I agree with what everyone else has said about the rattlesnake just doing a better job, rather than mimicry in either direction. There's probably no way to prove this, but that was the conclusion I came to pretty quickly once I gave it a little thought.



I would also like to throw out a couple of other ideas for discussion as to why non-venomous snakes "rattle".



1) Maybe it's more of a distaction than a warning. Lots of snakes that don't really "rattle" will hide their heads and wiggle their tails so that a predator will go after their tail which is less vulnerable. "Rattling" could have evolved as simply a more effective version of this.

2) Even many nonvenomous snakes can inflict a painful bite, and so a distintive warning associated with that bite could still deter at least some predators even without venom. This seems more likely to be the current use in some snakes since they may gape, hiss, inflate their necks and/or strike while rattling.

3) Dr. Fry's research suggests at least the possibility that in the evolutionary past a much larger percentage of snakes may have been venomous which would make a "rattling equals danger" warning more plausable, and maybe it's carried through even though the venom hasn't.

4) All of the above.

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