Why would animals evolve a venom strong enough to kill stuff too big for them to eat? I mean, what does a spider or a snake gain from being able to nearly kill me?
Note, I’m assuming here that stronger venoms are harder (i.e. require more resources to produce.
Also, AFAIK, most creatures’ venom is not lethal (or particularly dangerous) to animals many times the size of their preferred prey, so I’m pretty sure these are the exceprion rather than the rule.
A fighting chance at not being eaten by you.
Venom is for self-defense, too, not just predation.
The venom attacks systemic processes, and there is a lot of consistency in the biochemistry throughout the animal kingdom.
So, in most cases, the venom would attack, more or less uniformly, the body chemistry of all animals, or at least all within an order of animals, with the possible exception of a few that have evolved an immunity to it, by living alongside it with repeated exposure…
Which yields a paradoxical result that the species most likely to be a venomous animal’s prey, becomes the one most likely to evolve an immunity to it. Such are the odd quirks of nature.
Just musing, IANK. (I am not knowledgeable, someone who is will be along shortly,)
As has been said, being venomous isn’t only for subduing prey, it’s also for self defense. And your predators are going to be much bigger than your prey.
In addition, your usual prey may have evolved a tolerance for the poison. In order to have a lethal dose for the prey (and which kills them rapidly), you may need poison that is also lethal to much larger animals (which don’t have the same resistance.)
Plus predators have an unfortunate tendency to occasionally get injured by prey. Putting it down ASAP is preferable.
Apart from what others have said, I do not think that that is a very well founded assumption. Certainly some toxins are toxic, even to large animals, even in minute doses (Botulinum toxin is an obvious example, and one where, AFAIK the bacterium that produces it gets no advantage at all out of poisoning humans and other large animals). I doubt whether there is any general relationship at all between how toxic a substance is and the resources required to produce it biologically.
This has come up before, and I said then pretty much what njtt: said, the biochemical pathways don’t have to evolve to precise dosages. Its just as much an artifact of how a target behaves that determines what will happen.
Here’s the thread: http://boards.straightdope.com/sdmb/showthread.php?t=556448&highlight=toxin
There are also some venoms which just coincidentally happen to be really bad for us for no particular reason, but because they’re bad for us, we notice them. For instance, I’ve heard that brown recluse venom is a very mild toxin for the vast majority of mammals, but through some quirk of biochemistry we happen to be particularly vulnerable to it.
There is folk wisdom (and maybe scientific as well) that it is better to be bitte by an adult Copperhead than a juvenile - the theory being that the young have not learned to conserve resources, and will unload their entire supply in a single strike; the adult knows there may be another predator coming along before the kid has a chance to recharge - the adult can still take out the next predator (or meal).