Venom resistance in kingsnakes

A kingsnake eating a cottonmouth

Individuals can acquire resistance through repeated exposure to low doses of a toxin. The immune system recognizes the toxin as foreign and attacks it. It forms a memory of each attack and stores the pattern for later, which makes later responses to the same toxin quicker and more effective. If the toxin dose is later increased, the memory is reinforced & may become stronger. This is how antivenom is made, how people become resistant to snake venom, and also how vaccines against infectious diseases work.¹

It is not how kingsnake resistance to viper venom works. Kingsnake resistance is evolved rather than acquired. This means that kingsnakes are born resistant to venom. As far as we know, their resistance levels are fixed for life & don’t change with age or exposure. This has happened over a long time through natural selection, over many generations of kingsnakes. We don't actually have a very exact  understanding of the physiological and molecular mechanisms behind how kingsnakes resist the toxic effects of viper venom. At least some of their resistance comes from antibodies—chemicals in their blood that interfere with the venom—because mice injected with kingsnake blood survive viper venom better than those that aren't, and the chemical composition of kingsnake blood changes after exposure to viper venom.

A kingsnake eating a western hognose snake

Any time a weapon appears, there is potential for counter-weapons (i.e. resistance and immunity) to appear in response. This happens through a process called a co-evolutionary arms race². Just as the United States and the Soviet Union were involved in an arms race centered around nuclear weapons during the Cold War, so are venomous snakes and their prey & predators involved in arms races centered around their primary weapon—venom.

A major difference is that, unlike nations or humans, animals cannot plan for the future and decide to invest more energy in research & development of novel or better weapons technology for future generations. Instead, co-evolutionary arms races happen through natural selection. What start out as tiny variations in toxin resistance can be magnified over many generations. 

A kingsnake and a copperhead biting one another

When vipers were first evolving their front fangs, defensive bites became a new option for them. At first, their predators were probably not very good at resisting the effects of the venom, especially if the predator’s physiology was similar to that of their prey, and venom would have made a very good defense mechanism. Vipers would sometimes be killed and eaten, but many predators later died from their bites. Kingsnake predators that were slightly better able to tolerate the effects of the venom were more likely to survive. Eventually, all the kingsnakes without these venom resistance traits had been killed by vipers that they tried to eat, and only the resistant ones remained. On the other side, vipers that had venom with toxins that were, for example, slightly more painful or fast-acting, might have been more likely to survive a predatory attack. Thus, the arms race escalates. Vipers also exhibit flipping, jerking, “body bridging” and other escape behaviors as a defense against kingsnakes—suggesting, since they do not try to bite kingsnakes in defense, that their venom is essentially useless as an anti-kingsnake defense mechanism by now and that kingsnakes have “won” this arms race.

A mongoose eating a boomslang

This is why kingsnakes are immune to the venom of copperheads, cottonmouths, and North American rattlesnakes, but not to the venom of, for example, king cobras or black mambas. Because they live on different continents, there has never been an opportunity for kingsnakes and black mambas to enter into a co-evolutionary arms race (although both prey and predators of black mambas in Africa, such as honey badgers, and of king cobras in India, such as mongeese, have probably accomplished much the same thing).

Kingsnakes also eat coralsnakes, but amazingly they are not immune to the venom of Eastern Coralsnakes (Micrurus fulvius)—kingsnakes injected with coralsnake venom die quickly, and kingsnake blood is 0% effective at neutralizing venom proteins from coralsnakes. Presumably they are able to catch and consume coralsnakes without getting bitten. This could be because coralsnakes often eat other snakes, so perhaps their venom is more difficult for kingsnakes to evolve resistance against. Or, perhaps coralsnakes are rare or dangerous prey for kingsnakes, and it’s possible but not worth it for them to evolve resistance.

A milksnake constricting a Dekay's brownsnake

Not every kingsnake species has been tested against every venom, but we do know that there is variation in which species are immune to which venoms. The only study to compare species in depth injected mice with mixtures of venom & snake blood and measured mouse symptoms and survival. They found that blood from Eastern Kingsnakes (Lampropeltis getula) had the widest spectrum of protection against the venoms tested and was the most effective at neutralizing many rattlesnake venoms, but the least effective against copperhead venom. Blood from kingsnakes from Florida & the Gulf Coast was the most effective at neutralizing the venom of copperheads & cottonmouths. Mole Kingsnake (Lampropeltis calligaster) blood is about 75% as effective at neutralizing Mojave Rattlesnake (Crotalus scutulatus) venom as the blood of Eastern Kingsnakes. Gray-banded Kingsnakes (L. alterna) have moderate neutralization potential against Western Diamondback (C. atrox) venom, but none against Eastern Diamondback (C. adamanteus) venom. Blood from milksnakes (formerly all called L. triangulum) from various locations had intermediate neutralization capacity, with blood from North American milksnakes being about 70% more effective against rattlesnake venom than blood from Central American milksnakes. Another study found that an eastern milksnake injected with copperhead venom died, and one injected with pigmy rattlesnake venom had "no noticeable ill effects", but a lack of replication prevents these results from being particularly meaningful. Somewhat surprisingly, blood from Long-nosed Snakes (Rhinocheilus lecontei), Cornsnakes (Pantherophis guttatus), Mussuranas (Clelia clelia), and Japanese Four-lined Ratsnakes (Elaphe quadrivirgata) was also effective at protecting mice from viper venoms, but blood from pinesnakes (Pituophis) and gartersnakes (Thamnophis) was not. Both vipers and elapids appear to have at least some level of resistance to their own venom, although detailed studies are lacking for most species.

Fight of the Mongoose and the Serpent Armies An 1850 folio from the Mahabharata

Kingsnakes are just one of many species that have partial immunity or resistance to venom. Hedgehogs, skunks, opossums, and possibly snake-eagles also have resistance to viper venoms, and eels are resistant to sea krait venom. Unlike kingsnakes, we have actually figured out exactly which proteins in opossum blood are responsible for its snake venom neutralization capacity. We also know that mongeese have followed a different route, changing the shape of the toxin targets in their cells rather than putting molecules into their blood to combat the toxins (which means that their immunity cannot be transferred). Other predators of venomous snakes, such as indigo snakes (genus Drymarchon), appear to have gotten away with not evolving immunity, although I was unable to find any actual data on physiological responses of indigo snakes to venom, just statements saying they were not resistant, so it's possible that actual tests have not been carried out.

A mountain kingsnake constricting a skink

Opossum resistance to copperhead venom probably evolved in a similar way to kingsnake resistance, but vipers are also involved in co-evolutionary arms races with their prey. Many rodent prey of North American vipers are resistant, including wood rats, prairie voles, and ground squirrels. Think of how the U.S. during the Cold War had to balance foreign policy not just with the Soviet Union, but also with other nations. The emerging foreign policy is a compromise, just as the venom that evolves is a compromise of selection pressures from predators and prey. Resistant prey may select for venoms that are better at quickly incapacitating, whereas resistant predators may select for venoms that are less deadly and more painful; it’s difficult to predict exactly what will happen without knowing the exact mechanism of resistance. Sometimes selection from predators and prey may act in the same direction, other times in opposite directions. The details of these processes are what evolutionary biologists study on a day-to-day basis.