SCIENTIFIC AMERICAN (New York, New York) 29 January 07 Snake Bites the Toxic Toad That Feeds It--and Spreads Its Poison - Snake loads up on noxious predator repellant by consuming poisonous toads (JR Minkel)
Photo: Poison Ooze erupts from the neck ridges of the Asian snake Rhabdophis tigrinus when the skin is squeezed. By studying hatchlings , researchers have identified the poison's source: toxic toads. (Deborah A. Hutchinson)
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.
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.
Photo: Protect Your Neck: In its defensive posture, the juvenile Rhabdophis tigrinus displays its poison-filled neck to aggressors. (Alan H. Savitzky)
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.
http://www.sciam.com/article.cfm?chanID=sa003&articleID=6FD96483-E7F2-99DF-3C8A12ACF6BB25D8

NEW SCIENTIST (London, UK) 29 January 07 Snakes eat poisonous toads and steal their venom (Rowan Hooper)
Toads on the Japanese island of Ishima seem to be losing their evolutionary battle with snakes. Most snakes, and indeed most other animals, avoid eating toads because of the toxins in their skin. Rhabdophis tigrinus snakes, however, not only tolerate the toxins, they store the chemicals for their own defensive arsenal.
Deborah Hutchinson at Old Dominion University in Norfolk, Virginia, US, and colleagues, found that snakes on Ishima had bufadienolide compounds – toad toxins – in their neck glands, while those snakes living on the toad-free island of Kinkazan had none.
The snakes are unable to synthesise their own toxins, so they can only have derived bufadienolide compounds from their diet. Hutchinson’s team confirmed this by feeding snake hatchlings either a toad-rich or a toad-free diet. Toad-fed snakes accumulated toad-toxins in the nuchal glands on the back of the neck; snakes on a toad-free diet did not.
“Rhabdophis tigrinus is the first species known to use these dietary toxins for its own defence,” says Hutchinson.
What is more, when attacked, snakes on different islands react differently. On Ishima, snakes stand their ground and rely on the toxins in their nuchal glands to repel the predator. On Kinkazan, the snakes flee.
“Snakes on Kinkazan have evolved to use their nuchal glands in defence less often than other populations of snakes, presumably due to their lack of defensive compounds,” says Hutchinson.
Moreover, baby snakes benefit too. The team showed that snake mothers with high toxin levels pass on the compounds to their offspring. Snake hatchlings thus also enjoy the toad-derived protection.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas0610785104)
http://www.newscientist.com/article/dn11048-snakes-eat-poisonous-toads-and-steal-their-venom.html

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