- GENERAL ARTICLES -
Hilton Jr., B. 1996. Animal colors. South Carolina Wildlife 43(4):10-15.
(Note: The draft below was submitted to South Carolina Wildlife magazine; the article that actually appeared in print may have been edited.)
"Bluebirds aren't really blue."
"Bullfrogs aren't really green."
"And cardinals can't really make red feathers!"
Comments like these would raise a few eyebrows and might even lead to heated argument among observers of the natural world. But they're true statements and are just two of many fascinating facts to be learned when we examine the role of color in the lives of South Carolina animals.
Most humans seem to take colors for granted. With its rods (sensitive to varying intensities of light) and cones (which help differentiate between various colors), the human eye is a marvelous receptor that allows our brains to see the world. Except for those of us with certain inherited traits, most humans have no trouble differentiating between shades of red and green, and everywhere we look we see a world enriched with bright, or even subtle, hues. Many of our fondest memories of natural events have colors at their core: the rosy pink of an autumn sunset, blue blossoms of a handful of spring violets, or the brilliant scarlet of a winter Northern Cardinal munching on sunflower seeds.
In the animal world, colors certainly did not evolve for the viewing pleasure of humans. Each brightly colored feather, each pigmented hair or scale has a meaningful function for the animal that bears it, and the significance of colors and the ways they are produced are as varied as the different species of animals themselves.
Among South Carolina animals, from tiny bugs to lumbering bears, external coloration is usually produced by pigments--small molecules or granules that reflect specific wavelengths of light. The most common animal pigments are melanins--brown or black polymers that occur in skin or fur. Melanin absorbs most color wavelengths and therefore appears very dark to the eye.
Animals synthesize their own melanin at the cellular level, laying down varying amounts in different cells. In a Carolina Chickadee, for example, there is plenty of black melanin in the bird's cap and bib feathers, much less in its gray back and wings, and none at all in its white cheek patches. Melanin is the predominant pigment in many small mammals, hence their muted earth-tone fur.
Although animals do manufacture their own melanin, they can't make many other pigments. This makes it hard to explain the brilliant yellow of the male American Goldfinch or the startling orange skin of a juvenile Red-spotted Newt. These two species, like nearly all animals, have no ability to produce yellows or oranges or reds on their own. Despite his appearance, even our male cardinal at the feeder cannot synthesize his own red pigmentation.
Plants, on the other hand, are very good at producing carotenoids--tiny pigment molecules that come in varying hues of red and yellow. Sometimes a plant lays down carotenoids in its flowers, sometimes in its roots, and often in its fruits or seeds. Flowering Dogwood, for example, produces an autumn berry that starts out green but turns brilliant red when ripe. The dogwood thus advertises when its seed is mature, attracting the attention of the cardinal at a time when he is most likely to be able to eat the berry, pass the seed, and disseminate another generation of dogwoods. At the same time, the cardinal picks up a nutritious snack from the berry's pulp, and--lo, and behold--he has also found a source of red carotenoid pigments!
Even though the cardinal hasn't synthesized carotenoids on its own, he is able to lay down red pigments in his feather follicles that, in turn, produce the bird's familiar plumage. Cardinals also get orange and red and yellow colors from other kinds of seeds, including those that we put out in our feeders each winter. The variety and intensity of fresh pigments in natural, wild food sources are what keep the cardinal's colors so vibrant. If one were to capture a cardinal and raise him indoors on a diet of bird seed, with each successive feather molt his plumage probably would become increasingly drab.
Roseate spoonbills---rare wading-bird visitors to the Carolina coast-or the more familiar pink flamingos at Riverbanks Zoo are both carnivorous filter-feeders; they eat planktonic animals such as brine shrimp that also cannot make their own carotenoids. The tiny shrimp, however, DO eat microscopic algae that manufacture red and yellow pigments, so when the spoonbill or flamingo consumes its prey it is able to acquire its carotenoids second-hand. (Most zoos supplement the diet of their flamingos with plant pigment extracts to provide intense coloration. After all, who would want to visit a zoo to find "gray" flamingos that hadn't been getting their minimum daily requirement of carotenoids?)
STRUCTURAL COLORS IN ANIMALS
Just as few animals are able to manufacture their own red and yellow pigments, most are also unable to make blues and greens. Although green chlorophyll pigments are common components of an animal's diet, green or blue coloration in animals is produced in a very different way--when light is altered by microscopic attributes of the animal's external covering.
In the case of our "not-really-blue" Eastern Bluebird, the explanation goes like this. Bluebirds manufacture melanin pigment and lay it down in growing feathers; this alone would make a bluebird a black bird. However, the bluebird's feather also contains tiny air sacs. When light strikes the feather, air sacs scatter the light and make it appear blue in much the same way that earth's atmosphere scatters sunlight and makes the sky blue against the black void of space. And, just as space appears black at night when there's no sunlight to be scattered by the atmosphere, a male bluebird in a dimly lit location will look almost black--except for its rusty, carotenoid-laden breast!
The green appearance of a bullfrog is due to an even more complex set of factors. Frog skin contains three kinds of highly branched color cells--collectively called chromatophores--that occur in discrete layers. The top layer is made of xanthophores bearing orange, red, or yellow pigments; the middle layer includes iridophores and their silvery, light-reflecting pigment; and the bottom layer has melanophores with black or brown melanin. In tropical frogs, red colors occur when iridophores reflect light back through a brightly pigmented red chromatophore layer. Among our "drabber" Carolina frogs, however, atmospheric blue light scatters back through yellow in the xanthophores--blending to form various shades of green.
A similar phenomenon occurs in the Carolina Anole, a common lizard erroneously called a "chameleon" because its color changes from green to brown. When an anole is aroused by activity, territorial intrusion, or the potential for mating, its yellowish xanthophores expand so that light reflected through their carotenoids appears bright green. At other times, the xanthophores close up, and the reflected light reveals only the brown coloration of underlying melanin.
Perhaps the most breathtaking structural colors occur in the insect world, particularly in beetles. Anyone who has handled a "Junebug"--which is really a scarab beetle--will remember the intensity of the insect's metallic green sheen. In close-up view, even stinkbugs and common flies reveal a pleasing array of metallic colors, as do many dragonflies and leafhoppers.
FUNCTIONS OF ANIMAL COLORS
The infinite variety of animal colors certainly suggests that coloration plays a significant role in the lives of animals. Humans use animal colors as a way of differentiating one species from another, and it's likely that this also happens among the animals themselves. In closely related species, coloration may be the initial cue for species identification.
Color also provides a way for animals to determine the sex of another individual. In Ruby-throated Hummingbirds, for example, only the adult male has throat feathers that form a red gorget; females--and young males--have a white throat. When a territorial male ruby-throat encounters another hummer, he can quickly determine if the intruder is an adult male that he needs to chase away, or a female that he might like to woo.
A third function of animal coloration is evident in the juvenile stage, or eft, of the Red-spotted Newt--a common South Carolina salamander. The juvenile's skin is fire-orange in color, with two rows of small red dots down its back. In its eft stage, this newt wanders the forest floor pursuing earthworms; its striking color is a warning to predators that its skin is loaded with toxins. As an adult, the newt retains its spots but the rest of its skin becomes dark green--just at the time when it returns to the safer haven of a small pond to mate and live out the rest of its days as an aquatic organism.
Rather than advertising their presence with warning colors, other animals take just the opposite approach--camouflage--to defend against predation. These organisms have evolved a variety of unobtrusive colors and patterns that allow them to blend in with their habitat. A Timber Rattlesnake is almost invisible among fallen leaves until its tail begins to shake. Sunlight dappling the spotted coat of a newborn White-tailed Deer fawn helps it disappear into the ground cover, while the neutral brown pelt of the Bobcat makes it very difficult to detect as it approaches its prey from downwind.
Camouflage, warning, sexual attraction, and species recognition are all among the functions of colors in animals. Even within a species, different colors may play different roles. Our male cardinal's bright red plumage declares his presence to other males, and shows a prospective mate that he is adept at finding foods rich in carotenoids; this also indicates he could be a worthy provider with necessary skills to help feed a female and her nestlings. The female cardinal herself, however, is distinctly less colorful, with only a tinge of red and orange mixed in with otherwise tan feathers. This coloration works in her favor as she sits on her nest for three weeks; after all, an incubating female with bright red feathers would only attract predators bent on taking eggs, nestlings, and mother bird alike.
UNUSUAL ANIMAL COLORS
Among wild animals, color aberrations sometimes occur. The most familiar is albinism, a genetically determined trait in which no pigments are produced. Even the iris is unpigmented in a true albino, and the eye looks pink. ("Leucistic" animals such as polar bears, which are normally all white, have pigmented eyes.) Varying degrees of albinism occur in all kinds of animals; in birds, sometimes only individual pairs of feathers on opposite sides of the body are affected. Albinos are so obvious against a natural background of green or brown that they often are easy targets for predators, so there are few true-breeding albinistic populations in the wild.
Other color variations in animals include melanism (an "over-abundance" of melanin pigment that makes an animal abnormally dark); xanthism (abundant yellow pigments); and erythrism (abundant reds). One species of South Carolina reptile--the Southern Hog-nosed Snake--is known to produce individuals with any of these colorations. In several suburban locations in the U.S., there are all-black colonies of melanistic gray squirrels maintained by humans who selectively cull out "less desirable" gray individuals.
COLOR VISION IN ANIMALS
A common question among nature lovers is whether animals have color vision like humans. As a general rule of thumb, one only needs to look at a taxonomic group's coloration for a probable answer. Since most mammals are primarily brown or black or gray, it's likely they have not evolved color vision and depend instead on acute powers of smell and hearing. Likewise, most bird species are brightly colored and probably see colors as least as well as we do (although they also seem to have hearing that is superior to ours).
To take this logic further, frogs and toads are probably "color-blind," but many insects such as butterflies and beetles have noticeable colors and patterns that seem to indicate their color vision is well-developed. There's the possibility, of course, that animals see different colors as shades of gray that the human eye could not differentiate, or that their eyes see colors in some amazing way that we do not yet understand. Regardless, we humans can rejoice in the fact that we are among those organisms that can see color, and that we have the capacity to observe and appreciate the wondrous hues that exist around us in the colorful world of South Carolina animals.
Bill Hilton Jr. is a science education consultant, writer, naturalist, and Macintosh computer enthusiast who lives in York, South Carolina.
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Hilton Pond Center for Piedmont Natural History is a non-profit research & education organization in York, South Carolina USA; phone (803) 684-5852. Directed by Bill Hilton Jr., aka The Piedmont Naturalist, it is the parent organization for Operation RubyThroat. Contents of this website--including articles and photos--may NOT be duplicated, modified, or used in any way except with the express written permission of Hilton Pond Center. All rights reserved worldwide. To obtain permission for use or for further assistance on accessing this Web site, contact the Webmaster.