ODU Researchers Discover Unusual Defenses of Asian Snakes

A Juvenile Rhabdophis tigrinus snake from Ishima, a Japanese island, curls in a defense posture. Credit: Old Dominion University, Alan Savitsky

Vipers are born with a poisonous bite they can use for defense. But what can nonpoisonous snakes do to ward off predators? What if they could borrow a dose of poison, perhaps by eating a toxic frog and recycling the toxins?

This might sound far-fetched, but it can happen, according to an international team of researchers including two Old Dominion University herpetologists. Their findings could be good news for endangered species of amphibians, and for humans with ailments that might be treated with heart-stimulating compounds in the toxins.

A paper written by Deborah A. Hutchinson, a postdoctoral research associate in the Department of Biological Sciences, together with her mentor, Alan H. Savitzky, professor of biological sciences, will appear this week on the Web site of Proceedings of the National Academy of Sciences (PNAS). Advance media alerts from the journal last week stirred the interest of The New York Times, Science magazine, National Geographic magazine and many other publications.

For more than seven years, Hutchinson has been studying the Asian snake Rhabdophis tigrinus and its relationship with a type of toxic toad. In the PNAS article, she and her co-authors describe dietary sequestration of toxins by the snakes. The process allows the snakes to store in their neck glands some of the toxins from the toads they have eaten.

When under attack, the snake thrusts the back of its neck toward the would-be predator. The snake’s dorsal skin often ruptures during a confrontation, causing liquid to leak from the glands. The liquid irritates mucous membranes and contains toxic steroids.

The researchers made their case by testing Rhabdophis tigrinus on several Japanese islands, one with a large population of the toads and another with none of the toads, and compared them with snakes from the main island of Honshu, where toads are scattered here and there. The presence of toxins in the snakes’ neck glands depended upon their access to the toads. Laboratory tests with snake hatchlings confirmed the fieldwork.

Snakes without the borrowed toxins were more likely to turn and flee from danger than to stay their ground and perform the neck-thrusting defensive maneuver, according to the researchers.

Surprisingly, the researchers also found that some of the mother snakes with high concentrations of the toxins in their glands were able to pass on defensive toxins to their offspring. This finding shows how maternal diet can bestow a survival advantage on offspring.

Hutchinson said the research identified six defensive compounds in Rhabdophis tigrinus that are new natural products and may hold promise in medical treatments for humans suffering from hypertension or related blood pressure disorders.

Also, she said, “The demonstration that a snake is dependent on a diet of toads for chemical defense is highly unusual and, therefore, important to the field of biology. (And) the fact that R. tigrinus is dependent on toads for defense has implications for conservation because amphibians are currently undergoing a global decline.”

Hutchinson, who earned a master’s degree in biology in 2001 and doctorate in ecological sciences in 2006 from ODU, used the research project for her doctoral dissertation.

Savitzky said the project was proposed by Akira Mori from Kyoto University in Japan, and would not have been possible without the chemical analyses provided by two Cornell University faculty members, Jerrold Meinwald and Frank Schroeder. The research team also included Gordon Burghardt, a biologist at the University of Tennessee, and Xiaogang Wu, another chemist at Cornell University. These individuals are co-authors of the research paper.

Funding from the National Science Foundation was critical in supporting the important chemical analyses, Savitzky said. The findings may have been released sooner, Hutchinson added, “but we spent additional time ensuring that our unique findings would be published in a highly respected journal.”

Source / Credit: Old Dominion University