Sparks fly when a female nematode meets her mate in a Petri dish. Tracking him by smell, she beelines over and is pregnant within moments of physical contact. But for the hermaphroditic version of these tiny roundworms, it's a very different story. Anatomically female but capable of self-fertilizing with their own supply of sperm, hermaphrodites remain emphatically uninterested in mating—until their sperm supply runs dry. Only then will they seek out males.