Poisonous Animals

animals that contain some constant or periodically occurring substance that is toxic to individuals of other species. Even in small amounts, the toxin can cause painful disturbances in other animals and can sometimes lead to death.

There are approximately 5,000 species of poisonous animals: about 20 protozoan species, about 100 coelenterate species, about 70 species of worms, about 4,000 arthropod species, about 90 mollusk species, about 25 species of echinoderms, about 500 species of fish, about 40 species of amphibians, about 100 reptile species, and one mammalian species. In the USSR there are approximately 1,500 species of poisonous animals.

The best studied poisonous animals are snakes, scorpions and spiders, and blister beetles, and the least studied are amphibians, fish, mollusks, and coelenterates. Some species have special toxin-secreting glands, while others contain toxic substances in the body tissue. Some of the animals have a piercing device (venomous animals), which facilitates the introduction of the poison into the body of the enemy or prey. In protozoans (for example, infusorians), the trichocysts serve as the piercing apparatus, and in the coelenterates (hydras, actiniae, medusas), the stinging cells; in urticating caterpillars the body has single-cell cutaneous glands with delicate stinging hairs, a number of arthropods (scorpions, bees, wasps) have multicelled cutaneous glands connected to a stinger, and fish have similar glands joined with spines on the fins (for example, the scorpion fish) or gill slits (Trachinus). In many animals (myriapods, spiders, some dipterans, hemipterans, and snakes) the poisonous glands are linked to the mouth, and the venom is introduced into the victim’s body by means of a bite or sting.

In venomous animals, the venom serves as a means of defense or offense. In poisonous animals that have venom glands but no specialized piercing apparatus, such as amphibians (salamanders, tritons, toads), the glands are located in various parts of the skin: when the animal is aroused, the venom is secreted onto the surface of the skin and acts on the mucous membranes of the prey. In poisonous animals that have no special venom glands, the toxicity is caused by the properties of some body tissue or other. The toxicity is manifested only when such animals are eaten by others. Toxic body parts can include sexual glands (in some coleopterans and annelids), the eggs of some fish (barbel, marin-ka), and the blood serum (eels, morays, members of the suborder Batoidei). Many internal parasites of animals are poisonous. For example, the parasitic roundworms of the genus Ancylostoma secrete toxic substances that dissolve the erythrocytes.

Upon introduction into the organism, the toxin first has a local effect, and the more it is absorbed into the body, the more general is its effect on the organism. In some cases, the local effect is very strong and the general effect weak (bee sting), while in other cases, the opposite is true (cobra bite). Local effects include swelling in the vicinity of the bite, pain, the formation of blisters, and tissue damage (necrosis). The general effect usually involves the nervous or cardiovascular system and is manifested in arrest of the heartbeat or respiration, inflammation of the kidneys, and abnormally rapid clotting, sometimes with fatal results. For instance, the bite of the female karakurt spider causes serious local and general reactions; the latter includes excitement, convulsions, and partial paralysis and sometimes ends in death. The effect of some toxins is almost instantaneous. For example, paralysis occurs immediately after a caterpillar is stung by the poisonous wasp ammophila, when the venom reaches a nerve node, and a mouse will die three to four seconds after being bitten by a Levantine viper.

The strength of a toxin’s effect depends on the nature of the toxin, the amount introduced, and the way the toxin is introduced into the organism; venom that enters the blood directly usually acts much more quickly than venom deposited in tissues, which have few blood vessels (in this case, the absorption of the venom is very slow). The sensitivity of different animals to the same toxin varies (the same quantity of rattlesnake venom is fatal for 24 dogs, 60 horses, 600 rabbits, 800 rats, 2,000 guinea pigs, or 300,000 doves). The degree of poisoning also depends on the size and age of the animal. Some animals are not sensitive to some toxins; for example, swine are not affected by rattlesnake venom, hedgehogs by the venom of vipers, and desert rodents by the venom of scorpions. Some birds, such as storks, ravens, condors, and secretary birds, eat poisonous snakes. Some poisonous snakes, for example, Lachesis, eat other poisonous snakes (coral snake), while the nonpoisonous snake mussurana eats poisonous snakes. Some birds can eat urticating caterpillars. Chickens eat karakurts, and the karakurt itself can eat Spanish flies, whose cantharidine is not dangerous to the spider.

Thus, there are no poisonous animals that are dangerous for all other animals; the degree of toxicity is relative. Human beings and animals can become desensitized to a particular toxin when it is given in small doses over a long period of time. Beekeepers sometimes develop a tolerance for bee venom. Small doses of snake venom, bee venom, and the toxins of some other animals are used for therapeutic purposes. (See alsoTOXIN.)