Tetrodotoxin affects survival probability of rough skinned newts (Taricha granulosa) faced with TTX resistant garter snake predators (Thamnophis sirtalis)
Lethal chemical defenses in prey species can have profound effects on interactions with predators. The presence of lethal defenses in prey can correct the selective imbalance suggested by the life dinner principle in which the fitness consequences of an encounter between predator and prey should be much greater for the prey species than the predator. Despite the apparent adaptive advantages of lethality the evolution of deadly prey presents a fundamental dilemma. How might lethal defenses confer an individual fitness advantage if both predators and prey die during interactions? We examined the interaction between the rough skinned newt (Taricha granulosa) which contains a powerful neurotoxin called tetrodotoxin (TTX) and the common garter snake (Thamnophis sirtalis) In some sympatric populations, Th. sirtalis have evolved physiological resistance to TTX. Whether the newts' toxin confers protection from snake predators or has been disarmed by the snakes' physiological resistance has not yet been directly tested. In predator prey trials, newts that were rejected by snakes had greater concentrations of TTX in their skin (4.52 3.49 mg TTX/g skin) than those that were eaten (1.72 1.53 mg TTX/g skin) Despite the plethora of taxa that appear to use TTX defensively, this is the first direct and quantitative demonstration of the antipredator efficacy of TTX. Because the survival probability of a newt (and thus fitness) is affected by individual TTX concentration, selection can drive the escalation of toxin levels in newts. The variable fitness consequences associated with both TTX levels of newts and resistance to TTX in snakes that may promote a strong and symmetrical coevolutionary relationship have now been demonstrated.
Chemoecology