The current American Naturalist study argues that natural selection is favoring intermediate testosterone levels in the population of male juncos the scientists examined, and because testosterone-mediated behaviors are often inherited, the prediction is that moderation will prevail over time.
"As for why, we don't know for sure what's going on yet," McGlothlin said. "We expected that high testosterone would lead to lower survival rates, and that's what we observed. But it's not clear why low testosterone also led to lower survival rates. We would have expected the opposite. We have some ideas, and that's something we're going to investigate soon."
Employing a great deal of student help, the scientists mapped out all junco nesting sites in their study area. From spring to fall, the researchers caught males multiple times. Immediately after capture and prior to release, the juncos were given a gonadotropin releasing-hormone "challenge," which causes the birds' bodies to produce testosterone. Past studies have shown the levels of testosterone induced by the challenge are proportional to the birds' natural tendency to produce testosterone in response to a rival in the wild. The challenge therefore gives scientists a sense of what levels of testosterone production are normal for each bird.
The researchers also collected DNA samples from the males and their mates. They then visited each nest and collected DNA from the nestlings. Comparisons of the male genotypes with offspring genotypes revealed paternity and told the scientists just how successful each particular adult male junco was at reproducing. Juncos are semi-monogamous in that males pair and nes
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