Feeding Ecology and Tetrodotoxin Resistance in Thamnophis sirtalis

Why do predators consume toxic prey and evolve potentially costly resistance to prey toxin? Ecological and behavioral factors may explain why the benefit of consuming toxic prey might outweigh the cost of experiencing the toxin and result in resistance evolution. Toxic prey may be an important resource temporally, spatially, or for different classes of predators, and may be mediated by the predator’s condition (e.g., hungry predators or those with high energetic requirements). Evolved prey recognition behaviors may constrain predators to consume, or may facilitate inclusion of toxic prey into the predator’s diet. Through experience, predators may also learn to prefer certain prey. In an eastern population of garter snake (Thamnophis sirtalis), I measured resistance to tetrodotoxin (TTX) to determine if the trait had evolved from the ancestral condition as observed in western populations. I examined the ecological context in which Th. sirtalis preys upon sympatric Notophthalmus viridescens (red-spotted newt), which are chemically defended by TTX. I also explored behavioral hypotheses for why toxic newts are consumed.

At Mountain Lake Biological Station (MLBS) in Virginia, Th. sirtalis exhibit moderate resistance to TTX (7.66 mass adjusted mouse units (MAMU), range = 1.5 - 72.7 MAMU) compared to other populations of garter snakes, but do not display the expected molecular variation in a gene known to confer resistance in other snakes. On average, Th. sirtalis are predicted to experience minimal effects of TTX after consuming local newts, but yearling and juvenile snakes may be more susceptible to the amount of toxin found in sympatric efts. Notophthalmus viridescens made up 3.3% of the observed diet of Th. sirtalis at MLBS and newts were consumed only by females. A combination of congenital responsiveness toward, and learned preference for N. viridescens contributes to the inclusion of newts in the diet of those Th. sirtalis with sufficient resistance to consume them. The availability of N. viridescens during critical periods for Th. sirtalis, such as during gestation or prior to overwintering, may provide the ecological context that drives evolved consumption of toxic newts and may also be a means of selecting for TTX resistant snakes in this population.