Poisonous animals

Substances that cause disease symptoms, injure tissues, or disrupt life processes on entering the body are poisons. When ingested in large quantities, most poisons kill, including those from minerals, vegetable foods, or animal attacks. Any poison of animal origin is venom. Venoms are delivered by biting, stinging, or other body contacts. These animal poisons are used to capture prey or in self-defense. In some animals, the ability to produce venom arose in those too small, slow, or weak to otherwise maintain an ecological niche. The mechanism for developing the ability to make venom is unclear.

The most familiar poisonous animals are snakes, insects, spiders, and some other arachnids. Poisonous species, however, occur throughout the animal kingdom, including a few mammals,lizards, and some fish. The severity of venom effects depends on its chemical nature, the nature of the contact mechanisms, the amount of venom delivered, and victim size. For example, almost all spiders are poisonous. However, their venom is usually dispensed in small amounts that do not affect humans, though they kill prey and use venom in self-defense very effectively.

Chemically, venoms vary greatly. Snake venoms are mixtures of enzymes and toxins. Studies of their effects led to the identification of hemotoxins, which cause blood vessel damage and hemorrhage; neurotoxins, which paralyze nerves controlling heart action and respiration; and clotting agents, which excessively promote or prevent blood clotting. Cobras, coral snakes, and arachnids all have neurotoxic venoms.

Poisonous Lizards, Arachnids, and Insects

The poisonous lizards are useful to explore first because only two species are known: Gila monsters and beaded lizards (both holoderms). They inhabit the southwestern deserts of the United States and Mexico. They do not strike like snakes; rather, they bite, hold on, and chew to poison. Holoderm bites kill prey but rarely kill humans. Beaded lizards grow to three feet long, and Gila monsters grow to two feet long.

Most poisonous arthropods are spiders and scorpions. Both use venom to subdue or kill prey. As stated earlier, few spiders endanger humans because their venom is weak and is not injected in large quantities, but some species have very potent venom and harm or even kill humans. The best-known of these are black widow spiders. Though rarely lethal to humans, black widow bites cause cramps and paralysis.

There are approximately six hundred scorpion species of sizes between one and ten inches. All have tail-end stingers. Large, tropical scorpions can kill humans, while American scorpions are smaller and less dangerous. Scorpions are more dangerous than spiders because they crawl into shoes and other places where their habitat overlaps that of humans.

Many insects, such as caterpillars, bees, wasps, hornets, and ants, use venom in self-defense or to paralyze prey to feed themselves or their offspring. Caterpillars use poison spines for protection. Bees, wasps, hornets, and ants use stingers for the same purpose. The venom of insects also kills many organisms that seek to prey on them. Humans, however, are rarely killed by insect bites. Such bites are usually mildly to severely painful for a few minutes to several days. However, severe anaphylaxis occurs in some cases, followed by death. This occurs in humans when an individual has a specific allergy. Allergies may be mild or life-threatening.

Poisonous Snakes

Of all snake species, around 725 are venomous, and around 250 can pose a threat to human life, especially those in rural areas. Poisonous snakes are found on every continent except Antarctica and are primarily those in the Colubridae, Elapidae, or Viperidae families. All have paired, hollow fangs in the front upper jaw. Poisonous snakes have fangs that fold back against the upper palate when not in use, and when a snake strikes, they swing forward to inject a venom that attacks the victim’s blood and tissues. The heads of poisonous snakes are usually scale-covered and triangular. Among them are pit vipers, named for the pits on each side of the head that contain heat receptors. The pits detect warm-blooded prey, mostly rodents, in the dark. Pit vipers include rattlesnakes, moccasins, copperheads, fer-de-lance, and bushmasters.

Poisonous snake populations in America and Europe differ. In North America, about thirty such snake types occur across the continent except in Alaska, including three elapid coral snakes, two copperheads, twenty-three rattlers, and two cottonmouths (all vipers). Rattlers have the widest habitat. They are abundant in the snake-rich Great Plains, Mississippi Valley, and southern Appalachia. In contrast, copperheads and cottonmouths are abundant in Appalachia and the Mississippi Valley, respectively. Mexican poisonous snakes are divided into two ranges—the northern, from the US-Mexican border to Mexico City, and the southern, south of Mexico City. In the north, snakes are mostly rattlers, as in the contiguous United States, while coral snakes and pit vipers are plentiful in the south. Most perilous are the five- to eight-foot fer-de-lance, whose venom kills many humans. In South America, all vipers but rattlers are tropical. Bushmasters, the largest South American vipers, and elapid coral snakes are nocturnal and rarely endanger humans. Tropical rattlers and lance-headed vipers, somewhat less nocturnal, kill many. Europe has few snakes due to its cool climates and scarce suitable habitats. Its few vipers range almost to the Arctic Circle. Eastern Mediterranean regions hold most of the European vipers.

There are many poisonous snakes in Africa and Asia. North Africa, mostly desert, has few snakes. Central Africa’s diverse poisonous snakes are colubrid, elapid, and viper types. Elapids include the dangerous black mambas (Dendroaspis polylepis), twelve to fourteen feet long, and smaller cobras, which also occur in South Africa. Among diverse vipers, the most perilous are gaboon vipers (Bitis gabonica) and puff adders (Bitis arietans). The boomslang (Dispholidus typus) is one of Africa's most deadly snakes. In the Middle East and Central Asia, the Inland taipan (Oxyuranus microlepidotus) and Egyptian cobra (Naja haje) are known to be the most deadly snakes. On the Indian subcontinent, the Indian saw-scaled viper (Echis carinatus), Indian cobra (Naja naja), Common krait (Bungarus caeruleus), and Russell's viper (Daboia russelii) comprise the group of well-known deadly snakes called the BigFour. Southeast Asia has the most poisonous snakes in the world—elapids, colubrids, and vipers. This is due to snake habitats that range from semiarid areas to rainforests. The huge human population explains why this area has the world’s highest incidence of snakebite and related death due to vipers, cobras, elapids, and sea snakes. Vipers bite most often, but elapids cause a larger portion of deaths. The Far East snake population is complex, and its snakebite incidence is also high. Its important poisonous snakes are pit vipers.

Australia and New Guinea have large numbers of poisonous snakes. Australia has 65 percent of the world’s snakes, while New Guinea has 25 percent. Also, sea snakes occur offshore and in some rivers and lakes. However, these countries have few snakebite deaths due to the small size and nocturnal nature of most indigenous snakes. In Australia, poisonous snakes include the common death adder (Acanthophis antarcticus), Inland taipan (Oxyuranus microlepidotus), the eastern brown snake (Pseudonaja textilis), and Mengden's western brown snake (Pseudonaja mengdeni), among many others.

Poisonous Fish and Amphibians

Venomous fish are dangerous to those who enter the oceans, especially fishermen who take them from their nets. The geographical distribution of these fish is like that of all other fish. The highest population density is in warm temperate or tropical waters. The number and varieties of poisonous fish decrease with proximity to the North and South Poles, and they are most abundant in Indo-Pacific and West Indian waters.

A well-known group of poisonous fishes is the stingrays (family Dasyatidae). They inhabit warm, shallow, sandy-to-muddy ocean waters. Dasyatids lurk almost completely buried, awaiting prey that they sting to death with barbed, venomous teeth in their tails. The tail poison is made in glands at the bases of the teeth. Small, freshwater dasyatids occur in South American rivers, such as the Amazon, hundreds of miles from the river mouths. Stingrays near Australia grow to fifteen-foot lengths. Emphasizing the wide distribution of stingrays and their danger to humans is their mention in Aristotle’s third-century BCE writings and the death of John Smith in 1608, killed by a stingray while exploring the Chesapeake Bay. Though stingrays also made the news media in 2006 when Australian wildlife expert Steve Irwin was inadvertently stung by the animal, stingrays generally are not a threat to humans.

Also well known is the venomous Scorpaenidae fish family, many members of which cause excruciating stings. Scorpaenids have sharp spines supporting dorsal fins. The spines, used in self-defense, have venom glands. The deadliest fish venom is that of the stonefish, which, when stepped on, can kill humans. Some poisonous animals like zebrafish, stonefish, and poisonous frogs or toads can use their poison from a distance. For example, Africa and South America's poison dart frogs (Dendrobatidae family) and cane toads (Rhinella marina) secrete toxins through their skin. Contact with the poisons can cause effects ranging from severe irritation to death, but the poisons frighten away or kill most predators that attempt to eat them. One ecological function of poisonous animals is to help control the population of insects, rodents, arachnids, and small fishes and maintain balance in ecosystems. Poisonous land animals, such as scorpions and many snakes, are often nocturnal and add another dimension to pest control by nighttime predation.

Other Poisonous Animals

While poison in animals is usually associated with insects or reptiles, many animals do not fit the category's stereotype. The only venomous primates are nocturnal creatures in the genus Nycticebus called slow lorises. They have a toxin gland under their arm, and when they lick it, their bite becomes poisonous. The male platypus of Australia and New Guinea has a sharp spur on its heel that contains venom for defending territory. Short-tailed shrews (Blarina brevicauda) and geography cone snails (Conus geographus) are also poisonous.

Principal Terms

Anaphylaxis: hypersensitivity to a foreign substance, such as venom, that causes discomfort and can even kill

Arachnid: an arthropod having eight legs; a spider

Arthropod: an organism with a horny, segmented external covering and jointed limbs

Hemotoxin: a substance that causes blood vessel damage and hemorrhage

Neurotoxin: a substance that damages the nervous system, most often nerves that control breathing and heart action

Venom: a poison made by an animal

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