Amphibans are poisonous to prevent other animals from eating them. However, a new study published this week in the open-access journal PLoS Biology investigating the toxic rough-skinned newts (Taricha species) finds that, despite being among the most poisonous known animals (because they contain the deadly tetrodotoxin), in some regions these newts have no effect on their main predator, the garter snake, as the snakes have evolved resistance to a poison so strong that one-newtsworth can kill thousands of mice.
The garter snake and the rough-skinned newt are an example of a co-evolutionary arms race: two groups, in this case predators versus prey, competing to survive and reproduce optimally. The two groups evolve interacting traits as each struggles to out-compete the other; the prey animal may evolve poison and, in response, the predator evolves resistance and then the prey evolves a stronger poison, etc. The researchers found that in the newt-snake system, some snakes have managed to escape from this battle by becoming super-resistant. Charles Hanifin and colleagues studied the interaction between the two species all the way from British Columbia to Southern California. As expected, if the two traits have evolved in a tit-for-tat way, areas where the newts were most toxic were also those where the snakes were most resistant. However, surprisingly, they found that often the snakes had won by becoming resistant to even the most toxic local newt. Where traits are mismatched in this snake-always-wins way a third of the studied locations there is no scope for further evolution of the conflict.
What has allowed the snakes this crushing victory? Hanifin found that some of the most resistant snakes differed only in one unit of their genetic code and that this tiny change had altered the structure of a snake protein enough that the toxin could no longer bind to it. This is an example of how a single random mutation can generate a major change in an animal.