The Zoque people of southern Mexico greet the rainy season with a religious ritual that involves poisoning a stream that runs into the nearby Cueva del Azufre, and gathering up the bounty of cave fish that float, anesthetized, to the surface. Those fish—considered gifts from the gods of the underworld—help keep the Zoque fed until crops grown in the rainy season can be harvested.
But centuries of annual die-offs, caused by a single, locally sourced poison, have functioned as a driver of natural selection. Today, researchers found, fish that live in Cueva del Azufre—downstream from the point where the Zoque poison the water—are becoming resistant to that poison.
Fish exposed to the annual ritual indeed proved more resistant to the toxin than fish that lived elsewhere, able to swim in poisoned waters for roughly 50 percent longer. As such, the poison from the ceremony apparently has over time helped select fish that can tolerate it — fish that cannot get captured and killed by the Zoque.
This is more than just a fascinating look at evolution in action. The local government recently banned the fish harvest ritual, out of concern over its impact on the fish population. The researchers hope their data will help explain what the ceremony actually does to local fish, and maybe lead to a compromise that would allow the Zoque to keep their traditions, and their rainy season food source.