eli5: Why do some tiny animals contain enough venom to kill thousands of humans, isn’t that just overkill?
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They’re not doing it to kill thousands of humans.
They’re doing it to kill *anything* big that might annoy it (like a gorilla, or an elephant).
Humans just aren’t that big.
Its not just about quantity.
Its more like what little venom a small creature has is highly toxic/potent enough to kill even large animals.
A threat can be either small or large, and theire venom can deal with the largest ones, so it appears overkill for the small threats.
For americans, its like instead of a 9mm, you carry a desert eagle. They’re about the same size, but 1 of these packs a much bigger punch
Most venom is used to kill prey, not predators. A mouse isn’t much against a human, but to a snake they are vicious, with sharp teeth and strong (for their size) legs with claws. Spiders are generally tiny, spindly, and fragile with very fragile and costly webs. They regularly take on prey bigger than themselves.
In the wild, every little scratch can lead to death from an infection. Or death from being just that much slower than normal and unable to escape another predator. Or being just a little too slow to catch more prey. *Every* time an animal gets hurt it needs to be worth it for what they get – escaping with their lives or definitely getting a meal, so they really would rather avoid getting hurt.
Venom is a great tool for that because it can very quickly knock out prey and stop them from struggling, biting, clawing, stinging, etc. Bigger predator animals don’t need venom because they have raw strength, speed, group tactics, and they’re usually predating on things smaller than they are. Incidentally, you’ll still rarely see big predators going for the adults of their prey animals – you won’t see lions attacking adult water buffalo or adult zebras unless the lions are really hungry or the animal is sick, wounded, and alone.
The stronger the venom is, the faster the prey will stop fighting back. That’s still true when the venom is used primarily for defense, too. However, *most* animals that use venom for defense don’t have venom that is deadly strong, just *extremely* painful. Often, humans don’t die directly from the venom, but because they are in a dangerous place, like out in the ocean or alone in the wilderness and being unable to swim or move is what kills them.
Potent venoms are usually the result of an evolutionary arms race between the predator and the prey.
A venom’s primary purpose in most animals is to incapacitate prey. This applies an evolutionary pressure on prey animals to become more resistant to said venom, as being able to survive even just a single dose is often enough to escape and go on to mate. This in turn applies an evolutionary pressure on the predator to possess a more potent venom, as those with more potent venoms will end up being more successful at hunting.
Repeat this evolutionary back and forth over hundreds of millions of years, and you end up with a single dose of venom capable of bringing down elephants a hundred times over.
As for why humans or other large animals don’t develop these same tolerances, well when’s the last time you saw a venomous snake or spider actively hunt down a human or an elephant?
When we look at really small animals that have crazy strong poisons used for defense, such as cone snails or poison dart frogs, the animal itself is very vulnerable outside of it’s poison. Stronger toxins like that do more than just kill larger creatures, they also kill them faster. If a fish tries to eat a cone snail, even if the poison kills the fish in a few minutes the snail is still dead, so it has to kill the predator almost instantly to make a huge difference, and as such leads to crazy overkills.
It just happens that the mutation they came up with happens to be an effective neurotoxin and neurotoxins are very effective at killing pretty much anything.
Also you need to remember that these animals still have to deliver the venom to a potentially aggressive opponent. They are not always going to get a good bite or sting where they can deliver a full dose but they still want it to be a deterrent to stop and would be predator from continuing to attack them and would be prey from getting away. So having a strong venom is highly beneficial to survival and having enough venom for 2-3 attacks is good in case you miss or mess up.
> isn’t that just overkill
Yes. It would also mean a predator is more sure to die, thus leading to it being a better deterrent. It also ensures that any prey animals are sure to die, meaning less energy from chasing it down or from injury.
How evolution works is over thousands of years minor mutations occur that provide a small advantage over others of its species. For example if there was a serial killer killing only female blondes, it would be advantageous to be a brunette…. which would then create a large cluster of brown haired children.
Evolution is generally about energy conservation. Every species in the world does as little as humanly possible to reduce its total use of calories so that they become efficient creatures. This is why insects are the worlds best survivors (with very little evolutionary changes over the years) and cats are those prime mammals in every environment.
With that in mind snakes with lower amounts of venom would have died and not carried on their genes. The snakes we have today would have had to survive a lot of predators and inter-species competitors. Keep in mind, snakes have been around for 112 million years. That’s some massive animals you gotta be able to kill.
Who are you, Karl Pilkington?
There are some great answers already, but I’ll add one more – evolution is a process by which a mutation leads to any successful breeding, it stays.
Having a mutation that caused the venom to be more useful than was necessary never reduced the creature’s ability to breed, so it didn’t get weaker.
Also, humans simply didn’t evolve the resistance to the kinds of venom that were not used against our ancestors. The two lack of pressures combined makes us especially vulnerable to a venom that happened to be more powerful than it’s necessary to accomplish the task.
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