SGER: Sequestration of Dietary Toxins in Toad- and Mollusk-Eating Snakes

Old Dominion University Research Foundation, Norfolk VA

Investigators

Abstract

Many animals employ toxic or distasteful chemicals as a defense against predators. Such chemicals may either be synthesized from nontoxic molecules or obtained directly from the animal's diet and sequestered in the tissues. Sequestered defensive compounds (SDCs) have been documented in many invertebrates. Herbivorous insects often sequester toxic molecules from host plants, thereby becoming distasteful themselves. In contrast, SDCs have been reported rarely from vertebrate animals. The only known cases involve a few groups of specialized ant-eating frogs, such as poison dart frogs, that store potent toxins obtained from their prey. However, recent evidence suggests that other specialized vertebrate predators sequester defensive toxins from their diets. The investigators will explore the evidence for sequestration, and its physiological consequences, in two such groups, snakes that consume toads and snakes that feed on slugs. This project will be the first to study toxin sequestration in any amniote vertebrate (reptile, bird, or mammal) and the first to study sequestered toxins derived from vertebrate prey (toads). Toxin sequestration has important implications for behavior, community ecology, and conservation biology. Ecologically, SDCs link species across three trophic levels; the protection an individual enjoys against its own predators depends upon chemicals obtained from its prey. Therefore, if its natural prey becomes rare or extinct (as with the current decline in amphibian populations), a predator that relies upon SDCs can no longer acquire essential defensive chemicals. This study also has implications for understanding the physiological mechanisms underlying certain human cardiovascular disorders. Hypertension and related conditions have been linked to compounds that are chemically similar to toad toxins and are stored in the adrenal glands of mammals. Curiously, toad-eating snakes exhibit greatly enlarged adrenal glands, but the relationship between those glands and the snakes' diet has not been investigated further. The investigators will determine whether these specialized snakes sequester prey toxins and, if so, how they tolerate high levels of such normally debilitating compounds. Comparisons of the chemistry, physiology, and anatomy of toxin-consuming snakes will be made to the conditions in generalized species that consume nontoxic prey. Chromatographic and spectroscopic methods will be used to analyze tissues from these snakes to determine whether prey toxins accumulate or are simply detoxified and eliminated by them. Second, the impact of prey toxins on snake physiology will be determined by comparing the effect of toxic prey on locomotor speed and stamina in specialized versus generalized predators. Third, electrocardiography will be used to investigate the effect of toad toxins on heart function in toad-eating specialists. Toad toxins normally have an adverse effect on heart muscle, yet toad-eating snakes must withstand high concentrations of those toxins. Finally, the investigators will examine the fine structure of the adrenal glands in snakes that specialize on toxic prey to determine which tissues within that complex gland are enlarged. Those results will provide clues to mechanisms underlying tolerance of toxins and will assist in understanding mammalian cardiovascular responses to similar compounds. These integrated studies will reveal whether these specialized vertebrate predators sequester prey toxins for defense, and how they tolerate such normally dangerous compounds.