Natural Selection for Moderate Testosterone Surprises Scientists

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 report in the June issue of The American Naturalist (now online) that extreme testosterone production -- high or low -- puts male dark-eyed junco at a disadvantage in both survival and reproduction outside their semi-monogamous breeding pairs.

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