The common garter snake is locked in an evolutionary arms race with the rough-skinned newt, a creature so toxic, the poison in a single newt can kill a dozen or more humans. Yet garter snake populations that share the rough-skinned newt’s range have evolved the ability to eat this lethal amphibian. Snakes that have developed resistance to the poison force the newts to produce more powerful toxin, which in turn prompts the snakes to develop more resistance—and the evolutionary escalation spirals onward.
Garter snakes in some regions that scientists recently studied have gained the upper hand in the arms race. The snakes have developed such extreme resistance, the newts can’t increase their toxicity fast enough to keep up. One researcher declared that in some geographic areas “the snakes have won.” But the snakes cannot be complacent in their victory, for resistance to newt poison comes at a high price. Super-resistant snakes have slow crawl speeds; a sluggish garter snake is easily snagged by the talons of a hawk, the teeth of a fox, or the claws of a bear. If the evolutionary price of super-resistance is too high, selection pressure will favor snakes with less resistance that can slither safely away from predators.
When the poison resistance of a snake population lessens while its escape speed increases, rough-skinned newts will slay some garter snakes that can’t tolerate the toxin—and the arms race between predatory snakes and their poisonous prey will again ramp up. There is no lasting peace in the universe, and nature never rests.