Natural Camouflage

The military camouflage of World War II--let alone World War I--is so crude that it would shame innumerable snakes, caterpillars, birds, fishes. For the unhappy fact is that man has failed to master many of the primary principles of protective coloration practiced by the lower animals.

So writes Zoologist Hugh B. Cott of Cambridge University in the preface of his plentifully illustrated new book, Adaptive Coloration in Animals (Oxford; $8.50). When it appeared in Britain, Cott was at once snapped up by the British armed forces to make their guns as inconspicuous as woodcocks, their tanks as bush bucks, their planes as pickerel. Sternly scientific, the book is more readable than popular "wonder books" of nature.

Though the green frog among his lily pads and the dappled deer in the sun-flecked forest are familiar to everyone, adaptive coloration has always confused biologists. Extremists like G. H. and Ab bott H. Thayer (whose Concealing Coloration in the Animal Kingdom is often consulted by U.S. Army camouflage experts) have claimed that all animals are camouflaged, "the most gorgeous costumes being, in their own way, climaxes of obliterative coloring." Obliterative climax of the Thayers' theories--which made Theodore Roosevelt gnash his teeth and boom "Nature Fakers!"--was the idea that flamingos are concealingly colored because their foes mistake them for sunsets. Other biologists have been skeptical of all claimed adaptations, especially mimetic postures. Cott goes to neither extreme. Proof of adaptive coloration, he agrees, "is one of the most notable triumphs ever won by the great (Darwinian) Theory of Natural Selection." In Cott's book it is a triple triumph.

Concealment. That concealing coloration is not accident is in part proved, Cott says, by the astonishing variety of color-causes. Some caterpillars are green because their blood absorbs chlorophyll from their food; others because they are transparent, revealing the green food inside them. A South American sloth acquires a concealing greenish hue from symbiotic algae which live on his fur.

That color changes with environment is strikingly illustrated by the varieties of U.S. horned toads. A brightly splotched variety lives in Arizona's Painted Desert, a drab single-toned variety on the drab soil of Oregon, a white variety on the white alkali soil of the Amargosa Desert, a black variety on the West's black lava belts. Such adaptations sometimes occur before the very eyes of biologists: during the industrialization of Germany a black moth replaced a former pale variety in factory areas.

Cott describes a number of statistical experiments in which assorted birds and insects have been exposed purposely to predators. Results show clearly that concealing colors and confusing patterns are of major importance in the survival of many species.

Advertisement. "Many animals, far from being concealingly colored, are very conspicuous objects in nature." Brightness is often used by animals for sexual or signaling purposes, but Cott's concern is only with the relations of prey and predator. As a rule, if an animal advertises its presence, it is a good bet that a predator wouldn't want it anyway. Birds and fishes are likely to mistake inanimate objects for the insects on which they feed, so they can easily mistake an unpalatable insect for a tasty one--unless the former distinguishes himself by loud red, orange or yellow markings (although some other palatable animals adopt conspicuous markings in order to resemble unpalatable or poisonous varieties). Studies of 200 kinds of insects eaten by U.S. birds show that none of the palatable varieties is conspicuously marked. Among California salamanders, those eaten by snakes are concealingly colored and hide by day, and those which snakes avoid as nauseous are loudly marked and go about in daylight.

Disguise. Some animals, says Cott. "are impostors rather than self-effacers." Typical examples:

> The poor-me-one, a tropical American relative of the whippoorwill, nests on the fractured stump of a growing tree, where its coloration and its cryptic posture make it look like a broken branch (see cut).

> Many butterflies have gleaming spots painted on their wings, which "by their general deceptive resemblance to the eye of some large vertebrate such as an owl . . . would be mistaken in the gloom by insectivorous birds and mammals for something on no account to be meddled with."

> A South American bug wears a masklike hollow structure on its head which makes it look like a distant alligator. "The resemblance extends to a number of independent details, including the nasal prominence in front, a large false eye behind, bearing a white mark which simulates light reflected from a real eye, and false teeth which are present not merely in color but in relief."

> The Brazilian snake Oxybelis acuminatus mimics a liana (vine) so closely that it is but a quarter-inch in diameter though four feet long. It is thus 160 times as long as its greatest width.

> Cott quotes a fellow naturalist: "In . . . Tanganyika a small moth resembling a bird-dropping was not uncommon. On one occasion I observed what I thought to be one on a leaf, but after a close examination from a distance of only a few inches I discovered (to my own satisfaction) that it was after all only a bird-dropping. Just as I turned away the said bird-dropping flew off!"

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