Rough-skinned newt

At noon, when most salamanders are hidden under a log, a rough-skinned newt does the opposite. It walks across the open trail in no hurry at all, dark brown on top with a belly the color of a traffic cone, and it does not care who is watching. That confidence is not an accident. Its skin makes tetrodotoxin, the same poison found in pufferfish, in a dose strong enough that scientists estimate the skin of one newt could kill 25,000 mice. A raccoon, a bird, a curious dog: almost nothing that tries to eat this newt walks away from it.

Almost nothing, except one animal that has spent generations catching up. Common garter snakes in parts of the Pacific Northwest have evolved resistance to the toxin, a change in the same nerve channels the poison targets, so that swallowing a newt does not kill them the way it kills everything else. The newts in those same populations have answered back by making even more toxin, and the snakes there have kept evolving further resistance in turn. Biologists studying the two species have tracked this back-and-forth for decades and call it a coevolutionary arms race, one with no finish line: in a few populations the snakes have pulled ahead enough to eat newts that would drop any other predator in minutes.

The newt does not need the arms race everywhere. At Crater Lake, a local population called the Mazama newt had the whole lake to itself for thousands of years, the only real predator in water too new and too isolated for fish to reach on their own. It evolved less red coloring and less toxin than its relatives outside the crater, because inside the crater it did not need either. That changed once people started stocking the lake with fish starting in 1886, then crayfish in 1915. The crayfish have since spread across a growing share of the shoreline, eating newts and changing what grows on the lake bed along with them. A newt that walked around safely for millennia now shares its water with something that finally bites back.