From Scientific American:
It sounds like something straight out of a video game: A snake collects toxin by biting a poisonous toad and uses that venom as a defense against hawks and other predators. But that is exactly what researchers say the Asian snake Rhabdophis tigrinus does, based on studies of glandular fluid from hatchlings and adult snakes on two Japanese islands.
Some R. tigrinus snakes carry toxins called bufadienolides in their nuchal glands, sacks located under a ridge of skin along their upper necks. When threatened, they arch their necks, exposing the poisonous ridge to an antagonist. The clawing and biting of hawks and other predators most likely rips the skin and lets the poison ooze out, potentially blinding the snake’s attackers, says herpetologist Deborah Hutchinson of Old Dominion University in Norfolk, Va. “It might not kill the predator but it would be noxious enough to deter predation,” she says.