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Reading this question, I wondered why is it that we associate vertebrate venoms so often with snakes and fish, and more rarely with lizards, amphibians, mammals, and birds (apparently never, in birds?).

Are venoms more advantageous for snakes and fish, or more expensive or dangerous to produce for the other taxa? Or is it connected to evolutionary history – is venom somehow more 'evolvable' for snake and fish lineages?

Or, perhaps, is it just chance?

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This write up by Carl Zimmer basically covers anything I could have said. He links to a number of resources, in particular this pdf, which at a cursory glance looks utterly fascinating and very well done. Figure 1 in that pdf sums it all up, I guess, or to quote Carl:

"Each lineage of venomous animals became deadly on its own, independent of all the others. And yet, in the end, their venoms echo each other... These results show that there are a limited number of ways to kill your victim quickly. No matter what genes you borrow for the evolution of venom, they will end up very similar to other venoms."

Another Zimmer piece specifically points out research purporting to show that snake venom genes are much older than snakes, maybe 200 million years old. That gets around but doesn't quite answer your question. An older review tried to coalesce things (as best as he could in '92), spending time focusing on insectivora and dealing with mammals. The theory is that while venom is an excellent advantage, it requires a significant investment and is often slow-working. In a world with sharp teeth capable of tearing, venom may not be necessary. Mammals, for example, might evolve to not use venom, as it may not be suitable for their high daily energy demands. The pdf linked above briefly touches on the concept of "reverse recruitment," where venom genes may be usefully re-purposed for other biological processes.

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Venom production is a hard adaptation to achieve. So it is conceivable that simpler organisms had enough evolutionary time to get the adequate mutations for this to happen, which would explain why venom is almost absent (I know only of a platypus as being poisonous) in mammals and birds, and seemingly abundant in fishes, amphibians (such dart frogs) or reptilians.

Also, as I understand there should be great selective pressure and mutation in simpler organisms is more likely to produce viable offspring than in more complex ones, which can be a contributing factor.

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I note that both fish and snakes lack limbs and in fact many venomous and poisonous creatures would be almost helpless without venom/being poisonous to eat or touch. There are cases of poisonous creatures whose sole defense is their coloration advertising their poisonous nature and the poison itself (and of course their are false advertisers); these creatures rely on predators choosing not to eat them, not any kind of active defense at all.

So I suggest lacking limbs and/or being slow and/or being small/fragile favors evolving toxins as a means of self-defense or for killing.

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