COLUMBUS, Ohio -- What makes a normally monogamous male bird stray from his mate to copulate with neighboring females?

High levels of testosterone may be part of the answer, according to a new study. Researchers found that male dark-eyed juncos that were given extra testosterone during the breeding season were more likely than normal males to fertilize females other than their mates.

The junco’s family at the home nest paid the price for these sexual encounters. Results showed that high-testosterone male juncos spent less time providing care for young in their own nests. Researchers suspect this is why high-testosterone males, when compared to normal males, produced fewer offspring with their own mates but more offspring with the mates of their neighbors.

The study was conducted by Samrrah Raouf, a graduate student, and biologists Ellen Ketterson and Val Nolan, the project’s directors, all of Indiana University. They collaborated with Patricia Parker, associate professor of zoology at Ohio State University, who is an expert in mating systems of birds and in determining parentage; and Charles Ziegenfus of James Madison University, who has studied juncos for many years. Their findings were published in a recent issue of the Proceedings of the Royal Society of London Series B - Biological Sciences.

“Researchers all over the world are using new genetic tools like DNA fingerprinting to understand animal mating behavior,” Parker said.

“In many cases, the results reveal that clandestine matings are taking place, but we often know little about the conditions under which they take place, or the costs and benefits of different mating tactics,” Parker explained.

“This study suggests testosterone plays an important role in determining how often males wander from their home nests to fertilize other females,
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Contact: Patricia Parker
Parker.3@osu.edu
(614) 292-0378
Ohio State University
10-Feb-1998