Thank you Desmostylus; I already knew that, but the reference helps. The problem is that there are other venomous snakes in their ranges, just not coral snakes. I didn’t know what Blake would make of that.
FWIW, I am a herpetologist. I don’t have a PhD yet, I occasionally make mistakes, but I am published and generally know what I’m talking about RE: snakes (if little else!).
So: Tamerlane, you are correct. That is certainly a case of the secondary development of defense uses for venomous snakes. As for the coral snakes, the debate isn’t necessarily settled as to the reasons for the predator avoidance. I suspect that predators avoid them due to the fact that they have had bad experiences with bright colors in the past, i.e. monarchs and whatnot. The primary reason I don’t believe that it necessarily has anything to do with their venom is due to their behavior. Eastern coral snakes are nocturnal and fossorial. They are active mainly at night, and are hardly ever active on the surface. Additionally, they are very inoffensive animals. They are difficult to provoke. They also cannot really strike in the typical sense. They turn their head and jerk their upper body backwards towards whatever the threat is. They don’t strike in the traditional sense, and if grabbed near the rear portion of the body, they have trouble making contact. They do have tendency to flip wildly about after being provoked, though, this may be a deterrent to the predator.
Their colors do, however, do an excellent job of breaking up the line of their body as they dart away, making them difficult to follow. I suspect that this is their primary function, due to the secretive nature of the snakes in general. As to the predator avoidance, I am not convinced (nor are many of the herpetologists I know) that it is due primarily to their venom. The defenses of the supposed “mimics,” for instance, are radically different. The scarlet kingsnake (Lampropeltis triangulum elapsoides) begins, after being cornered, immediately attacking whatever has grasped it, biting and chewing (those little teeth hurt!). The scarlet snake (Cemophora coccinea) begins slowly writhing back and forth on the ground and autohemmorhaging from the mouth (quite disturbing). Combine this with the fact that almost the entire Lampropeltis triangulum complex (ranging across about 60% of the continental U.S.) shares the supposed mimicry pattern, while overlapping with the coral snake range in about 15% of it’s range, and the scarlet snake overlapping with the coral snake in about 30% of it’s range, and you begin to suspect claims of mimicry. All of these snakes are fossorial, however, and the pattern breaks up the line of their body when they are escaping. Avian predators may avoid them, but terrestrial predators like rats, raccoons and many other snakes don’t. I suspect that the pattern similarities are the result of some forces of convergent evolution for predator escape, rather than mimicry for predator avoidance.
As for the rest of the venomous snakes, yes, it is possible that their venom occasionally gives them some defensive advantage. This is, however, purely and distantly secondary, as evidenced by their reticence to even strike (as per the paper I linked) and their reticence to use venom when they strike. This, combined with the fact that venomous and nonvenomous snakes are eaten with gusto and little apparent regard for their toxicity by myriad predators, provides little evidence for their use of venom as a defense system. The idea that it is so effective as a defense that nonvenomous snakes are using Batesian mimicry to try to fool predators into believing they are venomous is ridiculous.