natural selection

The downside to tons of testosterone

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Researchers studying dark-eyed juncos found that the males that were most likely to survive produced intermediate levels of testosterone. Likewise, the male juncos that produced the most offspring were also more likely to produce moderate levels of testosterone. High and low testosterone production, on the other hand, were associated with failure in both survival and reproduction. (Credit: Joel McGlothlin)

INDIANA U. (US)—Too much—or too little—testosterone may put some males at an evolutionary disadvantage, according to new research involving an American songbird.

Researchers found that extreme testosterone production puts male dark-eyed junco at a disadvantage in both survival and reproduction outside their semi-monogamous breeding pairs. The results—reported in the June issue of the American Naturalist—are based on a wild population of juncos studied near the University of Virginia’s Mountain Lake Biological Station.

“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,” says Joel McGlothlin, a Virginia postdoctoral fellow who conducted the project as an Indiana University Ph.D. student. “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 trade-off, by exerting opposing effects on survival and reproduction in wild populations. High testosterone is often associated with aggression in male animals and sometimes with suppressed immunity. These effects can harm a male’s chances of survival but also yield more opportunities for him to mate.

Low testosterone production is presumed to have the opposite effect—increased survival and fewer mating opportunities.

But that isn’t what the research group found.

Instead, they saw “stabilizing selection” on testosterone production when looking at both survival and reproduction. The male juncos that were most likely to survive produced intermediate levels of testosterone. Likewise, the male juncos that produced the most offspring were also more likely to produce moderate levels of testosterone. High and low testosterone production, on the other hand, were associated with failure in both survival and reproduction.

The 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 says. “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.”

Researchers from Indiana University Bloomington, the University of Virginia, Indiana University-Purdue University Indianapolis, and the University of Southern Mississippi contributed to the work. The study was funded by grants from the National Science Foundation, the American Ornithologists Union, the University of Virgina’s Mountain Lake Biological Station, Sigma Xi, and the Wilson Ornithological Society.

More news from Indiana University: http://newsinfo.iu.edu/

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