SciFri 2.11.05

A male peacock has reason to be proud. He's so healthy, he can afford to squander energy accumulating and arranging pigments in his beautiful tail.

Color me essential: From stars to atoms, colors make the difference

By Deane Morrison

Published on February 11, 2005

On a walk through a meadow, you see a bee swooping down on a black-eyed susan. The flower, with its yellow petals and dark center doesn't seem unusually beautiful, but the bee finds it irresistible. In a nearby tree, two identical-looking birds are performing what can only be a mating ritual; how can they tell who's male and who's female? When trees turn color in the fall, red and yellow pigments become visible. Why are they there? And how about stars--our sun is yellow, but some glow red, white, or even bluish.

In nature, color serves multiple purposes, only a few of which are obvious to our eyes. A complete survey would fill several volumes, but a few instances will indicate the spectrum of roles color plays in the world around us.

As a bee flies over a green meadow with ultraviolet-reflecting flowers, it may see what looks like the night sky: a dark background with the lights of individual flowers zipping by underneath like tangible, close-up stars.

If there's a king of the pigments, it would have to be chlorophyll, the green molecule that traps the energy of sunlight and stores it in sugar, the basis of life for humans and other animals. The molecule appears green because it absorbs lots of blue and red light, but reflects green light, along with yellow and orange. Chlorophyll's job is to convert carbon dioxide and water into sugar and oxygen, and it absorbs the wavelengths of light that have the right energy to allow it do its job. (Blue light has lots of energy, red has less, and green is in between.)

But plants have accessory pigments called carotenoids, which absorb light wavelengths that chlorophyll can't, and which aid the process of photosynthesis. Carotenoids include the carotenes, which are usually red and orange, and xanthophylls, which are yellowish. They are responsible for the red of peppers and tomatoes, the orange of carrots, and the colors of animals that eat carotenoid-containing food, like pink flamingoes. Carotenoids help protect a plant's photosynthetic apparatus from light damage and serve as antioxidants and sources of vitamin A in our bodies.

Color coding

In addition to photosynthesis, plants use their pigments to signal pollinating species like bees, and some of those signals are invisible to us. Bees can see ultraviolet light, which is beyond the realm of human vision. But they can't see red; therefore, their vision is shifted toward the blue end of the spectrum.

Take that black-eyed susan. To a bee, its petals are dark toward the center, bright near the tips.

"In the ultraviolet, the flower looks like a bull's eye," says bee expert Marla Spivak, an associate professor of entomology. "Flowers that have nectar and pollen and require outcrossing (not self-pollination) often reflect ultraviolet light. It's like landing lights to a bee." Dandelions are another common flower that sends out a UV searchlight.

Red flowers are not often pollinated by bees, says Spivak. Red probably looks gray or black to a bee, whereas blue and yellow flowers stand out. Therefore, as a bee flies over a green meadow with ultraviolet-reflecting flowers, it may see what looks like the night sky: a dark background with the lights of individual flowers zipping by underneath like tangible, close-up stars.

Another aspect of bee vision: their eyes can respond to light much faster than ours. A bee would be able to pick up the flickering of a fluorescent light, and if bees went to the movies, they would see the individual frames rather than smooth motion. They can detect flashes up to 150 and maybe 200 times a second-at least three times faster than the average film speed.

Birds, too, can see ultraviolet light. For example, the male and female blue tit have blue crowns, but the male's crown looks brighter in UV light, says Muir Eaton of the University of Kansas, a former postdoc fellow of Bell Museum director Scott Lanyon.

"It appears a lot of species have some color differences between the sexes," says Eaton. "In many cases, our color discrimination isn't good enough to see them." But humans are a highly visual species; why can't we see in ultraviolet? "I think the reason is that our lens blocks out UV light," he says.

For many animals, it's not size, but color

Birds may signal their fitness by their color. A male bird that is well fed and strong can accumulate lots of carotenoid pigments from plants to give himself a colorful appearance. When a peacock displays to a pea hen, he is showing her how healthy he is-so healthy, he can afford to squander energy accumulating and arranging pigments in his magnificent tail. But melanin, a dark pigment that many animals can synthesize, also may signal fitness or dominance. For example, says Eaton, the size of the male house sparrow's black throat patch seems to signal his dominance in a flock. Mammals also use color signalling. For example, research by the U's Craig Packer and Peyton West indicated that the sexiest male lions are ones with long dark manes.

Animals use color for all kinds of other purposes, including camouflage and fooling predators. Mimicry between species is common. A classic example is the monarch and viceroy butterflies. Many monarchs are distasteful to birds, so a bird that eats one quickly learns to avoid this beautiful insect. The viceroy tastes yummy, but it has evolved to resemble the monarch enough that birds generally give it a wide berth, too. Other animals mimic their surroundings. Some octopi use specialized pigment-containing cells to blend into their surroundings. By contracting or expanding the cells, they can change color and patterns in a matter of seconds.

Many respiratory pigments, such as hemoglobin, are brightly colored. Hemoglobin is similar to chlorophyll in that it's a protein with a special structure that gives it color. That structure is called a porphyrin molecule, and its job is to hold an atom of iron, which holds the oxygen in our blood and makes it red. Another respiratory pigment, hemocyanin, is found in certain mollusks and arthropods. It contains copper instead of iron, and when carrying oxygen, it is blue instead of red.

Overhead, the stars appear in colors ranging from blue and white to yellow and red. Red stars are huge and cool, like Betelgeuse, the eastern shoulder of Orion, and Antares, the heart of Scorpius, both about 5,500 degrees F. Compare them to Sirius, the brightest star in the sky. It is a multistar system, but its color is white--bluish through a telescope--and its temperature is more than 16,000 degrees F. Our sun is yellow, and its surface temperature is a middling 10,000 degrees F. The colors of stars may not be apparent without a telescope, because the cone cells in our retinas--the ones sensitive to color--don't work very well at night.

But during the day, when we look at the birds, flowers, insects, plants, what a feast around us.