Party: For a religious feast, the Zoque people added poisonous barbasco plant roots to the water and ate fish that floated to the surface.
Hearty: Over centuries, a new study says, fish developed tolerance to the poison and passed the trait to offspring, leading to resistant populations. Michael Tobler from Oklahoma State University says "the fish responded to [the ritual] evolutionarily."
Lights Out: The festival has been discontinued to protect the cave.
Learn more about the Atlantic molly at the Encyclopedia of Life.