Genetically, Darnell is male. But after being raised in water contaminated with the herbicide atrazine at a level of 2.5 parts per billion—slightly less than what’s allowed in our drinking water—he developed a female body, inside and out. He is also the mother of his children, having successfully mated with other males and spawned clutches of eggs. Recently he was moved to an atrazine-free tank and has turned lanky, losing the plump, pincushion look of a female frog. But last March, when UC-Berkeley integrative biology professor Tyrone B. Hayes opened him up to take a look, Darnell’s insides were still female. “He still has ovaries, but there’s no eggs in them,” Hayes told me the next day as we stood watching the frog, who swam over and inspected us soberly, then turned and flopped away.