Mating mystery from testosterone in birds.

• Position: Natural Selection

A field study of the relationship between testosterone and natural selection in an American songbird, the dark-eyed junco, has defied some expectations and confirmed others. Scientists from the University of Virginia, Charlottesville, report that extreme testosterone production--high or low --puts male dark-eyed juncos at a disadvantage in both survival and reproduction outside their semi-monogamous breeding pairs. The results are based on a wild population of juncos studied near the state's Mountain Lake Biological Station.

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"Our goal in this study was to characterize natural variation in testosterone production in the wild and to learn how that relates to natural variation in survival and reproductive success," notes postdoctoral fellow Joel McGlothlin. "We learned there are far more complex things going on here than we expected."

Past studies of juncos (and other animals) have shown that testosterone presents something of a tradeoff by exerting opposing effects on survival and reproduction rates in wild populations. High testosterone often is associated with aggression in male animals and sometimes with suppressed immunity. These effects can harm a male's chances of survival but yield more opportunities for him to mate. Low testosterone production is presumed to have the opposite...