New York Times, Sean B. Carroll.
I can remember the first time I saw one, at the St. Louis Zoo, and the feeling that certain death was just on the other side of the glass. I could not get over the snake’s size — this one was about 12 feet long. I was used to looking at giant snakes in zoos (I always made a beeline for the reptile house), but pythons did not seem so scary to me because they rarely moved. This sleek, agile and very alert snake was a king cobra, the largest venomous snake in the world and an icon to all snake enthusiasts, including this writer.
The king cobra’s venom is not, ounce for ounce (or milligram for milligram, as the professionals would measure it), the most potent. Among land snakes, that honor appears to belong to the inland taipan of Australia. But what the king cobra lacks in potency, it makes up for in volume. Its half-inch fangs deliver a huge dose, up to seven milliliters of venom, or about one-quarter of a whiskey shot glass. The lethality of venom depends on a combination of its potency, the volume delivered and the size of the victim. A king cobra bite can kill a human in 15 minutes and a full-grown elephant in a few hours.
What makes these cobras kings is not just their size, or their deadliness — after all, they don’t eat humans or elephants — it is that they eat other snakes. Even deadly snakes like kraits or other cobras are prey. These snakes bite when attacked, of course, which raises the question: How does the king cobra maintain such an apparently high-risk lifestyle?
Krait and cobra venoms, including that of the king cobra, act very quickly by crippling the nervous system. Among the arsenal of weapons in the snakes’ venom is one especially potent neurotoxin that works by binding to receptors on muscle cells. The toxin blocks the ability of acetylcholine, one of the body’s chemical neurotransmitters, to control muscle contraction. The blocking of these receptors causes paralysis, respiratory failure and death.
But the king cobra is not fazed by bites from its victims. Biochemists have carefully mapped exactly how neurotoxins block the acetylcholine receptor of many species, and they have discovered that the toxins do not bind to the cobra’s receptor. Mutations have altered the snake’s receptor in such a way that, because the toxin cannot bind to the receptor, the acetylcholine function is undisturbed. The king cobra can subdue its dinner without suffering from any venomous counterattack.
This large snake, resistant to the very potent venoms of its prey, would appear then to be impervious. But we all know the story of the mongoose, if not from wildlife programs then from Rudyard Kipling’s tale “Rikki-Tikki-Tavi.” How does the mongoose defeat the king cobra?
The mongoose’s quick reflexes help it dodge the cobra’s defensive bite, and its powerful jaws can dispatch a snake in one blow. But there are also genetic grounds for the mongoose’s courage. Sometimes an attacking mongoose is bitten, but it has another line of defense against the venom — its acetylcholine receptor has also evolved so that the cobra neurotoxin cannot bind to it. A set of changes in the mongoose’s receptor makes it resemble the cobra’s own resistant receptor.
The mongoose’s evolutionary adaptation is not unique. Other small, humble creatures have evolved ways to endure what for most animals would be lethal snakebites, and some of these resistant animals turn the tables to conquer and consume their venomous foes.
Sea snakes generally possess very toxic venoms. While the snakes rarely bite humans in the water, fishermen are struck occasionally when sifting through their trawls and have died from the bites of some species. The potent venom is not meant for large animals like ourselves, of course. Sea snakes prey on small marine animals, and the powerful toxins in the venom quickly immobilize the prey before it can swim off. Eels are some of the favorite foods of the banded sea krait. Some eels, however, have been observed to be remarkably resistant to the sea krait’s venom.
Harold Heatwole and Judy Powell of North Carolina State University showed that undulated moray eels and liver-colored moray eels found in the waters around New Guineacan tolerate several hundred times the venom dose that kills spotted moray eels from the Bahamas.
The biologists’ explanation for the great disparity in the sensitivity of eel species is that sea snakes are not found in the Atlantic Ocean, so there has been no selective pressure on eels there, whereas in the Pacific, where sea snakes are abundant, selection has been so intense that some eel species have evolved resistance.
A similar situation has evolved among California ground squirrels with respect to the venom of northern Pacific rattlesnakes. This species and most other rattlesnakes kill their prey with a battery of toxins that is different from those of sea snakes and cobras. Rattlesnake venom toxins work by breaking down tissues and causing internal bleeding.
A good-size rattlesnake can deliver a hefty dose of venom that is sufficient to kill a human if the bite is left untreated. But ground squirrels in some parts of California, despite being one hundredth the size of humans, exhibit fairly mild effects from the venom. This resistance is not the result of altered receptors, but comes from the ability of proteins in their blood serum to neutralize the effects of venom.
But squirrels of the same species from Alaska, where the rattlesnakes are absent, exhibit much greater sensitivity to the venom, and their serum is much less effective at neutralizing the venom. These observations suggest that in some areas where the rattlesnakes are abundant, local squirrel populations have evolved a degree of resistance.
One family of completely harmless snakes, the kingsnake, has also evolved serum that neutralizes rattlesnake venom and uses that ability to greater advantage than ground squirrels do. Kingsnakes are a group of beautiful constrictors found in many parts of the United States. As their name indicates, kingsnakes eat other snakes — they do not hesitate to attack, kill and consume rattlesnakes.
These stories of evolution are a bit like Shakespearean dramas, where the one best able to carry out or to thwart poisonous schemes winds up becoming king. Snakes in general, let alone cobras, will never be much loved by humans, but these animals are so extraordinary, this enthusiast cannot resist one sentimental thought: Long live the kings.