Camouflage

Predation is an instinctive animal behavior that involves the pursuit, capture, and immediate killing of animals for food. Birds that capture insects in flight, starfish that attack marine invertebrates, and tigers that pursue gazelle are all examples of predators. Predatory animals may be solitary hunters, like the lion, or they may be group hunters, like wolves. Natural selection favors the development of a variety of quick defenses against predators including camouflage and predator avoidance behaviors.

Camouflage is a form of deceptive coloration that is essential to the survival of most animals. Camouflage can make it extremely difficult to spot an animal in its natural habitat because the animal appears to blend into its surroundings. This adaptation is beneficial because it can provide protection from predators. At the same time, it can also conceal an animal and allow it to be a stealthy predator able to inconspicuously hunt down or snatch its unsuspecting prey.

Animals camouflage themselves in many ways, including background matching, color changing, disruptive coloration, and countershading.

Background matching is probably the most common type of concealment. The animal and its surroundings are so close in color that they appear as one. Fish eggs, for example, often have very little pigmentation and appear transparent against the blue of the open sea. Polar bears appear to merge into the ice and snow of the Arctic, and grasshoppers blend perfectly with green grasses and shrubs.

Color changing is another way to achieve camouflage. Emotion seems to play a role in color change in some animals, such as cephalopods and certain fish, which are capable of rapid color changes completed in a half-second or less. These animals, when excited, can exhibit spectacular displays of color, with waves of color rippling across their bodies. As the animal's eyes register the colors in its immediate environment, hormonalreactions send chemical messages to chromatophores, pigment-bearing cells in the animal's skin. The chromatophores undergo rapid changes in pigment concentration, distribution, and position, allowing the animal to seemingly change color almost instantly. Most vertebrates, however, undergo color changes less rapidly, requiring several minutes to several hours.

Disruptive coloration may appear as patterns in which an animal's markings do not coincide visually with its body shape or outline. Flatfish, for example, are marked in such a way that their skin patterns do not reveal their contour when they rest on the ocean bottom. Many reef fish also have disruptive patterns in their coloration, which enable them to school safely over reefs during daylight hours. When a predator approaches, the fish form dense schools in which all of the individual fish orient themselves in the same direction. The movement of many fish, coupled with their similar disruptive coloration of vertical banding or horizontal stripes, presents an extremely confusing spectacle. This makes it difficult for a predator to attack any individual fish.

Some forms of disruptive coloration also function to hide movement. Forward movement of concentrically banded snakes, for example, is difficult to perceive when the animal moves between reeds or tall grasses.

Countershading, a type of camouflage coloration in which the upper surfaces of an animal's body are more darkly pigmented than the lower areas, gives the animal's body a more uniform darkness and lack of depth relief because the underside of the body is shadowed. Light-producing organs found in some deepwater fish provide a unique form of countershading. The light-producing organs often occur in bands along the fish's undersides and are directed downward. This unique arrangement, coupled with the utter darkness of the ocean at deep depths, may provide camouflage by obliterating the fish's silhouette when a predator views it from below.

Some animals camouflage themselves through mimicry by showing an imitative resemblance to inanimate objects in their environment, such as the leaves or twigs of a tree. Stick insects, for example, may resemble twigs when resting on trees.

In addition to camouflage, animals use predator avoidance behaviors or protective adaptations to avoid being killed. Warning calls and visual and chemical signals that are unique to different animal species may evoke avoidance behaviors such as freezing, crouching, fleeing, escaping, and stinging. For example, many perching birds will gather in a mob when stimulated by the sight of an owl.

Freezing or immobility usually makes detection less likely. Many animals, such as rabbits and squirrels, exhibit this reflex-like behavior when startled. Some groups of animals commonly keep in touch by calls or by movements such as tail flicks, which are exhibited during freezing.

Many animals posses protective reflexes, armor, and spines that enable them to avoid predation. Stick insects resembling twigs and leaves, for example, exhibit unusual reflex behaviors, such as swaying to imitate moving foliage. Mollusks, like oysters and clams, may retract their soft bodies intotheir shells when disturbed. Turtles and other slow moving animals may retreat into their armor for protection. Still other animals, such as porcupines, protect themselves from predators with a thick coat of sharp quills.

Chemical means of defense may help an animal escape predators. An animal may eject a poisonous substance from a body reservoir or spine. Jellyfish, for example, may sting to avoid being captured. Snakes may inject venom through their fangs to kill or deter menacing predators. Some animals, like skunks, may even squirt substances at their enemies. The skin of some toads contains substances that make them distasteful to predators. Ants produce strong substances that attract other ants at low concentrations and in high concentrations produce fast movement, defense postures, and even fleeing.

Fleeing and escaping are two of the most common predator avoidance reflex behaviors. When an animal is startled or subjected to pain, it may run or jet away. Squids, for example, use jets of water to propel themselves quickly out of danger. Bony fish have structures that initiate escape-swimming when agitated.

Habitat; Mimicry.

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Hanlon, Roger, and John B. Messenger. Cephalopod Behavior. London: Cambridge University Press, 1998.

Slater, P. Essentials of Animal Behavior. London: Cambridge University Press, 1999.

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