On a September afternoon in Tabasco in southern Mexico, the two of us made our way through the rain forest toward the sound of flowing water, in pursuit of a small but important fish. Iridescent blue morpho butterflies flitted by, and howler monkeys roared from the trees overhead, offering welcome distractions from the broiling heat and humidity. Soon we spotted a green kingfisher diving into the nearby creek and then returning to its perch to consume its catch. The bird had nabbed the same kind of fish we were after: an Atlantic molly (Poecilia mexicana), a member of a family of fishes called poeciliids, whose females give birth to live young and whose males have flashy colors that make them popular among aquarists around the world.