Snake venoms and myths
Some snakes are venomous, as we all know. Indeed, some of them can be dangerous to humans if we get too close to them and they feel threatened. Potentially dangerous animals fascinate people, engender some fear, and also lead to extraordinary myths and misunderstandings. So, let’s take a quick review of how snake venoms work and see how this can lead to misunderstandings.
There are two broad classes of snake-venom proteins. Hemotoxins, which primarily digest tissues and cause internal bleeding, and neurotoxins which primarily block nerve signals in the body. That distinction is pretty clear but, in thinking about any particular species of venomous snake, it is best to think of these as two ends of a continuum. This is because the venoms of most species comprise a mixture of these two classes of proteins. That is to say that few snakes are ”only neurotoxic” and few are “only hemotoxic”—snake venoms typically are a combination of these two types. The details of that combination varies among species; it can vary with the age of the snake; and sometimes even vary among populations of the same species. Venoms evolved primarily to enable prey capture; their use in defense clearly was an added benefit realized later.
Medically speaking, this means that different venoms affect the human body very differently. Highly hemotoxic venoms can cause enormous tissue damage and hemorrhage, which can sometimes be fatal. Highly neurotoxic venoms can cause paralysis of key organ systems, such as the respiratory system, and can sometimes be fatal. Snakes with a mixture of venom types can cause significant tissue damage and also paralytic effects such as numbness, drooping eyelids, etc. This is why we must use different anti-venoms in medicine to counter the effects of the different venoms of different species of snakes.
Ecologically speaking, the ends of this venom continuum tend to specialize on endothermic prey (hemotoxins) versus ectothermic prey (neurotoxins). This is because the physiology of and endothermic animal is very sensitive to sudden drop in blood pressure. Hence, a venom that will rapidly drop blood pressure will very quickly disable and kill a mouse or bird. Ectotherms, like fish, amphibians, or snakes and lizards, are not nearly so sensitive and can tolerate huge swings in blood pressure, so neurotoxins simply disable them through muscular paralysis. These differences are evident in the diets of the snakes. A snake-specialist cobra typically will have a high proportion of neurotoxins, and a mammal-specialist rattlesnake may be primarily hemotoxic. A generalist predator like a cottonmouth (Agistrodon piscivorus) has mixture of both types of venoms.
An interesting phenomenon occurs when a snake species changes its diet as it grows. For example, the southern Pacific rattlesnake (Crotalus helleri) of California preys primarily on lizards when it is young, and switches to rodents as it grows. This means that the venoms of the babies and adults are quite different. Perhaps this is the basis for the myth that “baby rattlesnakes are more venomous than are adults”? This is not true, because “different” does not necessarily mean “more dangerous.”
Another confusing aspect of envenomation by snakes is that the individual snake has complete control over how much venom they deliver in each bite. One can create all kinds of stories to explain how they make such decisions, but the fact is we do not know what goes on in their minds during a bite. In defensive bites to humans, this reality has the important consequence that the bite may be “dry” or “minimal” or “moderate” or “severe” with those terms correlating with how much venom was injected during the bite. This can lead to confusion in the emergency room, because a fully verified bite by a properly identified venomous snake may result in zero medical symptoms—because the snake simply decided not to inject any venom at all. Or, the bite may be severe. Surely this has led to many myths about cures for snake bites. One can imagine a scenario where a viper bites a person on the hand. The person rubs a green apple over the bite (not a red apple, mind you, as that would just be silly) and suffers no ill consequences from the bite. This may lead people to believe in baseless cures for snakebites, when in fact it was just a dry bite. On the other hand, a person suffering a severe envenomation may waste time with a useless remedy when they should be seeking immediate medical attention.
As you can see, snakebites are complicated and variable and sometimes quite unpredictable. Complex scenarios in any realm can lead to confusion and myths and fallacies. For example, there is no basis for the common myths that venomous snakes are more dangerous right after they emerge from hibernation. And, while a larger snake may be capable of injecting a larger dose of venom, it does not necessarily mean that it will choose to do so. All of this variation and nuance is why I discourage the oft-repeated question to identify the most venomous species of snake in the world or the most dangerous species. As you can see, it all depends …
We invite you to come regard the snakes at Zoo Atlanta and admire them for the complexity, as well as their beauty. And you can do so with a safe, solid pane of glass between you and these fascinating animals. We hope to see you soon!
Joe Mendelson, PhD
Director of Research