CHEMICAL DEFENSES IN ANIMAL

Some groups of animals that feed on plants rich in secondary compounds receive dr extra benefit, one of great ecological importance. When the caterpillars of monarch butterflies feed on plants of the milkweed family, for example, they do not break down the cardiac glycosides that protect these plants from most herbivores. Instead, they store them in fat bodies. As a result, these butterflies are themselves protecieil from predators! The cardiac glycosides in the leaves that the caterpillars eat are con centrated, stored, and passed through the chrysalis stage to the adult and even to ill;
eggs; all stages are protected from predators. A bird that eats a monarch butierfi quickly regurgitates it and thenceforth avoids the kind of conspicuous orange-and black pattern that characterizes the adult monarch. Locally, however, some birds have acquired the ability to tolerate the protective chemicals and eat ttit monarchs.
Insects that do feed regularly on plants of the milkweed family are, in general, brightly colored. Among them are brightly colored cerambycid beetles, whose larvae feed on the roots of the milkweed plants; bright blue or green chry-somelid beetles; and bright red true bugs of the order Hemiptera. In some parts of the world, there are also bright red grasshoppers and other very obvious insects, These herbivores clearly are "advertising" their poisonous nature by their bright colors, using an ecological strategy known as warning coloration. Similar relationships occur in marine communities; a recent investigation revealed that of the exposed common coral reef invertebrates at Lizard Island, on the Great Barrier Reef off the northeast coast of Australia, three fourths were toxic to fish; of the protectively camouflaged ones, only one fourth were toxic.

Insects that eat plants that lack specific chemical defenses are seldom brightly colored. In fact, many of these insects are cryptically colored-colored so as to blend is with their surroundings and thus to be hidden from predators. Thisis also true of insects such as the larvae of cabbage butterflies, which, although they fed on plants with well-marked chemical defenses, are able to do so because of their ability to break down the molecules involved, rather than store them.
Some marine animals-such as certain nudibranchs-acquire defensive chemicals or defensive cells from their prey. Hydroids often provioc' such stinging cells to animals that graze on them, for example. Off the coast of Panama, the large nudibranch Aplysia grazes selectively on red algae of the genus Lam-cia, which is protected by elatol, a powerful inhibitor of cell division. Perhaps ihisis the reason that few fish feed on Aplysia. An intensive investigation of marine animals, algae, and flowering plants for new drugs against cancer and other diseases, or as sources of antibiotics, is now being carried out. It holds great promise, because of tile enormous diversity of chemical compounds that occur in these organisms.
Animals also manufacture many chemicals that they use in their defense. In faci, animals manufacture and use a startling array of substances to perform a wide variety of defensive functions. Venomous snakes, lizards, and fishes are well known; in adm tion, bees, wasps, predatory bugs, scorpions, spiders, and many other arthropflai have chemicals that they use to defend themselves and to kill their prey.