Animal Camouflage

Science World, Nov 13, 2000 by Mona Chiang

Can you locate the animals hiding in these pages? They don't want you to. That's how they survive!

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).

MORPHING MOTHS

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!

FAKE-OUTS

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.

STRESS STORY

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.