Hunting can't be easy when you have no limbs at all. And what use is having the ability to swallow whole animals larger than your own head if your prey inflicts fatal injuries on you when you try to do so? Well, fortunately for many carnivorous snakes, evolution has provided two ingenious methods with which they can subdue amphibians, birds, rodents - and even larger mammals - before sucking them back: envenomation and constriction.

Fatal fangs

First of all, how did snakes develop venom? According to George Zug, amphibian and reptile curator with the American Museum of Natural History in New York, it probably started with a case of deadly saliva.

"Animals with toxic saliva would gain a benefit," Zug surmises. "They would be able to control prey, and have less injury. Eventually, venom and an apparatus for delivery would be produced.


Venom controls much larger prey.

And what an apparatus it is. According to Cornell University herpetology professor Harry Greene, the venomous snake's mouth acts remarkably like a hypodermic syringe. The oral glands hold the venom like a needle barrel, and when the snake bites, jaw muscles push the toxin like a plunger into the grooves of the fangs, which deliver the poison like a needle into the unlucky recipient.

These vicious-looking needles come in several varieties: short, fixed and in front, like the King Cobra; large, front fangs folded back when not in use (seen in vipers); and fangs in the rear of the mouth, used to chew poison into the prey. And some types of cobras are even able to spit jets of venom - with lethal accuracy - into their victims' eyes.

The venom itself is comprised of complex proteins that either destroy tissue (hemotoxins), or the nervous system (neurotoxins), or both.


Rattlesnakes are more likely to be tissue destroyers while cobras tend to inject neurotoxins.

"Rattlesnakes are more hemotoxic," says Darryl Frost, herpetology curator at the American Museum of Natural History. "The site of the bite dissolves - it becomes a big crater." Hemotoxins usually cause internal bleeding in human victims, with death resulting from shock.

Which venomous snake is the deadliest? Greene says there are many contenders, from the Inland Taipan, whose bite can have enough venom to kill 200,000 mice, to the Black Mamba and King Cobra, whose bites emit neurotoxins that cause muscle paralysis and eventually respiratory failure in humans - and can be life-threatening in minutes.

But as Dr. Wolfgang Wuster, herpetologist and research fellow at the University of Wales will tell you, judging the "deadliest snake bite" for humans is no easy task. While we usually try to quantify venom potency by injecting it into mice and then calculating on average how much will kill, many other factors affect whether a snake bite will prove fatal.


A couple of drops of venom of the Black Mamba can kill a human.

The sensitivity of different animals to different venom differs enormously, and these values cannot be translated directly into relevant figures for humans," Wuster explains. "Furthermore, the quantity of venom the snake injects when biting is also relevant - a snake that injects a teaspoonful of relatively weak venom may well be much more dangerous than a snake that injects a few milligrams of highly potent venom. Some snakes that look very impressive on paper are known to be clinically much less."

Herpetologist Whit Gibbons at the University of Georgia's Savannah River Herpetology Laboratory agrees.

"Venom is a complex protein," Gibbons points out. "[If bitten], different people will react differently to the venom of different species - even to different individual snakes, depending on where it bites you and what you might be allergic to."


Cobra bites can destroy the tissue around them if untreated (Photo Dr. Sherman Minton, E-medicine.com)

The snake's anatomy and behaviour is also a factor.

"Size makes a difference," Gibbons says. "[Also], is it biting only to scare, did it bite with only one fang? And if the snake has just eaten, it may have used up most of its venom."

Picking the most lethal snake to humans, say the experts, is probably more correctly done by seeing how many people die of bites from that species.

"The only measure of a snake's degree of danger is to study a good series of bites, and determine what percentage of the victims die, or come close to dying," suggests Wuster. "Since the amount of venom injected by a snake, and hence the severity of a bite, are highly variable, one needs a good series to assess the range of variation in severity. "

Among the highly-feared cobra species, this type of information is available for only a minority of species, Wuster says. Of those for which it is available, the Philippine cobra probably gets the prize for being the most lethal: nearly half of one series of patients required intubation (meaning they would have died of asphyxiation without treatment), and some were critical only 10 minutes after the bite. One could die from the bite of this species in 10-15 minutes, Wuster says, although longer periods of time are more usual.

"King cobras are also, of course, highly lethal," Wuster adds. "But there has been no well-studied series of bites that would allow one to make meaningful comparisons. Anecdotal accounts of individual bites tend to be highly selective."


But the experts emphasize that no cobra species could truly be termed "aggressive" towards humans and that holds true for other highly-venomous species as well. Luckily, the likelihood that a human will be bitten by a snake - at least outside of the tropics - is very low. Being bitten usually results from putting the snake on the defensive, trapping the snake where it can't get away, or deliberately getting near them or zoo handlers trying to feed them. In the U.S., the experts say, people are most likely to be bitten by Copperheads or Western Diamond-Back Rattlesnakes - a wide-ranging species that gets fairly agitated when on the defensive.

But even handling snakes doesn't mean getting bitten - and dying of snake bite is very rare. It is estimated that less than one percent of properly-treated snake bites in developed countries results in death.

"I've picked up lots of venomous snakes and I've never been bitten," claims Whit Gibbons. "In the U.S., Copperheads bite more people than any other snake species but to my knowledge, no one has died. That's pretty telling."

The Big Squeeze

If there's anything scarier than a King Cobra sinking its teeth into your arm it's probably a six-metre Anaconda throwing its coils around your body and squeezing you to death. But don't worry, says George Zug, curator of amphibians and reptiles for the American Museum of Natural History in New York. Death is quick for the constrictor's victim - and contrary to popular belief, it's not by suffocation.


Boa constricts a rat

"The constrictor collapses the chest cavity, constricting the heart so it shuts down," Zug explains. "Death is from a lack of oxygen to the brain. The prey dies faster than with suffocation - probably less than a minute. Suffocation would take several minutes."

Whew, that's a relief...at least for the snake.

"Animals that evolved into constrictors probably had no toxicity in their saliva [like the ancestors of venomous snakes]," says Zug. "They might begin to push prey against the ground. It would be a sequence."


According to Zug, this type of constriction is still used, while other snakes use the more popularly known method.

"Some snakes, like Bullsnakes, constrict by laying coils against an animal, pinning it to the ground," Zug says. "Some snakes, like Kingsnakes, lay coils around the animal and pull tighter and tighter."

Constrictors use a variety of ambush techniques, according to Cornell University herpetology professor Harry Greene. Diamond Pythons perch in trees waiting to seize arboreal animals. Puerto Rican Boas wait around the openings of caves to capture bats, while Green Tree Pythons lure their prey with their tails. Corn Snakes first bite their prey to get a good grip, then quickly coil themselves around and constrict.


Anaconda constricts caiman


But the constrictor most feared by humans is probably the Anacondas of South America. The Green Anaconda, which may reach 10 metres in length is the heaviest snake on the planet. Hiding in the watery lagoons it prefers, it will lie in wait for birds and mammals wading or drinking. Though it has poor eyesight, it has infrared sensors that can detect less than one degree difference in temperature. It strikes at six metres per second.

However, as fearsome as this might sound, Zug says he's never heard of anacondas being labelled aggressive -- and that to term any snake "aggressive" is really a misnomer.

"Snakes are reactive," Zug says. "They will defend themselves when they think there's no way out. But if that's aggressive..."