Rhabdophis Toxins: From Toad to Snake

The small colubrid snakes of the genus Rhabdophis have a shady past in the pet trade. Because they resemble certain harmless garter-snake like species, they were imported into the U.S. and U.K. under the wrong names, and ended up causing medically-significant emergencies when they bit their new owners. Rhabdophis are one of the few rear-fanged groups that can cause serious harm or even fatalities. A new study shows where they acquire this toxicity:

"A new study shows that the Asian snake Rhabdophis tigrinus becomes poisonous by sequestering toxins from its prey which consists of venomous toads. The research is published in the current issue of The Proceedings of the National Academy of Sciences.

"Analyzing differences between snakes living on toad-rich and toad-deficient islands in Japan, Researchers lead by Deborah A. Hutchinson of Old Dominion University in Norfold, Virginia, found that Japanese grass snake or Yamakagashi, as the snake is known locally, did not manufacture its own venom, but instead relied on that found in toxic toads:
"'The researchers found that snakes living on Japan’s toad-free island of Kinkazan lacked the toad’s toxic bufadienolide compounds completely. Snakes from Ishima, where toads are plentiful, had high levels of bufadienolides. R. tigrinus from Honshu, where toad numbers vary, displayed a wide range of bufadienolide concentrations. Feeding R. tigrinus hatchlings toad-rich and toad-free diets confirmed these results.'

"The study also found that snake mothers with high concentrations of the toxin are able to pass bufadienolide toxin on their offspring helping protect them from predators.
"The Yamakagashi stores the sequestered toxins in 'a series of paired structures known as nuchal glands in the dorsal skin of the neck,' according to the researchers. When threatened the snake takes a defensive position that exposes the toxin-containing nuchal glands to predators.

"While sequestering defensive toxins from prey is unusual among terrestrial vertebrates it is not unknown. Research published last year by Valerie C. Clark of Cornell University showed that poison dart frogs (Dendrobates species) and their Madagascar counterparts, the Mantella frogs, sequester toxic skin chemicals, called alkaloids, from the ants they eat. These alkaloids protect the frogs from predation. Similarly, some garter snakes are known to store tetrodotoxin from ingested newts while birds in New Guinea appear to sequester poisons from insects.

"Citation: 'Dietary sequestration of defensive steroids in nuchal glands of the Asian snake Rhabdophis tigrinus' by Deborah A. Hutchinson, Akira Mori, Alan H. Savitzky, Gordon M. Burghardt, Xiaogang Wu, Jerrold Meinwald, and Frank C. Schroeder."

NewScientist notes:

"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."