Poisons, or toxins,
substances that chemically interfere with the normal physiology and behavior of
organisms, are pervasive
in nature. Some form of poison is produced by almost every major group of
organisms. Because plants do not have the options of running, hiding, or fighting to avoid being eaten, many plant species have developed some form of chemical deterrent to prevent their being fed upon by herbivores. Chemical weaponry appears in all major animal groups. At one time, birds were believed to be the exception, but the pitohui of New Guinea has been found to have an extremely potent neurotoxin on its feathers and skin. Toxins manufactured by animals are called venoms, plant-synthesized poisons are collectively known as secondary
substances, and the toxins generated by microorganisms (algae, bacteria, and fungi) are technically referred to as antibiotics, which have proved useful in the development of drugs.
Natural poisons are rarely random products of living organisms that happen to be biologically active. They are diverse, usually complex chemicals, many with highly specific modes of action that have evolved by natural selection in the contexts of defense, competition, and exploitation. In most cases, these substances confer advantages on their producers. Natural poisons function in three ways: to prevent predation, to protect resources, and to capture prey.
Natural poisons are of enormous importance to humans because of the dangers some pose but more significantly because of the many benefits humans enjoy that are either directly or indirectly attributable to these substances.
Kinds of Poisons
Natural poisons may be classified in several ways: by their chemical composition, how they are produced or acquired, by the organisms they affect, by their mode of action, and by their method of delivery.
Natural toxins are found in all four of the major classes of organic compounds: carbohydrates, lipids, proteins, and nucleic acids. They range in molecular complexity from cyanide (HCN), a simple compound produced by certain millipedes and found in peach pits, to complex protein molecules containing scores of carbon atoms.
Toxins are synthesized directly from raw materials by both plants and animals, selectively concentrated from the soil by plants, or sequestered from plants by herbivores. Soil nitrate minerals in toxic concentrations are accumulated from the soil by many plant species. The monarch butterfly, Danaus plexippus, is noted for its larvae having the ability to sequester cardiac glycosides contained in the milkweed plants (Asclepias) upon which they feed. This provides a chemical defense for the larvae, the pupae, and the adult butterfly.
Natural poisons directed against animals are called zootoxins; chemicals that are active against plants are called phytotoxins. Most phytotoxins are fungicidal and defend green plants against fungi. Mature creosote bushes produce substances that inhibit the growth of and prevent the establishment of potentially competitive seedlings in their vicinity.
Natural poisons range in effect from mild to severe, and are sometimes fatal. They include skin irritants, emetics (vomit-inducing agents), proteolytic (protein-digesting) agents, hemotoxins (blood poisons), neurotoxins (nerve poisons), muscle contractants and relaxants, and physiological regulators. Some affect specific organs, such as cardiac glycosides that impinge on the heart; others interfere with critical life processes, such as molting in insects. Some may have long-term effects and may result in the development of cancer.
Virtually all poisons produced or concentrated by plants and many produced or sequestered by animals may be considered passive poisons in that they are delivered only when a target organism (the victim of the poisoner) has contact with or consumes the producer. Active poisoners include all animal species that use their poisons aggressively or have special active mechanisms to deliver them.The subjects of passive and active poisoners are elaborated in the following sections.