June 17, 2008
How They Shine
By Milius, Susan
Iridescence could be pretty meaningful-or maybe just pretty Believe it or not, science has barely begun to fathom the peacock's tail. Subtle as a pink tuxedo, one mightthink. Bigflashything. Peahens love it. What's not to understand.
With their tails a wall of shimmer, they sidestep or sometimes strut backward to their audience. Dakin is testing her idea that there's a method here. For the final act of the show, males vibrate the big eye-bearing feathers so vigorously they make a rattling sound, and Dakin hypothesizes that the males' footwork maneuvers them and their audience to line up with the sun for the finale.
A female with sun right behind her gets the most dazzling angle on the feathers, and for a peacock, angles are everything. The fiery greens and blues that have become a symbol of extravagant ornament have no green or blue pigment in them. There's black pigment, but the rest is all just the play of light.
The trick for conjuring colors out of nothing depends on structure at the scale of hundreds of nanometers. At this scale, the smallest branchings within peacock feathers reveal themselves coated with arrays of rods. When light bounces off, certain wavelengths combine to intensify a color as other wavelengths interfere with, and cancel out, each other. The effect of this symphony of light shifts with the angle of view, the definition of iridescence.
Dakin described her work in February at a conference on iridescence held at Arizona State University in Tempe. The physicists who attended have been discovering that birds, beetles, butterflies and plenty of other creatures evolved cutting-edge optical systems long before modern technology did. Dakin and other biologists are now trying to figure out what the animals do with their light shows. These nanomarvels make excellent systems for testing ideas about how animal communication systems evolve.
One of the questions under lively debate at the meeting was whether iridescence has signaling power because it is difficult to manufacture or maintain. Only the best males would flaunt the brightest colors, and females would evolve to favor the flashiest fellows.
In contrast, Richard Prum of Yale University, a biologist at the conference, argues that searching for such clues to quality could be just wishful thinking. Iridescent glitter could appeal to female animals all right. But the driving force for evolving that preference could have nothing to do with the male's health or any other quality. The majority of iridescence, he says, could be arbitrary, or "merely beautiful."
Controlling color, naturally
Mere prettiness is no slur on the marvels of iridescent structures. A longtime iridescence specialist, developmental biologist Helen Ghiradella of the University at Albany, State University of New York, has published pages and pages of scanning electron microscope images revealing huge variety in the fine details of the textures of animal surfaces: bumpy surfaces like rows of Christmas trees, fields of lattice-work honeycombs, bristles that work like fiber optic cables (but better). She reels off examples of the cutting-edge developments in optics that she has observed in nature: thin films, photonic crystals ordered in one, two and three dimensions, plus surfaces that combine techniques.
She protests the unfairness of questions about which species flaunt the showiest iridescence. When pressed, though, she offers examples that include the Southwest's scarab beetle Chrysina gloriosa. The naked human eye can't detect the full light show, alas, so people have to make do with admiring the beetle's shimmery greenback. Equipped with the right instruments, though, an observer realizes that the beetle reflects the controlled spirals of both right- and left-handed circularly polarized light.
Even one of the field's old classics, the Morpho butterflies that Ghiradella studied during the 1970s, still hold surprises. In 2007, she contributed to aMorpho article in the February Nature Photonics published by a General Electric research team led by Radislav Potyrailo of the company's Niskayuna, N.Y., lab. Potyrailo had seen pictures of a Morpho wing nano-structure and realized that vapors of different gases should subtly alter the butterfly's iridescence. The GE team and Ghiradella analyzed the effects, which Potyrailo says suggest new options for developing sensors that change color with a whiff of a certain vapor.
Natural structures for controlling colors certainly should be an inspiration for engineers, and physicists should pay attention, says Andrew R. Parker of the University of Oxford in England. His group studies optical biomimetics, or nature-inspired technology. The animals' devices come from millions of years of evolutionary trial and error and, as he puts it, "the average physicist has rather less time."
Wings of wood
Imitating nature isn't easy. Peter Vukusic, who estimates his research group at the University of Exeter in England has looked for these structures in 500 to 600 species of insects, still uses words like "unbelievable."
He and his Exeter colleagues have attempted to replicate the surface complexity of a butterfly wing. Starting almost a decade ago, they experimented with building large-scale models of these structures, at first just for show-and-tell but then in the hopes of doing experiments to understand the novel optical properties.
Vukusic, a veteran of restoring old houses, started trying to create repetitious elements in wood the way a router shapes chair rails. He wasn't even trying to build a whole wing, since he'd scaled up so much that a single butterfly would spread more than a kilometer.
Even at that extreme magnification, the skilled and inventive fabricators for Exeter's laboratories struggled to produce even grossly simplified versions.
Then, while driving home one day, Vukusic says, he "experienced a moment of clarity -suddenly the mist rises." Vukusic abandoned several years' worth of wooden butterfly parts and used a rapid prototyping system to bring wings into the era of computer- controlled polymer shaping. He and his colleagues finally created chunks of opaque white plastic that mimic a fleck of wing surface accurately enough for research purposes.
"This thing looks like a dinner plate," he says. At this large scale, the model bit of a Morpho butterfly wing, for example, holds shapes that resemble a row of white Christmas trees, each a few centimeters high. At this scale, the models do nothing to light but can manipulate the longer wavelengths of microwaves as stand-ins. Vukusic's team is using these models and microwaves to study how insect wings create a silvery effect. His models starred at the February workshop in Tempe.
Animals might have a hard time with these specialized structures too. If they do, some biologists suggest that the challenges give iridescence its value.
In one scenario, the structures represent a handicap. Growing them might sap energy from other developmental processes. Or flying around as a living disco ball might stir up predators. Costly iridescence would become the male butterfly's Porsche, says Darrell Kemp of James Cook University in Cairns, Australia. In a related scenario, "iridescence is just plain difficult, not necessarily costly, for all males to generate, like a good sense of humor in human males," Kemp says.
Earlier work on what female butterflies like had resoundingly shown that color matters. When researchers blotted out the iridescent ultraviolet markings on the wings of male Colias butterflies, the researchers found that the males had a pretty lonely existence.
Yet Kemp argues these earlier experiments had created such drastic changes in male finery that researchers couldn't say in what way the color mattered. The female might have rejected the male because she no longer recognized him as the right species. He revised experimental procedures and worked with Hypolimnas bolina butterflies. The upper surface of their wings are iridescent in ultraviolet wavelengths, which females ofthat species can see. The males must look like flashing beacons as they flap their wings.
To avoid the extremes of earlier experiments, Kemp used a screening substance to dull the males' wings to about half their former UV brilliance. For comparison, he also blacked out the UV patches with a pen on some of the males. In tests in fields and enclosures, marked males failed to attract the attention that females bestowed on the full-UV fellows. The loss of brightness matters to female butterflies in choosing mates, he concluded last year in Proceedings of the Royal Society B.
A similar experiment finds the same dynamic in Eurema hecabe butterflies. Dulled males meet with less success in mating, particularly in attracting the supposedly more desirable large females, Kemp reports in the January/February Behavioral Ecology.
So Kemp says he's convinced that females pay attention to males' iridescent light shows. Now he's working on understanding what kind of information those shows might contain. He has raised caterpillars under sorry conditions and checked to see if their displays changed. Both those that had to make do with skimpy rations and those that as pupae endured great swings of heat and cold grew poorly. As adults, their wings did not flash as brightly. Also, he noted that the iridescence seemed to diminish more than other traits he checked, such as pigment colors. Thus the intensity of iridescence could serve as a sensitive indicator of a male's history. One theory had also proposed that color signals could carry information about genetic quality, perhaps identifying certain males with the built- in resistance to laugh off slings and arrows of developmental stress. Kemp looked for signs that clusters of related individuals looked pretty good despite the stresses. Nice idea, but in this case, no support.
Prum says he accepts that animals use traits like iridescence as signals. What he objects to is what he describes as a widespread presumption that signals routinely carry information pertinent to the decision at hand. Some human signals, like onomatopoeic words, do carry clues to their meaning. Pop, snap, murmur. But plenty of human signals, like the words plenty of human signals, don't. Genetic modeling, says Prum, shows that animal signals can easily arise without some innate relevant clue, such as a connection to male quality. So he hypothesizes that most animal signals will turn out to belike plenty of human signals.
A Morpho butterfly wing shines bright blue but contains no blue pigment. The intensity of blue varies as the butterfly flaps its wings, reflecting light at different angles. "At a certain point that blue color turns on like a bright LED light," says evolutionary biologist Nathan Morehouse of Arizona State University in Tempe.
A Morpho wing steers light into iridescence. Light hits the wing's nano-structures (scanning electron microscope image at left). Different colors will take different paths back out, steered by the structures' texture. The light waves (red dots) in the middle image are constructively interfering, moving such that they amplify each other and the wing appears intensely blue. Colors in the last image destructively interfere and cancel each other out.
Peacock feathers carry a black pigment but no green, blue or brown. The shimmer and extra color arise from the play of light over arrays of minuscule rods in the feather's surface.
* Darreil J. Kemp. "Female mating biases for bright ultraviolet iridescence in the butterfly Eurema hecabe (Pieridae)," Behavioral Ecology. Jan./Feb. 2008.
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