Why are polar bears white and grizzly bears brown? Why do zebras have stripes, and why do chameleons switch color from green to brown in a flash? It's called camouflage, or concealment by blending into an immediate environment. "Camouflage enables animals to survive by not becoming prey, or by being good predators," says Bruce Grant, a biologist at the College of William and Mary in Williamsburg, Va.
While many species have displayed camouflage over millions of years, scientists are still puzzling to figure out this complex process of concealment. And with so many species using camouflage in as many amazing ways, biologists like Grant often focus on a sole species in his case the peppered moth (Biston betularia).
Peppered moths produce one generation per year. In mid-19th-century England, scientists believed the species was monomorphic (having one known form)--white speckled with black scales on the wings and body. But by the end of the century, more than 90 percent of peppered moths in some regions had become virtually solid black! Why?
Nocturnal (active at night), the peppered moth rests on tree trunks by day. From the 1850s on, heavy industrial pollution in England blackened trees and contaminated the air in many parts of the country. The pale moths stood out brightly in their now-darkened habitat (home) and became easy meals for hungry birds. It turns out, darker forms of this moth species existed, but in very small numbers. On dark trees, crypsis (KRIP-sees)--or coloring that matches an immediate background--helped dark moths escape detection from predators: they survived and reproduced. Over time, the moth population turned mostly dark!
How living things fit into their environment is called fitness. "Animals that are fit survive better and have more offspring," says Neil Greenberg, an ethologist (biologist who studies animal behavior) at the University of Tennessee. Traits that increase an animal's fitness are called adaptations. Body coloring that matches the environment "is a great adaptation," adds Greenberg.
The animal is protected from predators as long as its environment remains stable. When an environment changes, however, a species' survival is determined by how well it evolves (changes its hereditary features over time) and adapts to the new environment; its survival is also dependent on how effectively new traits are passed on to future generations. This process is called natural selection.
But between 1959 and today, the number of dark peppered moths in England plunged from 90 to 5 percent of the population. How come? Under the Clean Air Acts of 1956, the British government began to enforce environmental cleanup. Thus, less soot and sulfur dioxide produced lighter trees and more favorable conditions for new generations of pale peppered moths to hide in!
Some animals camouflage themselves by disguise (resembling natural surroundings). For example, Kenya's flower mantis (see cover) looks like the plant it rests on. The insect sits patiently on hind legs, with its front legs ready to nab unsuspecting insects as they stroll by.
A zebra's brown-and-white stripes are another form of camouflage, called disruptive coloration. Stripes work like an optical illusion to safeguard the animal, letting a zebra meld into its natural habitat on grassy African plains. When a lion, the zebra's chief predator, hunts by the dim light of dawn or dusk, shadows on white stripes make a zebra very hard to spot. And if a lion pursues a zebra, the creature can disappear among its herd. Lines from many zebras become a giant blur to the lion.
For the ultimate fake-out, consider a critter that camouflages through mimicry (looks like another species). The viceroy butterfly is protected from predatory birds because it resembles a monarch butterfly. Monarchs feed on milkweed, which contains nasty-tasting compounds called cardiac glycosides. Through evolutionary trial and error, birds have learned that eating monarchs induces severe vomiting. So birds steer clear of them--and its lookalike, the viceroy butterfly, too.
The common chameleon from North Africa can alter its skin color to match its background from leafy green to brown within seconds. The reptile's eyes signal the nervous system to trigger chromatophores (pigment-producing cells) beneath the skin surface to contract or expand. Result: quick-change artist!
However, green anole (Anolis carolinensis) lizards found in the southern U.S. don't change color to match backgrounds. "They do it because they're stressed out," says Greenberg. "My lab technician kept saying to me, `Those little brown lizards are so cute.'" But Greenberg told him, "They're not brown. They're green!" It turns out, that when Greenberg visits his lab, he's careful not to disturb the lizards. But when the technician would breeze in, he'd yell, "Hello lizards!" and the agitated anoles instantly turned from green to brown.
Unlike chameleons, the color cells of the green anoles only respond to hormones, body chemicals that control body functions (see diagram, above). Greenberg observed that the lizard is green when it feels safe--the natural resting state of its chromatophores.
But he also noticed that when two male green anoles are caged together, they fight for dominance. The winner gains control of a leafy roost and remains green 95 percent of the time. The loser stays on the bottom of the cage and remains brown 95 percent of the time. In nature, a laid-back green lizard among leafy branches and a stressed-out brown lizard on earth trying to avoid predators have obviously worked out their camouflaging survival strategies!
One moth or two?
A melanic (dark pigmented) peppered moth sits on a birch tree by a light-colored one camouflaged by crypsis (color matching with background). Guess which moth is more likely to wind up as lunch for predators?
Plant or bug?
Some bugs are camouflaged by disguise--they resemble their surroundings. Javanese leaf insects look like leaves. The walking stick looks like, well, a stick.
One zebra or three?
Disruptive coloration, or camouflage by wild patterns, breaks up an animal's outline and helps it blur into its surroundings.
Where's the snake?
Disruptive coloration hides a venomous gaboon viper in leaves as it eagerly waits for a meal of birds, rodents, or frogs in Africa's tropical forests.
Which tastes nasty?
No, they aren't the same. One's a viceroy, the other's a monarch. Read the story to find the answer.
The epidermis (outer skin layer) of the green anole is transparent. Its bluish-pigmented cells, iridophores, reflect light through yellowish cells (xanthophores). The combination makes the skin appear green. When stimulated by hormones, dark pigments in the melanophores (dark pigmented cells) spread to the top. Like putting on a dark pair of sunglasses, pigments block the green color, and anoles turn brown!
Hide Your Own Anole
blue, brown, and yellow transparent acetate (gel) * off-white construction paper * tape * photo of leaves (from magazine, etc.) * photo of ground
1. Cut out a 2.5 cm (1 in.)-square of each color gel and construction paper.
2. Place blue gel square on construction paper. Tape corners. Tape corners of yellow gel over blue.
3. Using your square as an anole, place against a background of photo of leaves. Observe.
4. Place square against photo of ground. Observe.
5. Tape corners of brown gel over yellow gel.
6. Repeat Steps 3 and 4.
When is the green anole most noticeable to predators?
DON'T STOP NOW:
Explore how other animals use camouflage. Choose examples from different climates and locations. Find their photographs, cut out the animals, and swap backgrounds. How well does each animal hide under different conditions?
History: Read and report on the evolutionary research of scientists Henry Walter Bates or Charles Darwin.
Did you Know?
* In many bird species, males are more colorful than females, using their bright feathers to attract mates. Less vibrant coloring helps nesting females and young hide from predators.
* Dolphins use counter shading, a type of camouflage that plays against natural shadows and light. From above, the mammal's dark top helps it blend into the darkness of the sea. But when seen from below, the light side blends with the light of the sky.
* Many species change colors with the seasons--the ermine weasel is brown in the summer and snowy white in the winter. This keeps it camouflaged all year round.
National Science Education Standards
Grades 5-8: structure and function in living systems * diversity and adaptation of organisms * populations and ecosystems
Grades 9-12: behavior of organisms * biological evolution * molecular basis of heredity * interdependence of organisms
For more on Dr. Bruce Grant's research: www.wm.edu/biology/Grant.html
For more on Dr. Neil Greenberg's research: www.bio.utk.edu/neils.nsf
"Lepidopterans: Allure and Illusion," by Michael H. Robinson, Zoogoer, March/April 1996
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|Date:||Nov 13, 2000|
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