Seeing Colors in a New Light

Many animals advertise their attractiveness with showy colors beyond the limits of human perception

  • Cynthia Berger
  • Dec 01, 2001
Many animals advertise their attractiveness with showy colors beyond the limits of human perception

THE HUMAN VIEW (left): A citrus swallowtail butterfly under normal lighting, showing colors as people see them.

THE WILDLIFE VIEW (right): The same butterfly under ultraviolet light, a view available to many creatures.

WHEN IT'S TIME to redecorate your living room, color is the key to selecting your paint and fabric. In the natural world, color can also be the key to selecting a mate. In nature, as in home decor, often the female does the choosing--and often, she picks her partner for his pleasing color. Girl guppies go for the guys with the biggest orange spots; female house finches fall for fellows with the brightest red feathers. Scientists call this process female mate choice, and say it explains why, in some species, the two sexes look so different: The male that gets the mate passes on his traits to his male offspring.

Scientists who study female mate choice still occasionally quantify an animal's color using an interior decorator's tool: They hold up standardized color swatches next to fins or feathers. And often, they trust the evidence that's right in front of their eyes: If Male A looks bolder and brighter than Male B, it seems to make sense that females prefer Male A. Yet new research suggests a scientist's visual judgment can be as unreliable as that of a color-blind interior decorator. It seems a number of species advertise their attractiveness with showy colors we humans can't see.

In a landmark study published in the journal Nature, Andrew Bennett, Innes Cuthill and Julian Partridge of the University of Bristol in England showed that female zebra finches (striped songbirds from Australia) respond to ultraviolet (UV) light reflected from a male finch's feathers--a signal undetectable to human observers. Other studies yielded similar results for European starlings and the chickadeelike birds called blue tits. "We readily accept that a dog's sense of smell is far richer than our own," wrote the researchers, "[yet] we often ignore the possibility that other animals see things that are beyond our power to detect."

"The thing that has made this discovery possible is technology," notes Richard Prum, an ornithologist at the University of Kansas. "Fiber-optic spectrophotometers [instruments that measure the wavelength and intensity of light] are coming down in price and going up in ability. They're the first tool that really works well for this kind of study. Yet here in America, some ornithologists are still getting out their paint chips."

NOT TRULY BLUE: The feathers of a male blue grosbeak (left) reflect light that looks dazzling blue to us. While all males of the species look similar to humans, birds see the world differently. Scientists recently found that grosbeak feathers reflect most strongly in ultraviolet (UV) wavelengths, visible to birds and other creatures. What's more, male grosbeaks vary in the intensity of their UV colors, perhaps advertising their fitness to prospective mates.

If female birds can see UV light reflected from feathers, why can't we? Human eyes have three kinds of color receptors (cones) on the retina. Each cone responds to light of a different wavelength, corresponding to the colors red, green or blue. In contrast, birds (at least the species tested) have four types of cones. One of these responds to ultraviolet light, which is outside the range of human vision.

In light of the new findings, Mike Webster is rethinking his research methods. The Washington University scientist studies female mate choice in the black-throated blue warbler, measuring such male traits as the size of white wing spots. Now he's considering UV evaluations. "The fact that birds can see in the UV, and that their feathers reflect in the UV, throws a huge monkey wrench into the game of studying mate choice and sexual selection in birds," he says, "because we're not seeing what they're seeing."

For bird scientists, this is a fresh insight. But scientists who study insects and fish are not so surprised. "The media made a fanfare about Bennett's work," says John Endler, a former fish researcher at the University of California–Santa Barbara who is now working to develop a camera that will see the world the way birds do. "But nonbird people say, ‘We've known about ultraviolet signaling for the past 50 years!'"

Indeed, many species of fish, insects and reptiles see and respond to UV light. Bees follow UV-reflecting "nectar guides" on flowers much the way airplanes follow runway lights to the terminal. Desert iguanas mark the ground with UV-absorbing urine, leaving territorial signals that stand out against the UV-reflecting sand. "Ultraviolet's just one more color," says Union College's Leo Fleishman, who studies green anoles, little lizards with UV-reflecting throat pouches. "But if you can't see it, you're going to miss a lot."

Yet even if "invisible" ultraviolet signaling is fairly well-known in the natural world, the role of UV color in female mate choice is still relatively unexplored. Consider butterflies. What we know so far comes from research back in the 1970s. Harvard researcher Robert Silberglied suggested that ultraviolet signals keep two look-alike butterflies--the orange sulfur and the clouded sulfur--from interbreeding. These small butterflies look virtually identical to the human eye--both have bright yellow wings with black borders. But the wings of a male orange sulfur butterfly reflect UV light; the wings of a clouded sulfur do not. Silberglied proposed that females use their ultraviolet vision to identify a mate of the proper species.

SEEING MORE THAN RED: Research reveals that the healthiest males have the reddest feathers, and females prefer the ruddy look in mates. Red, yellow and orange pigmentation in feathers often derives from food and is easily visible to humans as well as birds. But such carotenoid colors can also produce vivid ultraviolet reflections. The scarlet ibis (left), for example, might be more correctly named the ultrapurple ibis. 

But female mate choice means more than getting the species right; it also means choosing the best mate from a range of same-species suitors. Recently, Randi Papke, a doctoral student at Arizona State University, took Silberglied's research a step farther, testing the idea that UV signaling plays a role in female mate choice for orange sulfur butterflies. Her experimental design was straightforward. "I'd throw a female butterfly in the air in front of a flying male and watch," she says. Papke found that males that managed to captivate a mate reflected ultraviolet light twice as brightly as males that got the brush-off.

It feels intuitively sensible that the best-dressed guy gets the girl. But inquiring scientists want to know: Can a female be sure that "flashiness" equals "great mate"? Is the butterfly or bird with the brightest wings really the equivalent of a Harvard-educated Olympic athlete/billionaire? Or is he just an average guy in a discount Armani suit? One idea is that colorful displays will endure among males in a population only if the displays are an "honest advertisement" of some superior quality that leads to better reproductive success. (Other scientists argue that females simply go for the guy who catches their eye, and that choice is enough to perpetuate flashy displays.)

One possible honest advertisement is carotenoid colors--the bright orange, red and yellow pigments found in carrots and many other plants. In general, animals get these from the foods they eat. So bright carotenoid colors can be a sign of a successful food finder. New research also reveals that carotenoids may help promote a healthy immune response. Therefore, males that use their store of carotenoid pigments to fight disease or parasites may not have color to spare for a brilliant courtship display.

Does a similar link between color and male quality exist for UV signals? Scientists aren't sure--but they have some ideas. "I haven't specifically looked at this yet," says Papke, "but I would speculate that making specialized reflective scales is an energetic investment," one that only high-quality males could make. Another idea is that females read in a butterfly's bright wings the promise of a special present.

The reasoning goes like this: When sulfur butterflies mate, Papke says, the male passes a little blob of protein to the female along with his sperm. Inside the female's reproductive tract, the protein will be packed into the eggs, to give the hatchling caterpillars a good start in life. Young butterflies mating for the first time pass along the biggest protein packets, Papke notes. Older males, which may be mating for the second or third time, give smaller gifts, since they can't reprovision their protein supplies. "Obviously, it's to a female butterfly's advantage to choose a young mate who will supply as much protein as possible," Papke says. And perhaps she does that by picking a male whose wings are shiny and new.

At Alabama's Auburn University, Geoffrey Hill has spent the past decade demonstrating that male house finches make honest advertisements with their carotenoid colors. Male finches usually have raspberry red feathers on the breast, head and rump, but individuals can vary from bright red to pale red to shades of orange or yellow. Research reveals that the best-fed males--and the males with the fewest parasites--have the reddest feathers. Not surprisingly, females prefer the reddest males in mate-choice tests.

Recently, Hill turned his attention to the blue grosbeak, a sparrow-sized, thick-billed bird of brushy fields and woodland edges. Female grosbeaks are plain-Jane brown, but males are a dazzling blue; this color comes not from pigments in the birds' food, but from the microscopic structure of the feathers, which reflect blue light. "I knew carotenoid pigments had this connection to diet quality," says Hill. "I wondered, does a completely different plumage signaling system also give females reliable information about a male? Nobody had ever studied blue birds for variation in color because the conventional wisdom was, ‘There is no variation.' They all look equally blue to us."

With graduate student Amber Keyser, Hill collected feathers from about two dozen male blue grosbeaks and put them under a spectro-photometer to measure peak wavelength (the wavelength at which feathers reflected the most light) and intensity (the amount of light reflected at the peak wavelength--what we would call brightness). Both fell in the ultraviolet wavelengths, suggesting that this bird may need a name change to "ultraviolet grosbeak." In a separate study, Keyser and Hill found individual male grosbeaks varied in the intensity of their UV reflectance. So although grosbeaks look alike to us humans, female birds can see the difference.

Do those ultraviolet differences tell a bird anything about a prospective mate? According to Keyser and Hill, feather growth patterns revealed that the males with the most intense UV colors had gotten plenty to eat while their flashy feathers were growing in--so, a bright bird is also good at finding food. In a recent study, however, Hill and graduate student Barbara Ballentine found no evidence that female blue grosbeaks select mates for their ultraviolet colors. Noting that the intensely UV males had the biggest bodies--plus, they had managed to grab the largest territories, where insect prey was most abundant--Keyser and Hill say flashy plumage may help the male who flaunts it compete with other males for territory. More research is planned.

Still, Hill sees his work as evidence that ultraviolet colors are an honest advertisement of a bird's overall quality. "Carotenoids and feather structure are completely different ways to get your feathers colorful," he says, "yet they both seem to be condition- dependent traits."

But other scientists disagree. Prum, an expert on the structure of feathers, says, "So far, there are no controlled data showing that birds raised under different conditions have different structural colors in their feathers. To say we think structural colors can be perturbed by diet is a huge leap."

Prum also points out that the distinction between carotenoid colors (visible to humans) and UV colors (invisible to humans) isn't quite so neat as it seems at first. "Birds have some carotenoid pigments that reflect vividly in the ultraviolet," he says. "For example, scarlet ibises look screaming red to us; that's a carotenoid color, but it also reflects UV. To each other, ibises look ultrapurple!" So past studies of the role of carotenoid colors in female mate choice may need to be evaluated in a new light.

Perhaps this report of ultraviolet signaling makes you wish you could see the world the way birds and butterflies do. To some extent, you can, but the means is radical: cataract surgery. Ordinarily, the lens in the human eye screens out UV rays; but when the natural lens is replaced with a plastic lens, as in cataract surgery, UV rays can pass through. And human cones do respond just a bit to near-ultraviolet light. "It happened to my mother," says Endler. "She started to see the nectar guides on flower petals!"

No one's recommending this drastic step as a research aid, however. And even after surgery, you'd still lack the fourth type of cone that birds possess. That means, says Prum, "We can never see what birds see. We can't conceive of it." But maybe he's wrong; after all, scientists have vision as well as eyesight.

Pennsylvania journalist Cynthia Berger wrote about pigeons in the August/September issue.

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