As predators, snakes are missing a few key attributes. They have no legs to chase down their prey, no paws to knock down quarry, and no claws to hold their victims. But none of these deficiencies matters much, because evolution has handed snakes the ultimate weapon: snake venom. With it, the several hundred types of venomous snakes can kill or debilitate before their victims escape.
Snake venom has given these reptiles the ability to be small yet effective hunters, and they have spread to fill every ecological niche—as long as the environment is warm enough for them to stay in motion. Snakes live everywhere from treetops to the forest floor, in deserts and in the oceans.
In general, scientists agree on how snakes’ venom glands evolved, but that’s not the case for the poisons themselves. Some scientists think that venom is composed of modified proteins from the snakes’ spit that already functioned to break down and digest the prey. Some also believe that the ability to produce poison has evolved independently among the different species of snakes.
But Australian researcher Bryan Fry, one of the world’s leading experts on venomous snakes, has another theory. He has discovered, with the help of DNA analysis, that the vast majority of the proteins and enzymes found in snake venom closely resemble substances found in other parts of the snakes’ bodies—substances, for example, that have a function in the liver, or in the digestive organs or some other system. The genes that control the production of these substances in other organs somehow became activated in the salivary glands, where they produce substances that, once modified and refined, are able to help kill the snakes’ prey in an increasingly effective manner.
Fry also believes that this ability arose a single time, well over 100 million years ago, in one of the earliest ancestors of modern snakes. It then spread to all snakes, but it was only highly developed and refined among the three taxonomic families that we now consider poisonous.
Venom, in other words, is older than snakes themselves.
Cytotoxin: Chewing Starts Digestion
Three Types of Venom
Snake venoms vary dramatically from species to species, but they have one thing in common: Every one of them is an extremely complex composition, made from sometimes thousands of different proteins and enzymes, each with a specific function.
Venoms can be roughly divided into three main groups: cytotoxins, neurotoxins and hemotoxins. But the lines dividing these groups are fluid, and most species employ a combination of the three.
The type of poison and how fast it works is adapted to the snake’s lifestyle and that of its prey. Sea snakes, for instance, have extremely fast-acting venom. They prefer to live around coral reefs, and their most important prey are fish. If the fish they bite don’t die almost instantly, the snake’s meal is likely to escape.
Other species live in environments where it doesn’t matter as much if the prey is able to move a bit before the poison takes effect. After the snake strikes, it lets the prey run off. The physical activity ensures that the venom is quickly pumped through the animal’s body. The snake then uses its sense of smell to follow the prey. As an additional guarantee, some snake venom contains a strong diuretic that causes the prey to urinate while running away. This makes it the easier for the snake to track down its quarry.
Many snakes produce only small quantities of weak poison that is just adequate for their various small prey. But other snakes’ venom can be deadly for large animals—including humans. This is certainly the case for the king cobra, which is the world’s largest poisonous snake and might be capable of killing an elephant with a single bite. The king cobra preys overwhelmingly on other snakes, which have developed resistance to its venom. And that’s the reason for its powerful poison: It takes more venom to bring down another snake than it does a mammal.
Hemotoxin: Acts in Two Ways
Diabolically Sophisticated Syringes
All venomous snakes have specially modified teeth that actively inject their prey, but the variety among those snakes is staggering. Most poisonous snakes fall into one of three families—Colubridae, Elapidae or Viperidae—and the injection system has developed differently in each. The most primitive fangs are found in colubrids—for example, the African boomslang. Their fangs are located far back in the mouth and are often very short. This means they have to get their prey into their mouths and start chewing before the venom is injected.
Among the elapids, the category that includes cobras, the front teeth have developed into fangs. But the most advanced system is found in the vipers, a category that includes the American rattlesnakes.
In vipers, the canine teeth in the upper jaw have been modified into sophisticated syringes. When the snake’s mouth is closed, they lie folded back against the roof of the mouth, but they drop down when the snake is hunting. And when they stab into prey, the venom shoots down from the salivary glands, through the hollow fangs, and into the prey.
The design of the fangs is just one of the many factors that determine how dangerous a snake is to humans. Even the composition and toxicity of the venom isn’t always the deciding factor—which is why lists of the world’s “most dangerous” snakes often don’t reflect reality. A snake’s lifestyle and temperament can be just as important.
Sea snakes, for instance, are thought to have the most toxic venom of all snakes, but they produce it in small quantities. Although many people have been bitten by them, few have died as a result. They are not aggressive unless immediately threatened, so they aren’t especially dangerous. African puff adders, on the other hand, have venom of average toxicity, but they produce it in large quantities and have extremely large, long fangs. All in all, they are far more dangerous than sea snakes.
Turning Deadly Venoms into Cures
It is hard to believe that substances that have been so well designed for killing could also be useful in medicine, but it’s true. The first medically active substance isolated from a snake’s venom came from a Brazilian pit viper, Bothrops jararaca, in 1949. The venom dilates blood vessels, causing prey to lose blood pressure so that they react more slowly or even collapse. The material later became the basis for a popular family of blood-pressure medications called ACE inhibitors.
Another useful blood-disorder drug comes from the Malaysian pit viper. In its pure form, the venom causes prey to die of massive hemorrhaging by preventing blood coagulation. Among humans, it is used to treat patients who suffer from blood clots.
Snake venoms often act only on certain types of cells, and this specificity has led to important research into treatments for cancer. Typical chemotherapy drugs cause many undesirable side effects because they don’t discriminate between cancerous and healthy cells in the body. Some research that is currently under way is experimenting with using snake venom to destroy only those blood
vessels that carry nutrients specifically to the tumor, thereby starving it to death.
Unfortunately, transforming snake venoms into medicine can be very time-consuming, because they consist of so many different components. In many cases, venom from a single snake has extremely diverse effects.
From the venom of the Siberian moccasin, for example, scientists have isolated three enzymes—phospholipases—that are nearly chemically identical except for their acidity levels, yet they do dramatically different things. The low-acid phospholipase inhibits blood coagulation, while the highly acidic enzyme destroys red blood cells. The neutral type is a form of neurotoxin.
Many neurotoxins work by inhibiting or completely blocking nerve activity, so they are interesting research targets for diseases, such as epilepsy, in which there is too much electrical brain activity; for the treatment of pain; or for helping drug addicts trying to escape their dependency. Remarkably, other substances have been found in snake venom that actually foster the growth of new neurons. These could be useful for Alzheimer’s and other diseases in which neurons in the brain die off.
Snakes may kill tens of thousands of people yearly, but their deadly venoms have the potential to save many thousands more.
Cytotoxin: Chewing Starts Digestion
Hemotoxin: Acts in Two Ways
Neurotoxins: Paralyzing Prey