Defensive Sequestration of Toad Toxins in the Asian Snake Rhabdophis tigrinus

Old Dominion University Research Foundation, Norfolk VA

Investigators

Abstract

Many animals employ toxic chemicals to deter predators. Most species synthesize those compounds from simpler molecules, but some acquire their toxins from an external source, usually their diet, and store those toxins in their own tissues. Such sequestered defensive compounds are well-known among invertebrates, but vertebrate examples are rare. This project investigates the first documented case of a specialized reptilian defensive system based upon sequestered toxins. The Japanese snake Rhabdophis tigrinus delivers defensive compounds known as bufadienolides from specialized structures called nuchal glands, located in the skin of the neck. The investigators have recently demonstrated chemically that the toxins are derived from toads that the snakes have consumed as prey. Bufadienolides are not only distasteful but also induce potentially lethal contractions of the heart muscle in many predators. Preliminary experiments have shown that most Rhabdophis lack bufadienolides when they hatch and must feed upon toads in order to accumulate the toxins. However, females with exceptionally high levels of toxins in their tissues produce offspring that possess bufadienolides at the time of hatching and thus are chemically defended immediately. This project examines the relationship between diet, defensive chemistry, and cardiac function in Rhabdophis. Sequestered defensive compounds forge an especially strong ecological link between predator and prey because of the direct relationship between diet and defense. The presence of sequestered toxins has profound implications for the behavioral and reproductive ecology of the snakes, including the potential impact of prey selection and the possibility that a female's diet may determine how well her offspring are defended. This project will provide the first detailed analysis of the relationship between diet and toxin accumulation in Rhabdophis. The research will involve close collaboration between biologists and natural product chemists, including the cross-training of both graduate students and postdoctoral researchers. Undergraduate students will obtain research experience through their participation as laboratory assistants. Collaboration between American and Japanese colleagues is an especially important element of this project, and American biologists will conduct experiments at one of Japan's major national universities.