New York’s American Museum of Natural History is set to launch an exhibition on the world’s largest dinosaurs. Among the unknowns about how such massive monsters lived, Slate tackles one particularly pressing question: how did dinosaurs have sex? Paleontologists don’t know much about how dinosaurs mated, because soft tissues rarely turned up in fossils, and even a specimen’s gender was unknown until a few years ago, when it was discovered that females had a special calcium reservoir to help with eggshel formation. Dinosaurs probably had a “cloaca,” a single opening for urination, defecation and reproduction, in which case they may have had sex from behind. It’s also possible that males had no penises and like some birds, reproduced by squirting semen from one cloaca to another. Large males like the Mamenchisaurus, which was 60 feet long, probably mounted from behind like giraffes and elephants. Dinosaurs probably had courtship rituals, too, like triceratops who locked their horns to fight over mates. Even penis size is unknown: judging from different sizes of dinosaurs’ relatives, a Tyrannosaurus rex might have had a penis that ranged in size from 10 inches to 12 feet.