Why is snake venom so toxic?

A black mamba bite is sufficient in toxicity and volume to kill an adult elephant.

Nairobi beetles are about 15 times more toxic than a cobra. (He will not kill you but you will feel a lot of pain if you brush against or slightly squash it).

But why? Why is so much wildlife so incredibly toxic?





Natural Selection

Black mamba – Dendroaspis polylepis

A lot of natural venoms, indeed, are not simply made by one chemical, but they are a mixture of nasty toxins with a whole range of activities.

But why evolving a whole array of nasty chemicals, when one is usually enough to kill most things? What is the purpose?

This question puzzled lots of people and many answers have been given with no one clear winner.

The two main answers proposed are:


  1. There is no effective natural selection on snake venom. Once evolved, venom toxicity just drifts along. 

2. Other researchers suggest exactly the opposite: it must be a feature that is under extremely high selection pressure (this is the answer I personally agree with).

Now, the first answer comes from the observation that if one species doesn’t have venom and another one has it , but both survive, it would seem clear that evolution has nothing to do with it.

Superficially, this is quite persuasive, but it seems there is dubious thinking here. Remember the acacia thorn evolution: many plants in savanna have thorns, but the herbivores eat them anyway.

So, even here evolution has nothing to do with it? We have seen in another post that it is not like this.

Puff adder during attack – Bitis arietans

So let’s look at the second suggestion, hence an evolutionary benefit to the extreme toxicity of venom.

Let’s start by asking ourselves why snakes have venom above all.

They use it for two things:

– to kill their prey;

– as a defence against threats.

Undoubtedly, for a defence mechanism alone, they don’t need to kill.


In fact, they might even feel better if they don’t do so. They just need to give a warning that can be passed on to other potential predators.

So let’s concentrate on what seems the initial reason to evolve venom firstly: to kill prey.

Evidently, if you evolve venom for attacking, it is reasonable to later use it for self defence, but it is still  possible the attack reason is driving the whole process.

Now, a venomous snake that finds a prey strikes quickly and then wait for the prey to die at a safe distance.

This is not a bad idea if the snake wants to take something larger or stronger than itself.

If it was too slow the prey would have the chance to run away and the poor snake goes hungry.

So although there might be enough venom in a mamba bite to kill an adult elephant, it takes very long to kill (not fast enough if the snake wanted to actually eat the elephant).

We also know that there is a large cost involved in producing venom.

Snakes that have  recently bitten and need to re-fill their venom glands have a metabolic rate 11% higher than snakes with no such need.

This also explains why some snakes don’t always inject venom when acting in self-defence.

In fact the bite itself is warning enough.

Some snakes do even adjust the dose of venom according to the mass of the prey chosen.

Although there is certainly a cost involved in producing venom, it’s not clear that the costs involved in producing a rather less toxic venom would be significantly lower than those of producing a much more toxic venom – once you’ve paid the initial cost, producing different variations on a theme might not be additionally costly.

So if it’s relatively easy to vary the basic molecules involved in toxicity (and it seems so), it would make perfect sense to evolve a whole suite of chemicals, each with slightly different action making the overall combination toxic in the extreme.

What’s more, this also fits with what we know about resistance.

Exactly like bacteria can evolve resistance to our antibiotics, prey can evolve resistance to snake venom too.

Now, resistance is most likely to evolve in bacteria when the doses are not high enough for long enough to kill everything. That is why it’s crucial to complete a course of antibiotics (if you don’t, some bacteria may survive when you stop, and you’ve just selected those ones that are close to evolving proper resistance).

It’s surely the same with snake venom: it’s much harder to evolve resistance to a massive dose of a venom, than it is to a smaller dose.

We also use multiple antibiotics when we’re trying to protect ourselves against bacteria developing resistance (it’s much harder to simultaneously evolve resistance to two antibiotics than just one).

This might be the reason why venom is full of a whole range of toxins too.

Rattlesnake – Mexico

Despite this, resistance might still evolve in prey animals, so the doses need to keep getting larger and larger – just like thorns do.

But when it is used in self defence against an animal that has no evolved immunity because it’s so rare to get bitten (like us human beings), the dose is incredibly toxic.

Now the question is:  why is only a minority of snakes venomous, if there is such an advantage to being venomous?

Here, we have to consider that species specialise in different prey.

Constrictors and venomous snakes have developed two different strategies: one for killing large prey, the other one for dealing with fast moving and dangerous animals.

Non-venomous snakes (which are, always remember, the majority of species) that aren’t constrictors tend to prey on smaller things that won’t run away or fight back, like invertebrates, small reptiles, amphibians, etc.

The venomous snakes can hit birds, bats and other larger mammals if their quick bite will instantly immobilise and kill their chosen prey. And then, once they have evolved toxic venom, there are many good reasons why they should make that venom as toxic as possible, especially if it doesn’t cost much more than the initial stage of having any venom.


I wish to end with an advice:  if you get bitten by a venomous snake, if you can identify it, great!, if not please don’t go thrashing about after it to kill it or get a better look.

Most snake-bites can be identified sufficiently from their toxic effects and many anti-venin nowadays can target a group of snakes.

If you or your friends go thrashing around after a venomous snake that is already scared, the most likely outcome is someone else getting bitten too.

And this doesn’t help things at all, and you could even harm an innocent animal that was just defending itself.

Better if you find anti-venin as fast as possible …

Estimates suggest about 43,000 venomous snake-bites each year in East Sub-saharan Africa and that is why you should always be accompanied by a professional guide when you go to Africa.

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