How Does Snake Venom Work? Neurotoxins, Hemotoxins and Fangs

What is snake venom?

Venom is a toxic substance that one animal injects into another through a wound, using fangs, stings or spines. Snake venom is a highly specialized form of saliva. It is made in a pair of venom glands, one on each side of the head, below and behind the eyes. These glands evolved from salivary glands and are wrapped in muscle that squeezes venom along a duct to the fangs.

Proteins and peptides make up about 90–95% of the dry weight of snake venom. A single venom can contain dozens to hundreds of different proteins, and the exact mix varies between species, between populations of the same species and even as a snake grows older. Some of these proteins are true toxins, while others are enzymes that help the venom spread or start digesting the prey from the inside.

The main types of snake toxins

Biologists group venom components by what they target. Most venoms contain several types.

Neurotoxins are typical of elapids, the cobra family. Many are three-finger toxins that fit into the receptors on muscle cells where nerve signals normally land, blocking them. Others, such as the taipoxin of the inland taipan, act on the nerve endings and stop them from releasing their chemical messages. Either way, the muscles stop responding and the prey is paralyzed.

Hemotoxins are typical of vipers. Enzymes called metalloproteinases break down the walls of small blood vessels, causing bleeding, while other enzymes interfere with blood clotting. Some venoms make blood clot uncontrollably and then use up the clotting factors so that it cannot clot at all.

Cytotoxins destroy cells directly, causing swelling and tissue damage around the bite. Phospholipase enzymes, found in almost all snake venoms, can attack cell membranes and contribute to several of these effects.

The four types of snake teeth and fangs

How venom gets into prey depends on the teeth. Herpetologists recognize four broad types.

• No fangs (aglyphous): solid teeth only, as in pythons, boas and most harmless snakes.
• Rear fangs (opisthoglyphous): enlarged, often grooved teeth at the back of the upper jaw. Venom seeps in as the snake chews. Most rear-fanged snakes are harmless to people, but a few, such as the boomslang of Africa, can be dangerous.
• Fixed front fangs (proteroglyphous): short, hollow fangs fixed at the front of the upper jaw, found in elapids such as the king cobra, mambas, coral snakes and sea snakes.
• Hinged front fangs (solenoglyphous): long, hollow fangs that fold back against the roof of the mouth and swing forward when the snake strikes, found in vipers and pit vipers.

Hinged fangs can be much longer than fixed ones, because they fold away when the mouth is closed. This lets vipers such as the eastern diamondback rattlesnake inject venom deep into prey. The Gaboon viper of Africa has the longest fangs of any snake, up to about 5 cm (2 in).

How venom helps snakes catch and eat prey

Snakes cannot chew or tear their food, and many eat animals that can bite back. Venom solves both problems. A cobra bites a rat snake and holds on until it is paralyzed. A rattlesnake strikes a mouse, lets it go to avoid being bitten, and then follows its scent trail to where it collapses.

Venom also starts digestion. Enzymes injected with the toxins begin breaking down tissue from the inside, which may help snakes digest large prey faster, especially in cool weather. Defense is a secondary use: venom can deter predators, but snakes usually rely first on warnings such as hissing, rattling or hood displays.

Venom is costly to produce, so snakes use it carefully. They often inject more into large prey and less, or none, in a defensive bite.

Potency and yield

Two snakes can be dangerous in different ways. Potency measures how strong a venom is per unit of weight, while yield measures how much a snake injects. The inland taipan has the most potent venom of any land snake tested on laboratory mice, but it lives in remote parts of Australia and rarely bites anyone. The king cobra’s venom is less potent per drop but is delivered in very large amounts. See how venomous is a king cobra? for the numbers.

Why did snake venom evolve?

Venom glands and fangs evolved in the advanced snakes, the group that includes vipers, elapids and most colubrids, likely tens of millions of years ago. Some scientists have proposed that venom evolved even earlier in a shared ancestor of snakes and some lizards, such as the Gila monster and monitor lizards, but this “Toxicofera” idea is still debated.

What is clear is that venom evolves fast. Snakes that eat different prey often have different venoms. Sea snakes, for example, have venoms that act quickly on fish, while some snake-eating snakes have toxins especially effective on reptiles.

A bite from a venomous snake is a medical emergency. Anyone bitten should seek emergency medical help immediately by calling 911 (US), 112 (EU) or 999 (UK).