Eli5: Why do some spiders have poison that can kill mammals several hundred times it’s size , if they only eat other insects/arachnids, ect
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It’s completely a coincidental side effect. Almost all spiders are venomous to some degree for the purposes of immobilizing prey, but it’s just that most of those venoms don’t affect human. A few do and it’s just simply a quirk of evolution.
Bigger animals may still live for hours or even days when stung. If they had weaker poison their prey might be able to escape still, or at least destroy the whole web before being taken down. The poison has to act extremely quickly to stop the prey from moving, so it has to be strong.
Some mechanisms are designed to kill things you eat. Some mechanisms are designed to kill things that eat you.
Larger animals prey on them, these large animals learnt the color and shape of these deadly creatures and will avoid eating them. Many snakes and insects will copy the looks of the venomous once to try and avoid being eaten.
Not all venoms work on humans as well as insects or small animals. Also, they need the venom to work immediately. That can mean having enough venom to potentially kill a human eventually, but it’s actually JUST ENOUGH venom to instantly incapacitate their typical prey.
The poisons are evolved for the prey they attack… the affect on humans and others is coincidental.
For example, black widows. They use a alpha-latrotoxin to disable their prey. Since the spiders are small, and they sometimes eat small lizards, mammals and birds… they need to disable these animals almost instantly. Any delay means the animal can hurt or kill the spider, or just walk/run away. So they evolved an insanely potent poison that kills the small animals instantly.
So while their bites can kill larger animals, its just a byproduct. It will kill a larger animal slowly.
Others have answered and got partial answers, but the truth is pretty simple.
A spider has a very limited window in which to bite and immobilise prey, or bite and hurt/kill a predator or larger animal that could kill it by simply stepping on it.
If your prey gets away and dies much later, you probably don’t get to eat it, so you wasted energy on creating venom and hunting. Do that often enough and you die of starvation. Packing a bigger punch in the venom means the prey is more likely to die or be immobilised quickly enough to secure the food, and live a little longer.
Equally, having weak venom means that the predator, or simply larger creature that doesn’t notice you, is more likely to kill you before you escape.
Having very effective and powerful venom (not poison – venom is injected and poison is ingested) gives you a better chance to survive both eating and being eaten, especially in small, fragile creatures like spiders.
Dosage.
If you use your poison to kill prey you want it dead right away. If it runs off and dies half a day later some other predator will find and eat it not you
So when it bites a nonintended prey like a human the dose is enough to kill slowly or just make very sick
There are something like 50,000 spider species, practically all of which have venomous bites that they use to poison prey. Of these, something like 30 are known to have actually killed people, and in most of _those_ species, fatalities are an unusual outcome (after all, a bee sting can kill you too, if you happen to have a bad reaction).
So we are talking about maybe a dozen species out of 50,000 that can actually kill big mammals. That’s vanishingly rare. It’s the sort of pattern you’d expect to see if there’s no actual benefit to it, and it’s just a side effect of some venoms happening to be stronger than others, and some being on the very outlying end of that spectrum. Sometimes, you just roll all sixes.
They’re competing with the natural defenses of their prey. Think of it as teo species one upping each other over millions of years.
Say a small lizard evolves to be resistant to a spider bit, the spidersneith more potent venom survive and breed. The lizards with better resistance survive and breed. The spiders that can paralyze *those* lizards survive and breed. Generations of this and some dummy picks up a spider and gets injected with venom that is strong enough to paralyze or kill super resistant lizards.
Basically, other animals like humans sometimes get caught in the crossfire of an evolutionary arms race.
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