NEW ORLEANS -- A new study indicating that portions of bird brains enlarge in response to social, as well as other environmental, factors adds to the mounting evidence showing that the brains of higher animals change over time.
Reporting on the first observed socially-induced change in the avian forebrain, a University of Washington researcher said here today that parts of the brains of male sparrows housed with females were distinctly larger than the brains of other birds living with males or in isolation. The size difference ranged from 15 to 20 percent in two brain regions that are involved in producing songs.
"This is a dramatic anatomical phenomenon. Previously detected socially-induced changes in other animals were much more subtle," said Tony Tramontin, a UW doctoral candidate in zoology, at the annual meeting of the Society for Neuroscience. Tramontin conducted the research in collaboration with Eliot Brenowitz and John Wingfield, UW professors of psychology and zoology, respectively.
In birds that breed seasonally, such as the white-crowned sparrows (Zonotrichia leucophrys gambelli) studied by the UW researchers, growth of the brain regions controlling the song system has been thought to result primarily from an increase in the length of days and an associated increase in the level of the hormone testosterone in the birds' bloodstream.
Environmental factors, such as social cues between mates, are known to influence the timing of the birds' breeding efforts, but less was known about how such cues might affect the song system and song behavior, according to Tramontin.
To find out, the researchers housed male sparrows in four different conditions. Seven males were paired in the same room under long-day conditions -- 20 hours of light -- with females in breeding condition. Eight other males were similarly housed without females. In addition, four males were isolated in separate rooms under long-day conditions and four males were isolated under a short-day regimen -- eight hours of light.
Three individual brain regions and the birds' forebrains were measured at the end of the study. The avian song system is composed of a group of interconnected regions or nuclei in the forebrain that control singing behavior in song birds. Two of these regions are called the HVc and the RA. A third brain region, the Rt, is not involved in song behavior and was measured to make sure that the effect of females was specific to the song system.
At the end of the 40-day period, the researchers found that volumes of the HVc and RA in the males housed with the females were significantly larger than in any other group. However, the size of the Rt and the forebrains were the same in all four groups. The UW researchers also analyzed blood testosterone levels in the sparrows and found they did not differ between social treatments. But the testosterone level was markedly lower in the males housed in the short-day condition.
Tramontin also noted that there were differences in song behavior among the groups. Males housed with females sang more than males housed together, who in turn sang more than the birds isolated under long-day conditions. The isolated birds living in short-day conditions didn't sing at all. This suggests that the seasonal changes in the sparrows' song system and the behavior it controls are regulated not only by the length of the day, but also by social cues, he said.
Scientists study the brains of birds and their song systems as models for understanding the human brain and how humans acquire language. The avian brain also serves as a model for looking at the changes in brain structure and function that take place throughout the lifetime of birds, animals and humans. Songbirds provide a system in which naturally occurring brain changes can be studied and related to a particular behavior --song -- that is crucial for communication between individuals
In addition, the avian brain also is a useful model for understanding the effects of hormones on the brains of vertebrate animals. The nervous systems of all vertebrates have receptors for the steroid hormones testosterone and estrogen. Because song learning and production are very sensitive to steroids, they are used as a model to learn how these hormones work on the nervous system to induce behavioral changes in humans and other animals.
The UW research was funded by the National Institutes of Health and the National Science Foundation.
For more information, contact Tramontin at the Andrew Jackson Hotel in New Orleans Oct. 25-30 at (504) 561-5881. In Seattle he can be reached at (206) 543-3356 or at email@example.com.