Take spiders: Black widows, brown recluses, and other spiders we would call “poisonous” don’t need to kill anything much bigger than a housefly to have a nice meal, but they have the power to kill a small human. Compare this with ordinary, “non-venomous” spiders harmless to humans, who I assume can kill a fly very nicely but can’t harm bigger animals. They seem have just the amount of venom they need, no more.
Similarly, venomous snakes like rattlers, cobras, coral snakes, copperheads would rarely need to kill anything bigger than a rabbit, but they can take down a cow that they have no use for.
Why the “overkill,” so to speak, in some of these venomous species but not others?
Brown recluses, at least, only have as much as is needed for their usual prey. It’s just that humans, specifically, happen to be much more susceptible to that specific venom than most animals are, for basically random reasons.
The Australian funnel-web spiders are among the deadliest spiders in the world in the effect their bites have on humans and our primate relations (although the bite has little effect on dogs and cats).
There’s clearly no evolutionary pressure behind that result – there weren’t any primates there until the first humans arrived an evolutionary eyeblink ago – so it’s just circumstance.
This makes some sense from an evolutionary perspective. The spiders are going to be in a predator-prey arms race. The prey that they frequently target are likely to evolve resistance to milder venoms, so the predators in turn evolve more potent venoms. Humans are only occasionally bitten in accidental encounters, so there has been minimal selective pressure for humans to evolve similar resistance.
More venom than they need for what? Killing prey is OK, killing prey faster has great advantages: the prey cannot go very far away and it will be able to defend itself for a shorter time, so the snake’s chances not to get hurt increase.
It is also a good defensive strategy to show how poisonous you are with bright colours (the rattle of the rattlesnake is also very effective), although not all very poisonous snakes and spiders do.
Note, of course, that this goes both ways: There are probably some venomous creatures that are nearly harmless to us, but which would kill some other animal of our size. We just don’t tend to notice those.
Long ago, I took a herpetology class through a museum. They told us that snake venom glands evolved from salivary glands and that the venom not only killed the prey, it started the digestion process from inside the prey.
There will also be situations where adaptations for resistance to venom may have some associated cost. The mongoose acetylcholine receptor is adapted to resist snake neurotoxins. It’s easy to imagine that this mutation might have concomitant negative consequences for normal neurological function, so even if the mutation arose in humans, perhaps our encounters with snakes are infrequent enough that it wasn’t favored by natural selection.
So snakes evolve larger doses of venom or more potent venom to deal with smaller animals that carry resistance, with fatal consequences for humans on the rare occasions we are bitten.
Several folks have noted this.
It is not the quantity but the quality.
A particular toxin is not necessarily about quantity. A tiny species may have a tiny quantity of toxic venom that will kill a much larger species. It is easier to evolve a wide range toxin than a specific one, if the cost of producing it is similar. It would likely be more evolutionary difficult to tailor a toxin and it’s production level to a specific size of prey, than a broader toxin in a small amount that would take out a wider range of sized prey.
It seems that in venom, a one size fits most all is more efficient to evolve. It does seem that many evolve a certain level of quantity and quality that is sufficient precisely to their prey. But there is also a type of venom that evolves that does not respect any size. There is no reason to devolve that.
I’ve been given to understand that a snake can inject a small amount of venom to make a creature leave the snake alone, and save some to kill his lunch.
