark-eyed juncos put them at a disadvantage in both survival and reproduction when breeding with birds other than their mates.

“The males that did the best at both survival and reproduction had testosterone production very close to average,” says Joel McGlothlin, a postdoctoral fellow in biology at UVA who was one of the project’s leaders.

The researchers injected the juncos with a hormone that temporarily increased testosterone levels (mimicking what happens naturally when males fight). The scientists then measured their survival and success at reproduction, both in their own nest and elsewhere.

High levels of testosterone make birds more aggressive in securing territory and in mating, but fights can be costly; past studies also have linked elevated testosterone with suppressed immunity. Birds with low testosterone levels spend more time parenting and less time challenging other males.

Moderate testosterone levels favor a stabilizing effect in natural selection, McGlothlin says, but the relationships between testosterone and survival and reproduction are complex. “Testosterone seems to underlie this delicate balance between competing traits and behaviors, and the right balance might be different for different males.”