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.
A few years ago, based on the snakes' defensive habits on different islands, Japanese researchers proposed that R. tigrinus may acquire its toxin from toads. Acquiring poison secondhand is not unheard of. The monarch butterfly is famous for harvesting its defensive poison from the milkweed plant, and certain brightly colored poisonous frogs collect their toxin from bugs, but Hutchinson says such cases in vertebrates are rare.
To find out if toads are indeed the source of the snake venom, Hutchinson and her colleagues raised R. tigrinus hatchlings on a diet of toads or fish. Most of the young snakes contained little or no poison in their nuchal glands at birth. They quickly accumulated toxin, however, after just days of feeding on poisonous Bufo quercicus toads, the researchers report in a paper published online January 29 by the Proceedings of the National Academy of Sciences USA.
The finding fits with the group's other principal observation: snakes living on the toad-free island of Kinkazan contain no bufadienolides in their nuchal glands, while those on the toad haven of Ishima are rich in poison.
Hutchinson says the snakes may have a hard time evolving a more sophisticated way of secreting their toxin. The nuchal glands come from a different embryonic layer than other vertebrate skin glands, which typically synthesize their own fluids and have ducts for emitting it, she says. The snakes, though, don't seem to be complaining about their peculiar glands. "They work pretty well for the snakes as they are," she says.