29 January 2007

Snake’s poison comes from toads

By
Cosmos Online
Toads in an Asian snake's diet provide toxins the snake uses to defend itself from predators, according to U.S. and Japanese researchers.
Snake's poison comes from toads

The tiger keelback snake, Rhabdophis tigrinus, has specialised glands on its neck that contain toxins known as bufadienolides. New research shows that these toxins are not produced by the snake, but instead are derived from toads in its diet. Credit: Deborah A. Hutchinson

SYDNEY: Toads in an Asian snake’s diet provide toxins the snake uses to defend itself from predators, according to U.S. and Japanese researchers.

“The demonstration that a snake is dependent on a diet of toads for chemical defence is highly unusual,” said lead author Deborah Hutchinson, a biologist at Old Dominion University in Virginia.

The phenomenon, known as dietary toxin sequestration, is well known among invertebrates but is much rarer in animals with a backbone, according to the study, which is published today in the U.S. journal Proceedings of the National Academy of Sciences.

The findings may have implications for human health as some of the compounds are new to science, and could potentially be used in the treatment of heart conditions.

The tiger keelback snake, Rhabdophis tigrinus, possesses specialised glands on its neck that contain toxins known as bufadienolides, which prevent a victim’s heart from functioning correctly. When in danger the snake directs the glands towards its attacker.

Several observations led Hutchinson and colleagues to suspect that these defensive compounds might not be synthesised by the snake itself. Notably, bufadienolides form a major part of the skin secretions of many species of toads. Also, snakes living on a toad-free island use the glands in defensive displays less frequently and flee more often when threatened.

To test their hypothesis, the researchers analysed the glandular fluid from snakes collected from two Japanese islands, one toad-free and the other with toads. They also performed feeding experiments in which hatchling R. tigrinus were reared on controlled diets that either included or lacked toads.

The researchers found that snakes living on the toadless island of Kinkazan lacked the bufadienolide compounds completely, while snakes from Ishima, where toads are plentiful, had high levels of bufadienolides. Snakes from Honshu (mainland Japan), where toad numbers vary, displayed a wide range of bufadienolide concentrations.

“Our results not only document a rare case in which defensive toxins are acquired from vertebrate prey but also provide a compelling example of the geographic, ecological, and evolutionary interplay between diet and antipredator defense,” they said.

Apart from finding that the defensive compounds were indeed derived from toads, the researchers showed that mothers were able to pass on toad-derived toxins to their offspring, giving them a handy advantage in early life.

Hutchinson believes the research findings may be of interest to medical scientists, as the toxins found in the glands of R. tigrinus are similar to plant chemicals that have been used to treat congestive heart failure for over 200 years.

“Six of the defensive compounds that we identified are new natural products, which may hold promise for the treatment of hypertension and congestive heart failure in humans,” Hutchinson said.

Now that the connection between R. tigrinus and the toads has been established, Hutchinson hopes to explore the mechanisms of uptake, modification, transport and storage of bufadienolides following the ingestion of a toad.

“Additionally, we are interested in exploring the possibility of a geographic correlation between the toxicity of toads and R. tigrinus, as well as the defensive chemistry in other species of Asian snakes that possess [these] glands,” she said.

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