May 27, 2010 — Do nice guys finish last, or will the meek inherit the earth? A new study published in The American Naturalist suggests that, at least for birds, the answer lies somewhere in between.
Individual male birds can differ dramatically in their behavior, and this difference is often due in part to how much testosterone they produce. In many bird species, some males produce high testosterone and are more aggressive, while others produce lower levels and are more parental.
Testosterone and the behaviors it mediates may predict how well a male bird succeeds at survival or reproduction. For example, an aggressive male may be more likely to obtain high-quality territories that attract females. At the same time, aggression poses a survival risk, because aggressive males might be more likely to engage in costly fights. These potential effects of testosterone on survival and reproduction are expected to lead to natural selection, which is the driving force of evolutionary change.
To test this idea, a team of researchers led by Joel McGlothlin, a postdoctoral fellow in biology at the University of Virginia, studied a common species of songbird, the dark-eyed junco, near U.Va.'s Mountain Lake Biological Station. They tested how much testosterone a male could produce by injecting birds with a hormone that causes the bird to increase its testosterone levels temporarily (testosterone levels peaked at 30 minutes and went back to normal two hours later). This mimicked what happens naturally when males fight other males.
The researchers then followed the birds, measuring their survival and success at reproduction, both in their own nest and those of their neighbors. The birds often "court" neighboring females and mate with them, achieving what ornithologists call "extra-pair copulations" that lead to "extra-pair fertilizations." This is very common in most birds, McGlothlin said, leading to a situation where a male feeding baby birds is not necessarily their genetic father.
This information allowed them to measure what levels of testosterone production were favored by natural selection. The exact pattern of selection they found was surprising, however.
"The males that did the best at both survival and reproduction had testosterone production very close to average," McGlothlin said. "Overall, it was bad to produce either really high or really low levels of testosterone.
"This creates a specific type of natural selection called 'stabilizing selection,' which doesn't lead to evolutionary change in either direction. Instead, it maintains the status quo."
Despite this stabilizing selection, high-testosterone males did have one advantage: they were more likely to be the genetic father of the offspring raised in their nests.
"One of the complexities of this study was that we broke overall natural selection down into its specific components," McGlothlin said. "Males with high testosterone production did better at successfully mating with their own partner, but this seems to have been balanced out by reduced success at extra-pair fertilizations."
These results indicate that the relationships between testosterone and survival and reproduction are quite complex. "It's not as simple as saying testosterone is good for reproduction and bad for survival," McGlothlin said. "Testosterone seems to underlie this delicate balance between competing traits and behaviors, and the right balance might be different for different males."