Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Natural Selection for Moderate Testosterone Surprises Scientists

06.05.2010
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 Indiana University Bloomington, the University of Virginia, Indiana University-Purdue University Indianapolis, and the University of Southern Mississippi 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. The results 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," said Joel McGlothlin, a Virginia postdoctoral fellow who conducted the project as an IU Bloomington 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 members of IU Bloomington Distinguished Professor of Biology Ellen Ketterson's research group saw.

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 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 nest with a particular female, but often attempt to mate with females mated to other males.

"We were able to pick apart what was going on there," McGlothlin said. "By splitting reproduction into two different components, we were able to see how successful male birds were at home versus how they did away from home."

After tallying within-pair and extra-pair matings, the scientists produced two more unexpected results. Higher testosterone was associated with more offspring from within-pair matings, and intermediate levels of testosterone were associated with more extra-pair offspring.

"Which may seem counterintuitive," McGlothlin said. "We think what's going on is a time trade-off. For a male to make sure he is fertilizing the offspring in his home nest, he must spend a fair amount of time near his nest and perhaps follow his paired female around to make sure no other males approach her. Testosterone makes males more territorial and more likely to engage in that behavior."

Low testosterone levels were shown by McGlothin, Ketterson, and others in 2007 to cause males to spend more time parenting offspring in their nests, but also to be less territorial. McGlothlin said that might explain why his group saw a balancing effect for testosterone on extra-pair offspring. Males with low testosterone may make fewer forays to other nests, reducing their likelihood of encountering other males, while those with very high testosterone may be staying home for a different reason -- to guard their territory against potential intruders. Alternatively, these high-testosterone males may be seeking extra-pair mates, but are unsuccessful, perhaps because the behavior they produce is unattractive to females.

In evolutionary biology, the fitness of an individual organism is dependent on its capacity for survival and reproduction, but it is also a relative thing -- for any given character, fitness depends on how one set of genes or attributes performs in relation to all the other individuals in a population. This study suggests that when it comes to testosterone and the behaviors it controls, moderation helps provide this competitive edge.

"These findings represent the culmination of 20-plus years of research directed towards understanding how variability in male testosterone relates to Darwinian fitness, and the outcome is deeply satisfying," said Ketterson, a principal investigator and coauthor. "Initially we took an experimental approach, which revealed that chronic elevation of testosterone reduced lifespan in the junco. But the story could not be complete without knowing whether males with naturally high or low levels of testosterone were at a survival advantage or disadvantage. Much of the literature on testosterone suggests that more is better. Our studies, experimental and correlational, now show that while 'high-T males', as we call them, may sometimes benefit in reproduction, too much testosterone leads to lower survival, and it's the average male that is the best performer."

Postdoctoral fellow Danielle Whittaker, Research Associate Sara Schrock, Ph.D., student Nicole Gerlach, (former) postdoctoral fellow Jodie Jawor, and (former) lab and field manager Eric Snajdr also contributed to this study. All coauthors began their participation while affiliated with IU Bloomington. Ketterson is a founding member and former co-director of the IU Center for the Integrative Study of Animal Behavior. Jawor is now an assistant professor at the University of Southern Mississippi, Snajdr is now a science librarian at IUPUI. The director of Mountain Lake Biological Station is former IU Bloomington biologist Edmund Brodie III.

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.

To speak with scientists Joel McGlothlin or Ellen Ketterson, please contact McGlothlin at 434-243-4338 or jmcgloth@virginia.edu, or contact David Bricker, University Communications, at 812-856-9035 or brickerd@indiana.edu.

"Natural Selection on Testosterone Production in a Wild Songbird Population," The American Naturalist, v. 175, no. 6, by Joel W. McGlothlin et al

Joel McGlothlin | Newswise Science News
Further information:
http://www.indiana.edu

More articles from Studies and Analyses:

nachricht Do microplastics harbour additional risks by colonization with harmful bacteria?
05.04.2018 | Leibniz-Institut für Ostseeforschung Warnemünde

nachricht Rutgers-led innovation could spur faster, cheaper, nano-based manufacturing
14.02.2018 | Rutgers University

All articles from Studies and Analyses >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Writing and deleting magnets with lasers

Study published in the journal ACS Applied Materials & Interfaces is the outcome of an international effort that included teams from Dresden and Berlin in Germany, and the US.

Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia...

Im Focus: Gamma-ray flashes from plasma filaments

Novel highly efficient and brilliant gamma-ray source: Based on model calculations, physicists of the Max PIanck Institute for Nuclear Physics in Heidelberg propose a novel method for an efficient high-brilliance gamma-ray source. A giant collimated gamma-ray pulse is generated from the interaction of a dense ultra-relativistic electron beam with a thin solid conductor. Energetic gamma-rays are copiously produced as the electron beam splits into filaments while propagating across the conductor. The resulting gamma-ray energy and flux enable novel experiments in nuclear and fundamental physics.

The typical wavelength of light interacting with an object of the microcosm scales with the size of this object. For atoms, this ranges from visible light to...

Im Focus: Basel researchers succeed in cultivating cartilage from stem cells

Stable joint cartilage can be produced from adult stem cells originating from bone marrow. This is made possible by inducing specific molecular processes occurring during embryonic cartilage formation, as researchers from the University and University Hospital of Basel report in the scientific journal PNAS.

Certain mesenchymal stem/stromal cells from the bone marrow of adults are considered extremely promising for skeletal tissue regeneration. These adult stem...

Im Focus: Like a wedge in a hinge

Researchers lay groundwork to tailor drugs for new targets in cancer therapy

In the fight against cancer, scientists are developing new drugs to hit tumor cells at so far unused weak points. Such a “sore spot” is the protein complex...

Im Focus: The Future of Ultrafast Solid-State Physics

In an article that appears in the journal “Review of Modern Physics”, researchers at the Laboratory for Attosecond Physics (LAP) assess the current state of the field of ultrafast physics and consider its implications for future technologies.

Physicists can now control light in both time and space with hitherto unimagined precision. This is particularly true for the ability to generate ultrashort...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Invitation to the upcoming "Current Topics in Bioinformatics: Big Data in Genomics and Medicine"

13.04.2018 | Event News

Unique scope of UV LED technologies and applications presented in Berlin: ICULTA-2018

12.04.2018 | Event News

IWOLIA: A conference bringing together German Industrie 4.0 and French Industrie du Futur

09.04.2018 | Event News

 
Latest News

Diamond-like carbon is formed differently to what was believed -- machine learning enables development of new model

19.04.2018 | Materials Sciences

Electromagnetic wizardry: Wireless power transfer enhanced by backward signal

19.04.2018 | Physics and Astronomy

Ultrafast electron oscillation and dephasing monitored by attosecond light source

19.04.2018 | Physics and Astronomy

VideoLinks
Science & Research
Overview of more VideoLinks >>>