A Trick Nature Invented Over 100 Times
Venom is one of nature's favorite tricks. It has evolved separately at least 104 times in animals, across 8 major groups called phyla. When unrelated animals land on the same solution like this, scientists call it convergent evolution.
Venom is not the same as poison. Poison is passive: it has to be swallowed, breathed in or soaked up through the skin. Venom is actively pushed into another animal through a bite, a sting or a similar wound. That delivery has its own name, envenomation. A third type, called toxungen, is sprayed or smeared onto the outside of another animal.
Some animals do not even make all of their own toxins. Several species get venom ingredients from microbes, which may live right inside their venom glands. Venoms also adapt to their targets. Over time, a venom can become finely tuned to the ion channels of an animal's usual prey, so it works best on its standard diet.
Four Ways Venom Goes to Work
Most venoms are not a single chemical. They are complex mixtures of many toxins, and each kind attacks a different part of the body. There are at least four major classes.
A neurotoxin targets the nervous system, often by blocking the ion channels that carry nerve signals. Mambas, black widow spiders, scorpions, box jellyfish, cone snails, centipedes and blue-ringed octopuses all use them. A myotoxin damages muscles by locking onto a receptor, and these small proteins show up in rattlesnake and lizard venoms. A cytotoxin kills individual cells, and honey bee venom contains them. A fourth group, haemotoxins, disrupts blood clotting.
Snake venom shows how layered the recipe can be. It packs proteases, which cut proteins apart, and nucleases, which break the bonds that hold DNA together. Scientists think snake venom may have started when genes once used in the salivary glands of snake ancestors got duplicated.
Stingers, Spurs and Venomous Ribs
Venom shows up in surprising places. In bees and wasps, the stinger is actually a modified ovipositor. Bees use an acidic venom to defend their hives and food stores. Many wasps use a different venom to paralyze prey, keeping it alive as fresh food for their young. One wasp species uses venom as an alarm signal that calls nearby wasps to attack a predator, and one ant species spreads venom on itself to wipe out germs.
Some salamanders can push out sharp, venom-tipped ribs. Two frog species in Brazil have tiny spines around the top of their skulls that deliver venom on impact.
Mammals join in too. Shrews, solenodons, the European mole, slow lorises and male platypuses are all venomous. Platypus venom glands evolved from modified sweat glands. Spurs like the platypus's appear in many groups of early mammal relatives, which suggests venom may have been an original trait of mammals.
A Chemical Arms Race
When a predator relies on venom, its prey is under pressure to resist it. Scientists describe this back-and-forth as a chemical arms race, and resistance has evolved many times across the animal kingdom.
California ground squirrels resist the venom of the Northern Pacific rattlesnake, and their resistance is higher where rattlesnakes are more common. The snakes have answered locally by making their venom more effective. Kingsnakes of the Americas eat other venomous snakes. They are immune to the venom of local species like copperheads and rattlesnakes, but not to king cobras or black mambas.
At sea, eels resist sea snake venom, especially from the sea snakes that feed on them. The cleverest trick belongs to clownfish, which live among the stinging tentacles of sea anemones. Their protective mucus works as chemical camouflage, so the anemone does not treat them as something foreign and does not fire its stinging cells. Clownfish may even adjust their mucus to match one particular anemone species.
From Fangs to the Pharmacy
Venom is not only dangerous. It is also a treasure chest for medicine. Venoms are found in over 173,000 species, and their medical potential has been explored in over 5,000 scientific papers.
Snake venom proteins are already used to treat thrombosis, arthritis and some cancers. Gila monster venom contains exenatide, a substance used to treat type 2 diabetes. Solenopsins from fire ant venom have shown possible uses ranging from cancer treatment to the skin condition psoriasis. A whole branch of science, venomics, now studies venom proteins and how each part could be turned into medicine.
Venom is not even limited to animals. Some bacteria use a tiny injection system to fire toxins into competing microbes. Stinging plants deliver toxins into their targets. Some fungi push into plants or insects and release cell-killing toxins. Even tiny single-celled creatures called Coleps have special parts that inject toxins into prey. In a sense, every kingdom of life has evolved some version of venom.
