and formation of feathers
Two recent studies by Yale researchers have focused on our winged friends -- with special emphasis on their feathers.
In the courtship dance of a male South American bird, the club-winged Manakin, (Machaeropterus deliciosus), rubbing and vibrating specialized wing feathers together creates a courting melody to attract their mates, according to a report in Science.
Since Darwin proposed his theories of selection and evolution, it has been suggested that sounds made by the feathers of some birds evolved by sexual selection. Richard O. Prum, the William Robertson Coe Professor of Ecology and Evolutionary Biology, and his former student Kimberly S. Bostwick, a curator in the birds and mammals division of Cornell's Museum of Vertebrates and a research associate in the Department of Ecology and Evolutionary Biology, describe a mechanism unique among vertebrates that supports this theory.
Using high-speed digital video recordings of the courtship displays, they show that males produce sustained, harmonic tones by rubbing together secondary wing feathers. These highly specialized feathers have shafts that are enlarged hollow, club-like structures. They are bent and slightly twisted so that the ends can touch each other creating a ringing "TickTick Ting" song when rubbed together. A video of this phenomenon can be viewed online at www.sciencemag.org/content/
As loud as a typical bird vocal song, the wing song is easily heard tens of meters away, note the researchers. The "tick" notes are sharp clicks, and the "ting" is a sustained, violin-like note that lasts about one third of a second. "It is a bit like running your finger over a comb," says Prum, who as thesis advisor of the project, was initially skeptical of the idea that the sounds could be created that way.
"We ruled out a lot of hypotheses," says Bostwick, "but when I realized that the wing feathers were twisted in a way that forced them to rub, I knew we had something."
Prum notes, "The extensive modification of feathers in these unusual structures implies a high degree of selective evolutionary pressure.
"It must be a very romantic song," he adds.
Bostwick, who studied sexual attraction behavior in several species of this family, previously reported one that sings less spectacularly, but claps and has a courting dance that resembles Michael Jackson's "moonwalk." A video of this phenomenon can be seen at www.pbs.org/wnet/nature/deepjungle/episode1_bostwick.html.
"Nature is more miraculous than we know," says Bostwick. "It is amazing that we can walk into the woods, or jungle, and find this kind of beauty and diversity."
A National Science Foundation Grant funded this study.
Biologists testing a mathematical model of the mechanism birds use to control the growth of complex feathers found that plumed feather structures involve the coordination of at least two genes that activate and inhibit barb growth.
"Understanding these mechanisms of feather growth gives a whole new perspective on the unique beauty of feathers," says Yale ornithologist Prum, senior author on the study.
Prum, who is also curator of ornithology and vertebrate zoology at the Peabody Museum of Natural History, led a team of biologists that used a combination of mathematical and molecular methods to reveal some of the secrets of branched feather growth and propose how the unique complexity of feathers may have evolved. Other team members included anatomists Matthew Harris and John Fallon at the University of Wisconsin, statistician Scott Williamson at Cornell and Hans Meinhardt at the Max Planck Institute.
Their findings provide the best experimental evidence for a classical theory for growth of complex biological structures. Alan Turing, a mathematician, pioneering computer scientist and code-breaker, proposed in the 1950s that repeated patterns could emerge through the interactions among chemical morphogens or molecules that cause things to develop -- an activator that makes things happen, and an inhibitor that suppresses the activator.
To test the model in feathers, Harris forced expression of the activator, Shh, or the inhibitor, Bmp2, in the skin of six-day old chick embryos by injecting them with a retrovirus. The results were seen in localized patches and demonstrated that a simple relationship between developmental genes could be the basis for formation of feather structures. The study was the first documentation, in any plant or animal, that signaling molecules in development can actually behave as envisioned by Turing 50 years ago.
This work provides a key to some of these most basic questions of biology, notes Prum. The findings also indicate that more complex shafted feathers evolved from the simpler downy tufts by the addition of new players to the original activator-inhibitor pair. Prum is now following up on several clues in the search for these other molecular signals.
-- By Janet Rettig Emanuel
T H I S
Wooing wing song
vol309/issue5735/images/data/736/DC1/bostwick-SOM-movieS1.mov.
Secrets of feather growth
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