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,” Ketterson said.
The results showed that the high-testosterone males and the normal males ended the breeding season with about the same number of genetic offspring. “However, the two groups of males got their offspring through two very different pathways,” Ketterson said.
The normal males raised more offspring with their mates than did high-testosterone males, according to Ketterson. But DNA fingerprinting showed that the high-testosterone males had more genetic offspring with females other than their mates.
“These high testosterone males were having more success with females other than their mates,” Raouf explained.
These results are helping scientists learn how biological factors -- such as hormone levels -- influence the reproductive success of birds, Raouf said.
The research used dark-eyed juncos, a sparrow-sized species of bird that commonly breeds in Canada and mountainous parts of the United States. This study involved four years of studying juncos breeding in the Appalachian Mountains of Virginia.
The juncos were caught in early spring just before breeding. They were banded and a small blood sample was taken in order to later determine paternity. A small plastic tube was inserted under the skin of the males. In 58 of the males, the tubes slowly released testosterone over the course of the season. In 67 of the males, the tubes were empty. After this initial procedure, the birds were released and allowed to live naturally on their breeding grounds.
After the offspring hatched, the researchers then took a blood sample from the fledgling birds to determine their fathers through DNA fingerprinting.
The results showed:
- Normal males had significantly more young that survived to leave the nest (an average of 3.9 fledglings) than did high-testosterone males (2.7 fledglings). That may be because normal males provide more attentive care to young than do high-testosterone males. But nests of normal males also produced more fledglings (an average of 1.4) that were fathered by other males. High-testosterone males averaged only 0.74 fledglings sired by other males.
- High-testosterone males sired significantly more fledglings (an average of 1.21) than did other males (0.36) through inseminations of females other than their mates.
- The net result was that there was no real difference in the number of genetic offspring between the groups. High-testosterone males averaged 3.19 genetic offspring per year, while normal males averaged 2.94 genetic offspring, which is not a significant difference.
Dark-eyed juncos are normally monogamous. Normal males usually show a rise in their testosterone levels early in the breeding season when they are seeking a mate for the year. Research has revealed that male birds, when experiencing this spike in testosterone, are more likely to sing vigorously and adopt other behaviors that help them find and attract a mate.
However, after finding a mate, normal males will show a dip in testosterone. They then show less interest in other females, and an earlier study in this project demonstrated that they are more likely to help feed and care for their young.
“In this study, we manipulated some males so they always had the high level of testosterone that they would normally only have early in the breeding season,” Parker said.
The results suggest that testosterone may drive the behaviors -- such as vigorous singing -- that help males attract more females. It may also play a role in males spending less time taking care of their own fledglings, according to the researchers.
“The exciting thing is the experimental demonstration that testosterone underlies how males allocate their reproductive effort at home or elsewhere,” Ketterson said. “Without the DNA fingerprinting done by Raouf and Parker we would never have known that our manipulations caused juncos to become secretly polygynous.”
The researchers will continue the study in coming breeding seasons in the hope of learning more about testosterone’s role in explaining why some bird species are monogamous and others are polygynous. The work was supported by the National Science Foundation, the Indiana Academy of Science and the Center for the Integrative Study of Animal Behavior at Indiana University.